diff --git a/.gitignore b/.gitignore
index 89b5f0b..21951ac 100755
--- a/.gitignore
+++ b/.gitignore
@@ -1,5 +1,6 @@
 **/target/
 **/Cargo.lock
+mers_language_server/mers_language_server
 tree-sitter-mers/log.html
 tree-sitter-mers/package-lock.json
 tree-sitter-mers/node_modules
diff --git a/mers_language_server/Cargo.toml b/mers_language_server/Cargo.toml
new file mode 100644
index 0000000..5690c5b
--- /dev/null
+++ b/mers_language_server/Cargo.toml
@@ -0,0 +1,12 @@
+[package]
+name = "mers_language_server"
+version = "0.1.0"
+edition = "2021"
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[dependencies]
+mers_lib = { path = "../mers_lib", features = ["parse"] }
+tokio = { version = "1.34.0", default-features = false, features = ["macros", "rt", "io-std"] }
+tower-lsp = "0.20.0"
+line-span = "0.1.5"
diff --git a/mers_language_server/src/main.rs b/mers_language_server/src/main.rs
new file mode 100644
index 0000000..6c277d3
--- /dev/null
+++ b/mers_language_server/src/main.rs
@@ -0,0 +1,484 @@
+use std::{collections::HashMap, sync::Arc};
+
+use line_span::LineSpanExt;
+use mers_lib::{errors::CheckError, prelude_compile::Source};
+use tokio::sync::Mutex;
+use tower_lsp::{
+    lsp_types::{
+        CodeAction, CodeActionKind, CodeActionOptions, CodeActionOrCommand, CodeActionParams,
+        CodeActionProviderCapability, CodeActionResponse, DidChangeTextDocumentParams,
+        DidOpenTextDocumentParams, DocumentChanges, Hover, HoverContents, HoverParams,
+        HoverProviderCapability, InitializeParams, InitializeResult, InitializedParams,
+        MarkedString, MessageType, OneOf, OptionalVersionedTextDocumentIdentifier, Position,
+        ServerCapabilities, TextDocumentEdit, TextDocumentItem, TextDocumentSyncCapability,
+        TextDocumentSyncKind, TextEdit, WorkDoneProgressOptions, WorkspaceEdit,
+    },
+    Client, LanguageServer, LspService, Server,
+};
+
+#[tokio::main(flavor = "current_thread")]
+async fn main() {
+    let stdin = tokio::io::stdin();
+    let stdout = tokio::io::stdout();
+
+    let (service, socket) = LspService::new(|client| Backend::new(client));
+    Server::new(stdin, stdout, socket).serve(service).await;
+}
+
+struct Backend {
+    client: Client,
+    main_document: Arc<Mutex<Option<tower_lsp::lsp_types::Url>>>,
+    current_document: Arc<Mutex<Option<tower_lsp::lsp_types::Url>>>,
+    documents: Arc<Mutex<HashMap<tower_lsp::lsp_types::Url, TextDocumentItem>>>,
+    last_compiled: Arc<Mutex<Option<ParseCompileCheckResult>>>,
+}
+impl Backend {
+    pub fn new(client: Client) -> Self {
+        Self {
+            client,
+            main_document: Arc::new(Mutex::new(None)),
+            current_document: Arc::new(Mutex::new(None)),
+            documents: Arc::new(Mutex::new(HashMap::new())),
+            last_compiled: Arc::new(Mutex::new(None)),
+        }
+    }
+}
+
+#[tower_lsp::async_trait]
+impl LanguageServer for Backend {
+    async fn did_open(&self, p: DidOpenTextDocumentParams) {
+        *self.last_compiled.lock().await = None;
+        let mut md = self.main_document.lock().await;
+        if md.is_none() {
+            *md = Some(p.text_document.uri.clone());
+        }
+        self.documents
+            .lock()
+            .await
+            .insert(p.text_document.uri.clone(), p.text_document);
+    }
+    async fn did_change(&self, p: DidChangeTextDocumentParams) {
+        *self.last_compiled.lock().await = None;
+        if let Some(document) = self.documents.lock().await.get_mut(&p.text_document.uri) {
+            for change in p.content_changes {
+                if let Some(_range) = change.range {
+                    self.client
+                        .log_message(MessageType::WARNING, "Received incremental document update")
+                        .await;
+                } else {
+                    document.text = change.text;
+                }
+            }
+        } else {
+            self.client
+                .log_message(
+                    MessageType::WARNING,
+                    "Received changes for a document that was never loaded",
+                )
+                .await;
+        }
+    }
+
+    async fn initialize(
+        &self,
+        _: InitializeParams,
+    ) -> tower_lsp::jsonrpc::Result<InitializeResult> {
+        // TODO: PositionEncodingKind, set char to 0-based *byte*-index
+        Ok(InitializeResult {
+            capabilities: ServerCapabilities {
+                hover_provider: Some(HoverProviderCapability::Simple(true)),
+                code_action_provider: Some(CodeActionProviderCapability::Options(
+                    CodeActionOptions {
+                        code_action_kinds: Some(vec![CodeActionKind::new("Test")]),
+                        work_done_progress_options: WorkDoneProgressOptions::default(),
+                        resolve_provider: None,
+                    },
+                )),
+                text_document_sync: Some(TextDocumentSyncCapability::Kind(
+                    TextDocumentSyncKind::FULL,
+                )),
+                ..Default::default()
+            },
+            ..Default::default()
+        })
+    }
+
+    async fn initialized(&self, _: InitializedParams) {
+        self.client
+            .log_message(MessageType::INFO, "server initialized!")
+            .await;
+    }
+
+    async fn shutdown(&self) -> tower_lsp::jsonrpc::Result<()> {
+        Ok(())
+    }
+
+    async fn hover(&self, params: HoverParams) -> tower_lsp::jsonrpc::Result<Option<Hover>> {
+        *self.last_compiled.lock().await = None;
+        let byte_pos = {
+            match self.main_document.lock().await.as_ref() {
+                Some(uri) => {
+                    let doc = self.documents.lock().await;
+                    let doc = doc.get(uri);
+                    let doc = doc.map(|doc| doc.text.as_str()).unwrap_or("");
+                    let pos_in_og = get_byte_pos_in_og(
+                        doc,
+                        params.text_document_position_params.position.line as _,
+                        params.text_document_position_params.position.character as _,
+                    );
+                    Some(
+                        mers_lib::prelude_compile::Source::new_from_string(doc.to_owned())
+                            .pos_from_og(pos_in_og, false),
+                    )
+                }
+                None => None,
+            }
+        };
+        let mut infos_at_cursor_hook_index = None;
+        let pcc = self
+            .parse_compile_check(|i1, i3| {
+                i1.global.enable_hooks = true;
+                i3.global.enable_hooks = true;
+                if let Some(byte_pos) = byte_pos {
+                    let mut i1sia = i1.global.save_info_at.lock().unwrap();
+                    let mut i3sia = i3.global.save_info_at.lock().unwrap();
+                    infos_at_cursor_hook_index = Some((i1sia.len(), i3sia.len()));
+                    i1sia.push((vec![], byte_pos, 2)); // 2 -> ignore outer scope
+                    i3sia.push((vec![], byte_pos, 2)); // -> global error doesn't show up in "local" section
+                }
+            })
+            .await;
+        Ok(Some(Hover {
+            contents: HoverContents::Scalar(MarkedString::String(match pcc.as_ref().unwrap() {
+                ParseCompileCheckResult::NoMainDocument => {
+                    format!("# No main document\n\n(open at least one file!)")
+                }
+                ParseCompileCheckResult::MainDocumentNotFound => {
+                    format!("# Main document not found\n\n(probably a bug)")
+                }
+                ParseCompileCheckResult::ErrorWhileParsing(_, _, e) => {
+                    format!("# Error (can't parse):\n```\n{e}```")
+                }
+                ParseCompileCheckResult::ErrorWhileCompiling(_, _, e, _, info1) => {
+                    let i1sia = info1.global.save_info_at.lock().unwrap();
+                    format!(
+                        "# Error (can't compile):{}\n## Global error:\n\n```\n{e}```",
+                        if let Some(i1) = infos_at_cursor_hook_index.map(|(i, _)| &i1sia[i].0) {
+                            format!(
+                                "\n\n## Local errors:{}\n---\n",
+                                i1.iter()
+                                    .filter_map(|(src_range, _info, res)| {
+                                        let src_snippet = src_range.in_file().src()
+                                            [src_range.start().pos()..src_range.end().pos()]
+                                            .replace("\n", "  ")
+                                            .replace("\r", "  ");
+                                        match res {
+                                            Ok(()) => None,
+                                            Err(e) => {
+                                                Some(format!("\n- `{src_snippet}`\n```\n{e}```\n"))
+                                            }
+                                        }
+                                    })
+                                    .collect::<String>()
+                            )
+                        } else {
+                            format!("")
+                        }
+                    )
+                }
+                ParseCompileCheckResult::DoesntPassChecks(_, _, e, _, _, _, info3) => {
+                    let i3sia = info3.global.save_info_at.lock().unwrap();
+                    format!(
+                        "# Error (doesn't pass checks):\n```\n{e}```{}",
+                        if let Some(i3) = infos_at_cursor_hook_index.map(|(_, i)| &i3sia[i].0) {
+                            format!(
+                                "\n\n## Local types:{}",
+                                i3.iter()
+                                    .map(|(src_range, _info, res)| {
+                                        let src_snippet = src_range.in_file().src()
+                                            [src_range.start().pos()..src_range.end().pos()]
+                                            .replace("\n", "  ")
+                                            .replace("\r", "  ");
+                                        match res {
+                                            Ok(local_type) => {
+                                                format!("\n- `{src_snippet}` :: `{local_type}`")
+                                            }
+                                            Err(e) => {
+                                                format!(
+                                                    "\n- `{src_snippet}` :: ! Err !\n```\n{e}```\n"
+                                                )
+                                            }
+                                        }
+                                    })
+                                    .collect::<String>()
+                            )
+                        } else {
+                            format!("")
+                        }
+                    )
+                }
+                ParseCompileCheckResult::Checked(_, _, out_type, _, _, _, info3) => {
+                    let i3sia = info3.global.save_info_at.lock().unwrap();
+                    if let Some(i3) = infos_at_cursor_hook_index.map(|(_, i)| &i3sia[i].0) {
+                        format!(
+                            "## Local types:{}",
+                            i3.iter()
+                                .map(|(src_range, _info, res)| {
+                                    let src_snippet = src_range.in_file().src()
+                                        [src_range.start().pos()..src_range.end().pos()]
+                                        .replace("\n", "  ")
+                                        .replace("\r", "  ");
+                                    match res {
+                                        Ok(local_type) => {
+                                            format!("\n- `{src_snippet}` :: `{local_type}`")
+                                        }
+                                        Err(e) => {
+                                            format!("\n- `{src_snippet}` :: ! Err !\n```\n{e}```\n")
+                                        }
+                                    }
+                                })
+                                .collect::<String>()
+                        )
+                    } else {
+                        format!("Program's return type: `{out_type}`")
+                    }
+                }
+            })),
+            range: None,
+        }))
+    }
+
+    async fn code_action(
+        &self,
+        params: CodeActionParams,
+    ) -> tower_lsp::jsonrpc::Result<Option<CodeActionResponse>> {
+        Ok(Some(
+            if let Some(doc) = self.documents.lock().await.get(&params.text_document.uri) {
+                let mut src = mers_lib::prelude_compile::Source::new_from_string(doc.text.clone());
+                let srca = Arc::new(src.clone());
+                match mers_lib::prelude_compile::parse(&mut src, &srca) {
+                    Err(_) => return Ok(None),
+                    Ok(parsed) => {
+                        let pos_start = srca.pos_from_og(
+                            get_byte_pos_in_og(
+                                srca.src_og(),
+                                params.range.start.line as _,
+                                params.range.start.character as _,
+                            ),
+                            false,
+                        );
+                        let pos_end = srca.pos_from_og(
+                            get_byte_pos_in_og(
+                                srca.src_og(),
+                                params.range.end.line as _,
+                                params.range.end.character as _,
+                            ),
+                            true,
+                        );
+                        let mut statements = vec![];
+                        iter_over_statements(parsed.as_ref(), &mut |s| {
+                            // Wrap in `( )`
+                            if s.source_range().start().pos() <= pos_start
+                                && pos_end <= s.source_range().end().pos()
+                            {
+                                statements.push(s);
+                                s.inner_statements()
+                            } else {
+                                vec![]
+                            }
+                        });
+                        let statements = statements
+                            .into_iter()
+                            .rev()
+                            .take(3)
+                            .map(|s| {
+                                (
+                                    s.source_range()
+                                        .in_file()
+                                        .pos_in_og(s.source_range().start().pos(), true),
+                                    s.source_range()
+                                        .in_file()
+                                        .pos_in_og(s.source_range().end().pos(), false),
+                                    s,
+                                )
+                            })
+                            .collect::<Vec<_>>();
+                        let mut actions = vec![];
+
+                        // Wrap in `( )`
+                        for (og_start, og_end, statement) in &statements {
+                            actions.push(CodeActionOrCommand::CodeAction(CodeAction {
+                                title: format!(
+                                    "Wrap `{}` in `( )`",
+                                    srca.src()[statement.source_range().start().pos()
+                                        ..statement.source_range().end().pos()]
+                                        .trim()
+                                        .replace("\n", "  ")
+                                        .replace("\r", "  ")
+                                ),
+                                edit: Some(WorkspaceEdit {
+                                    changes: None,
+                                    document_changes: Some(DocumentChanges::Edits(vec![
+                                        TextDocumentEdit {
+                                            text_document:
+                                                OptionalVersionedTextDocumentIdentifier {
+                                                    uri: params.text_document.uri.clone(),
+                                                    version: None,
+                                                },
+                                            edits: vec![OneOf::Left(TextEdit {
+                                                range: tower_lsp::lsp_types::Range {
+                                                    start: get_lsp_pos_in_og(
+                                                        statement.source_range().in_file().src_og(),
+                                                        *og_start,
+                                                    ),
+                                                    end: get_lsp_pos_in_og(
+                                                        statement.source_range().in_file().src_og(),
+                                                        *og_end,
+                                                    ),
+                                                },
+                                                new_text: format!(
+                                                    "({})",
+                                                    &statement.source_range().in_file().src_og()
+                                                        [*og_start..*og_end]
+                                                ),
+                                            })],
+                                        },
+                                    ])),
+                                    ..Default::default()
+                                }),
+                                ..Default::default()
+                            }));
+                        }
+
+                        actions
+                    }
+                }
+            } else {
+                return Ok(None);
+            },
+        ))
+    }
+}
+
+/// Recursively iterate over statements.
+fn iter_over_statements<'b, S>(root: S, for_each: &'b mut impl FnMut(S) -> Vec<S>) {
+    for i in for_each(root) {
+        iter_over_statements(i, for_each);
+    }
+}
+
+fn get_byte_pos_in_og(og: &str, line: usize, character: usize) -> usize {
+    let line = og.line_spans().skip(line).next();
+    if let Some(line) = line {
+        line.start() + character
+    } else {
+        og.len()
+    }
+}
+fn get_lsp_pos_in_og(og: &str, pos: usize) -> Position {
+    let mut fallback = Position {
+        line: 0,
+        character: 0,
+    };
+    for (line_nr, line) in og.line_spans().enumerate() {
+        fallback = Position {
+            line: line_nr as _,
+            character: line.as_str().len() as _,
+        };
+        if pos <= line.end() {
+            return Position {
+                line: line_nr as _,
+                character: (pos.saturating_sub(line.start())).min(line.as_str().len()) as _,
+            };
+        }
+    }
+    fallback
+}
+
+enum ParseCompileCheckResult {
+    NoMainDocument,
+    MainDocumentNotFound,
+    ErrorWhileParsing(Source, Arc<Source>, CheckError),
+    ErrorWhileCompiling(
+        Source,
+        Arc<Source>,
+        CheckError,
+        Box<dyn mers_lib::prelude_compile::ParsedMersStatement>,
+        mers_lib::program::parsed::Info,
+    ),
+    DoesntPassChecks(
+        Source,
+        Arc<Source>,
+        CheckError,
+        Box<dyn mers_lib::prelude_compile::ParsedMersStatement>,
+        Box<dyn mers_lib::prelude_compile::RunMersStatement>,
+        mers_lib::program::parsed::Info,
+        mers_lib::program::run::CheckInfo,
+    ),
+    Checked(
+        Source,
+        Arc<Source>,
+        mers_lib::data::Type,
+        Box<dyn mers_lib::prelude_compile::ParsedMersStatement>,
+        Box<dyn mers_lib::prelude_compile::RunMersStatement>,
+        mers_lib::program::parsed::Info,
+        mers_lib::program::run::CheckInfo,
+    ),
+}
+impl Backend {
+    /// is guaranteed to return `MutexGuard<Some(_)>`.
+    async fn parse_compile_check(
+        &self,
+        func_modify_infos: impl FnOnce(
+            &mut mers_lib::program::parsed::Info,
+            &mut mers_lib::program::run::CheckInfo,
+        ),
+    ) -> tokio::sync::MutexGuard<'_, Option<ParseCompileCheckResult>> {
+        let mut last_compiled = self.last_compiled.lock().await;
+        if last_compiled.is_none() {
+            *last_compiled = Some(
+                if let Some(source) = self.main_document.lock().await.clone() {
+                    if let Some(document) = self.documents.lock().await.get(&source) {
+                        let src_from = match document.uri.to_file_path() {
+                            Ok(path) => mers_lib::parsing::SourceFrom::File(path),
+                            Err(_) => mers_lib::parsing::SourceFrom::Unspecified,
+                        };
+                        let mut src =
+                            mers_lib::prelude_compile::Source::new(src_from, document.text.clone());
+                        let srca = Arc::new(src.clone());
+                        match mers_lib::parsing::parse(&mut src, &srca) {
+                            Err(e) => ParseCompileCheckResult::ErrorWhileParsing(src, srca, e),
+                            Ok(parsed) => {
+                                let (mut i1, _, mut i3) = mers_lib::prelude_compile::Config::new()
+                                    .bundle_std()
+                                    .infos();
+                                func_modify_infos(&mut i1, &mut i3);
+                                match parsed.compile(
+                                    &mut i1,
+                                    mers_lib::prelude_compile::CompInfo::default(),
+                                ) {
+                                    Err(e) => ParseCompileCheckResult::ErrorWhileCompiling(
+                                        src, srca, e, parsed, i1,
+                                    ),
+                                    Ok(compiled) => match compiled.check(&mut i3, None) {
+                                        Err(e) => ParseCompileCheckResult::DoesntPassChecks(
+                                            src, srca, e, parsed, compiled, i1, i3,
+                                        ),
+                                        Ok(out_type) => ParseCompileCheckResult::Checked(
+                                            src, srca, out_type, parsed, compiled, i1, i3,
+                                        ),
+                                    },
+                                }
+                            }
+                        }
+                    } else {
+                        ParseCompileCheckResult::MainDocumentNotFound
+                    }
+                } else {
+                    ParseCompileCheckResult::NoMainDocument
+                },
+            );
+        }
+        last_compiled
+    }
+}
diff --git a/mers_lib/src/data/function.rs b/mers_lib/src/data/function.rs
index c793c2e..6ce9c84 100755
--- a/mers_lib/src/data/function.rs
+++ b/mers_lib/src/data/function.rs
@@ -17,6 +17,10 @@ pub struct Function {
     pub info_check: Arc<Mutex<CheckInfo>>,
     pub out: Arc<dyn Fn(&Type, &mut CheckInfo) -> Result<Type, CheckError> + Send + Sync>,
     pub run: Arc<dyn Fn(Data, &mut crate::program::run::Info) -> Data + Send + Sync>,
+    pub inner_statements: Option<(
+        Arc<Box<dyn crate::prelude_compile::RunMersStatement>>,
+        Arc<Box<dyn crate::prelude_compile::RunMersStatement>>,
+    )>,
 }
 impl Function {
     pub fn with_info_run(&self, info: Arc<Info>) -> Self {
@@ -25,6 +29,10 @@ impl Function {
             info_check: Arc::clone(&self.info_check),
             out: Arc::clone(&self.out),
             run: Arc::clone(&self.run),
+            inner_statements: self
+                .inner_statements
+                .as_ref()
+                .map(|v| (Arc::clone(&v.0), Arc::clone(&v.1))),
         }
     }
     pub fn with_info_check(&self, check: CheckInfo) {
diff --git a/mers_lib/src/errors/mod.rs b/mers_lib/src/errors/mod.rs
index 050e5e1..d524ec5 100644
--- a/mers_lib/src/errors/mod.rs
+++ b/mers_lib/src/errors/mod.rs
@@ -39,8 +39,12 @@ impl SourceRange {
     pub fn end(&self) -> SourcePos {
         self.end
     }
+    pub fn in_file(&self) -> &Arc<Source> {
+        &self.in_file
+    }
 }
-pub struct CheckError(Vec<CheckErrorComponent>);
+#[derive(Clone)]
+pub struct CheckError(pub Vec<CheckErrorComponent>);
 #[allow(non_upper_case_globals)]
 pub mod error_colors {
     use colored::Color;
@@ -76,7 +80,8 @@ pub mod error_colors {
     pub const BadTypeFromParsed: Color = Color::Blue;
     pub const TypeAnnotationNoClosingBracket: Color = Color::Blue;
 }
-enum CheckErrorComponent {
+#[derive(Clone)]
+pub enum CheckErrorComponent {
     Message(String),
     Error(CheckError),
     ErrorWithDifferentSource(CheckError),
diff --git a/mers_lib/src/parsing/mod.rs b/mers_lib/src/parsing/mod.rs
index 9e68d27..e69770d 100755
--- a/mers_lib/src/parsing/mod.rs
+++ b/mers_lib/src/parsing/mod.rs
@@ -1,5 +1,7 @@
 use std::{fmt::Debug, path::PathBuf, sync::Arc};
 
+use line_span::{LineSpan, LineSpanExt};
+
 use crate::{
     errors::{CheckError, SourcePos},
     program::{self, parsed::block::Block},
@@ -306,9 +308,37 @@ impl Source {
         }
         pos
     }
+    pub fn pos_from_og(&self, mut pos: usize, behind_comment: bool) -> usize {
+        for (start, comment) in &self.comments {
+            if *start + comment.len() <= pos {
+                pos -= comment.len();
+            } else if *start <= pos {
+                return if behind_comment {
+                    *start + comment.len()
+                } else {
+                    *start
+                };
+            } else {
+                break;
+            }
+        }
+        pos
+    }
     pub fn src_og(&self) -> &String {
         &self.src_og
     }
+    /// If found, returns `Some((line_nr, line_str, byte_pos_in_line_may_be_out_of_string_bounds))`
+    pub fn get_line_and_char_from_pos_in_str(
+        byte_index: usize,
+        str: &str,
+    ) -> Option<(usize, LineSpan<'_>, usize)> {
+        for (line, line_span) in str.line_spans().enumerate() {
+            if line_span.start() <= byte_index {
+                return Some((line, line_span, byte_index - line_span.start()));
+            }
+        }
+        None
+    }
 }
 
 impl Drop for Source {
diff --git a/mers_lib/src/parsing/statements.rs b/mers_lib/src/parsing/statements.rs
index 8fd0f5a..8ad7cfe 100755
--- a/mers_lib/src/parsing/statements.rs
+++ b/mers_lib/src/parsing/statements.rs
@@ -123,7 +123,7 @@ pub fn parse(
                     .msg(format!("EOF after `:=`"))
             })?;
             first = Box::new(program::parsed::init_to::InitTo {
-                pos_in_src: (pos_in_src, src.get_pos(), srca).into(),
+                pos_in_src: (first.source_range().start(), src.get_pos(), srca).into(),
                 target: first,
                 source,
             });
@@ -133,7 +133,10 @@ pub fn parse(
             src.next_word_allow_colon();
             let source = parse(src, srca)?.ok_or_else(|| {
                 CheckError::new()
-                    .src(vec![((pos_in_src, src.get_pos(), srca).into(), None)])
+                    .src(vec![(
+                        (first.source_range().start(), src.get_pos(), srca).into(),
+                        None,
+                    )])
                     .msg(format!("EOF after `=`"))
             })?;
             first = Box::new(program::parsed::assign_to::AssignTo {
@@ -155,7 +158,7 @@ pub fn parse(
                 Err(e) => return Err(e),
             };
             first = Box::new(program::parsed::function::Function {
-                pos_in_src: (pos_in_src, src.get_pos(), srca).into(),
+                pos_in_src: (first.source_range().start(), src.get_pos(), srca).into(),
                 arg: first,
                 run,
             });
diff --git a/mers_lib/src/program/configs/with_base.rs b/mers_lib/src/program/configs/with_base.rs
index 2553b47..e2e82c9 100755
--- a/mers_lib/src/program/configs/with_base.rs
+++ b/mers_lib/src/program/configs/with_base.rs
@@ -112,7 +112,8 @@ impl Config {
                     }
                 }
                 unreachable!("try: no function found")
-            })
+            }),
+            inner_statements: None,
         }))
             .add_var("lock_update".to_string(), Data::new(data::function::Function {
                 info: Arc::new(Info::neverused()),
@@ -160,7 +161,8 @@ impl Config {
                     let func = func.as_any().downcast_ref::<data::function::Function>().unwrap();
                     *arg = func.run(arg.clone());
                     Data::empty_tuple()
-                })
+                }),
+            inner_statements: None,
             }))
             .add_var("sleep".to_string(), Data::new(data::function::Function {
                 info: Arc::new(Info::neverused()),
@@ -183,7 +185,8 @@ impl Config {
                         unreachable!("sleep called on non-int/non-float")
                     });
                     Data::empty_tuple()
-                })
+                }),
+                inner_statements: None,
             }))
             .add_var("panic".to_string(), Data::new(data::function::Function {
                 info: Arc::new(Info::neverused()),
@@ -195,7 +198,8 @@ impl Config {
             }),
             run: Arc::new(|a, _i|  {
                 std::process::exit(a.get().as_any().downcast_ref::<data::int::Int>().map(|i| i.0 as _).unwrap_or(1));
-            })
+            }),
+            inner_statements: None,
         }))
             .add_var(
             "len".to_string(),
@@ -222,6 +226,7 @@ impl Config {
                         unreachable!("called len on {a:?}, which isn't a tuple or a string")
                     }))
                 }),
+                inner_statements: None,
             }),
         ).add_var(
             "loop".to_string(),
@@ -270,6 +275,7 @@ impl Config {
                             }
                         }
                 }),
+                inner_statements: None,
             }),
         )
         .add_var(
@@ -303,6 +309,7 @@ impl Config {
                         false
                     }))
                 }),
+                inner_statements: None,
             }),
         )
         .add_var(
@@ -323,6 +330,7 @@ impl Config {
                         unreachable!("called deref on non-reference")
                     }
                 }),
+                inner_statements: None,
             }),
         )
     }
diff --git a/mers_lib/src/program/configs/with_command_running.rs b/mers_lib/src/program/configs/with_command_running.rs
index 4b1083a..b7de3a9 100755
--- a/mers_lib/src/program/configs/with_command_running.rs
+++ b/mers_lib/src/program/configs/with_command_running.rs
@@ -76,6 +76,7 @@ impl Config {
                         unreachable!("run_command called with non-tuple argument")
                     }
                 }),
+                inner_statements: None,
             }),
         )
     }
diff --git a/mers_lib/src/program/configs/with_get.rs b/mers_lib/src/program/configs/with_get.rs
index 3b0c643..125a773 100755
--- a/mers_lib/src/program/configs/with_get.rs
+++ b/mers_lib/src/program/configs/with_get.rs
@@ -66,6 +66,7 @@ impl Config {
                         unreachable!("get called with less than 2 args")
                     }
                 }),
+                inner_statements: None,
             }),
         )
     }
diff --git a/mers_lib/src/program/configs/with_iters.rs b/mers_lib/src/program/configs/with_iters.rs
index 3210ee0..80b705b 100755
--- a/mers_lib/src/program/configs/with_iters.rs
+++ b/mers_lib/src/program/configs/with_iters.rs
@@ -98,6 +98,7 @@ impl Config {
                         unreachable!("for_each called on non-tuple")
                     }
                 }),
+                inner_statements: None,
             }),
         )
         .add_var(
@@ -133,6 +134,7 @@ impl Config {
                     Ok(Type::new(IterT::new(ItersT::Enumerate, data)?))
                 }),
                 run: Arc::new(|a, _i| Data::new(Iter(Iters::Enumerate, a.clone()))),
+                inner_statements: None,
             }),
         )
     }
@@ -203,6 +205,7 @@ fn genfunc_iter_and_arg<T: MersType, D: MersData>(
                 unreachable!("{name} called on non-tuple")
             }
         }),
+        inner_statements: None,
     }
 }
 
@@ -408,5 +411,6 @@ fn genfunc_iter_in_val_out(
             }
         }),
         run: Arc::new(run),
+        inner_statements: None,
     }
 }
diff --git a/mers_lib/src/program/configs/with_list.rs b/mers_lib/src/program/configs/with_list.rs
index 2be9828..d0c3793 100755
--- a/mers_lib/src/program/configs/with_list.rs
+++ b/mers_lib/src/program/configs/with_list.rs
@@ -94,6 +94,7 @@ impl Config {
                         };
                         o
                     }),
+                inner_statements: None,
             }))
             .add_var(
                 "pop".to_string(),
@@ -140,6 +141,7 @@ impl Config {
                             None => Data::empty_tuple(),
                         }
                     }),
+                inner_statements: None,
                 }),
             )
             .add_var(
@@ -202,6 +204,7 @@ impl Config {
                             .push(Arc::new(RwLock::new(tuple.0[1].clone())));
                             Data::empty_tuple()
                     }),
+                inner_statements: None,
                 }),
             )
             .add_var(
@@ -225,6 +228,7 @@ impl Config {
                             unreachable!("as_list called on non-iterable")
                         }
                     }),
+                inner_statements: None,
                 }),
             )
     }
diff --git a/mers_lib/src/program/configs/with_math.rs b/mers_lib/src/program/configs/with_math.rs
index e017b5c..ceb40bd 100755
--- a/mers_lib/src/program/configs/with_math.rs
+++ b/mers_lib/src/program/configs/with_math.rs
@@ -75,7 +75,8 @@ impl Config {
                 } else {
                     Data::empty_tuple()
                 }
-            })
+            }),
+                inner_statements: None,
         })).add_var("parse_int".to_string(), Data::new(data::function::Function {
             info: Arc::new(program::run::Info::neverused()),
             info_check: Arc::new(Mutex::new(CheckInfo::neverused())),
@@ -95,7 +96,8 @@ impl Config {
                 } else {
                     Data::empty_tuple()
                 }
-            })
+            }),
+                inner_statements: None,
         })).add_var("signum".to_string(), Data::new(data::function::Function {
             info: Arc::new(program::run::Info::neverused()),
             info_check: Arc::new(Mutex::new(CheckInfo::neverused())),
@@ -118,7 +120,8 @@ impl Config {
                     } else { 0
                     }
                 } else { unreachable!("called signum on non-number type")}))
-            })
+            }),
+                inner_statements: None,
         }))            .add_var("div".to_string(), Data::new(data::function::Function {
                 info: Arc::new(program::run::Info::neverused()),
                 info_check: Arc::new(Mutex::new(CheckInfo::neverused())),
@@ -137,6 +140,7 @@ impl Config {
                         _ => unreachable!(),
                     }
                 } else { unreachable!() }),
+                inner_statements: None,
             })).add_var("modulo".to_string(), Data::new(data::function::Function {
                 info: Arc::new(program::run::Info::neverused()),
                 info_check: Arc::new(Mutex::new(CheckInfo::neverused())),
@@ -155,6 +159,7 @@ impl Config {
                         _ => unreachable!(),
                     }
                 } else { unreachable!() }),
+                inner_statements: None,
             }))
             .add_var(
             "sum".to_string(),
@@ -215,6 +220,7 @@ impl Config {
                         unreachable!("sum called on non-tuple")
                     }
                 }),
+                inner_statements: None,
             }),
         )
             .add_var(
@@ -288,6 +294,7 @@ impl Config {
                         unreachable!("sum called on non-tuple")
                     }
                 }),
+                inner_statements: None,
             }),
         )
             .add_var(
@@ -349,6 +356,7 @@ impl Config {
                         unreachable!("product called on non-tuple")
                     }
                 }),
+                inner_statements: None,
             }),
         )
     }
@@ -428,6 +436,7 @@ fn ltgtoe_function(
             }
             Data::new(data::bool::Bool(true))
         }),
+        inner_statements: None,
     }
 }
 #[derive(Clone, Copy)]
diff --git a/mers_lib/src/program/configs/with_multithreading.rs b/mers_lib/src/program/configs/with_multithreading.rs
index dc7bd81..0087f57 100755
--- a/mers_lib/src/program/configs/with_multithreading.rs
+++ b/mers_lib/src/program/configs/with_multithreading.rs
@@ -58,6 +58,7 @@ impl Config {
                         unreachable!("thread called, but arg wasn't a function");
                     }
                 }),
+                inner_statements: None,
             }),
         )
             .add_var("thread_finished".to_string(), Data::new(data::function::Function {
@@ -78,7 +79,8 @@ impl Config {
                         Ok(t) => t.is_finished(),
                         Err(_d) => true,
                     }))
-                })
+                }),
+                inner_statements: None,
             }))
             .add_var("thread_await".to_string(), Data::new(data::function::Function {
                 info: Arc::new(program::run::Info::neverused()),
@@ -103,7 +105,8 @@ impl Config {
                     };
                     *t = Err(d.clone());
                     d
-                })
+                }),
+                inner_statements: None,
             }))
     }
 }
diff --git a/mers_lib/src/program/configs/with_stdio.rs b/mers_lib/src/program/configs/with_stdio.rs
index bde449c..0b2d9d4 100755
--- a/mers_lib/src/program/configs/with_stdio.rs
+++ b/mers_lib/src/program/configs/with_stdio.rs
@@ -42,6 +42,7 @@ impl Config {
                     _ = std::io::stdin().read_line(&mut line);
                     Data::new(data::string::String(line))
                 }),
+                inner_statements: None,
             }),
         )
         .add_var(
@@ -55,6 +56,7 @@ impl Config {
                     eprintln!("{} :: {}", a.as_type(), a);
                     Data::empty_tuple()
                 }),
+                inner_statements: None,
             }),
         )
         .add_var(
@@ -68,6 +70,7 @@ impl Config {
                     _ = std::io::stderr().lock().flush();
                     Data::empty_tuple()
                 }),
+                inner_statements: None,
             }),
         )
         .add_var(
@@ -80,6 +83,7 @@ impl Config {
                     eprintln!("{}", a.get());
                     Data::empty_tuple()
                 }),
+                inner_statements: None,
             }),
         )
         .add_var(
@@ -93,6 +97,7 @@ impl Config {
                     _ = std::io::stdout().lock().flush();
                     Data::empty_tuple()
                 }),
+                inner_statements: None,
             }),
         )
         .add_var(
@@ -105,6 +110,7 @@ impl Config {
                     println!("{}", a.get());
                     Data::empty_tuple()
                 }),
+                inner_statements: None,
             }),
         )
     }
diff --git a/mers_lib/src/program/configs/with_string.rs b/mers_lib/src/program/configs/with_string.rs
index 90fe9fe..0ba4049 100755
--- a/mers_lib/src/program/configs/with_string.rs
+++ b/mers_lib/src/program/configs/with_string.rs
@@ -25,7 +25,8 @@ impl Config {
             }),
             run: Arc::new(|a, _i| {
                 Data::new(data::string::String(a.get().as_any().downcast_ref::<data::string::String>().unwrap().0.trim().to_owned()))
-            })
+            }),
+                inner_statements: None,
         })).add_var("concat".to_string(), Data::new(data::function::Function {
             info: Arc::new(Info::neverused()),
             info_check: Arc::new(Mutex::new(CheckInfo::neverused())),
@@ -35,11 +36,13 @@ impl Config {
                 Err(format!("concat called on non-iterable type {a}").into())
             }),
             run: Arc::new(|a, _i| Data::new(data::string::String(a.get().iterable().unwrap().map(|v| v.get().to_string()).collect()))),
+                inner_statements: None,
         })).add_var("to_string".to_string(), Data::new(data::function::Function {
             info: Arc::new(Info::neverused()),
             info_check: Arc::new(Mutex::new(CheckInfo::neverused())),
             out: Arc::new(|_a, _i| Ok(Type::new(data::string::StringT))),
             run: Arc::new(|a, _i| Data::new(data::string::String(a.get().to_string()))),
+                inner_statements: None,
         })).add_var("index_of".to_string(), Data::new(data::function::Function {
             info: Arc::new(Info::neverused()),
             info_check: Arc::new(Mutex::new(CheckInfo::neverused())),
@@ -52,6 +55,7 @@ impl Config {
                 Err(format!("wrong args for index_of: must be (string, string)").into())
             }),
             run: Arc::new(|a, _i| index_of(a, false)),
+                inner_statements: None,
         })).add_var("index_of_rev".to_string(), Data::new(data::function::Function {
             info: Arc::new(Info::neverused()),
             info_check: Arc::new(Mutex::new(CheckInfo::neverused())),
@@ -64,6 +68,7 @@ impl Config {
                 Err(format!("wrong args for index_of: must be (string, string)").into())
             }),
             run: Arc::new(|a, _i| index_of(a, true)),
+                inner_statements: None,
         })).add_var("substring".to_string(), Data::new(data::function::Function {
             info: Arc::new(Info::neverused()),
             info_check: Arc::new(Mutex::new(CheckInfo::neverused())),
@@ -113,6 +118,7 @@ impl Config {
                 Data::new(data::string::String(s[start..end].to_owned()))
 
             }),
+                inner_statements: None,
         }))
     }
 }
diff --git a/mers_lib/src/program/parsed/as_type.rs b/mers_lib/src/program/parsed/as_type.rs
index c188246..379680a 100755
--- a/mers_lib/src/program/parsed/as_type.rs
+++ b/mers_lib/src/program/parsed/as_type.rs
@@ -35,4 +35,10 @@ impl MersStatement for AsType {
     fn source_range(&self) -> SourceRange {
         self.pos_in_src.clone()
     }
+    fn inner_statements(&self) -> Vec<&dyn MersStatement> {
+        vec![self.statement.as_ref()]
+    }
+    fn as_any(&self) -> &dyn std::any::Any {
+        self
+    }
 }
diff --git a/mers_lib/src/program/parsed/assign_to.rs b/mers_lib/src/program/parsed/assign_to.rs
index aa81998..41ad834 100755
--- a/mers_lib/src/program/parsed/assign_to.rs
+++ b/mers_lib/src/program/parsed/assign_to.rs
@@ -31,4 +31,10 @@ impl MersStatement for AssignTo {
     fn source_range(&self) -> SourceRange {
         self.pos_in_src.clone()
     }
+    fn inner_statements(&self) -> Vec<&dyn MersStatement> {
+        vec![self.source.as_ref(), self.target.as_ref()]
+    }
+    fn as_any(&self) -> &dyn std::any::Any {
+        self
+    }
 }
diff --git a/mers_lib/src/program/parsed/block.rs b/mers_lib/src/program/parsed/block.rs
index 2797687..3c83cd6 100755
--- a/mers_lib/src/program/parsed/block.rs
+++ b/mers_lib/src/program/parsed/block.rs
@@ -32,4 +32,10 @@ impl MersStatement for Block {
     fn source_range(&self) -> SourceRange {
         self.pos_in_src.clone()
     }
+    fn inner_statements(&self) -> Vec<&dyn MersStatement> {
+        self.statements.iter().map(|v| v.as_ref()).collect()
+    }
+    fn as_any(&self) -> &dyn std::any::Any {
+        self
+    }
 }
diff --git a/mers_lib/src/program/parsed/chain.rs b/mers_lib/src/program/parsed/chain.rs
index b08c9e1..6898586 100755
--- a/mers_lib/src/program/parsed/chain.rs
+++ b/mers_lib/src/program/parsed/chain.rs
@@ -30,4 +30,10 @@ impl MersStatement for Chain {
     fn source_range(&self) -> SourceRange {
         self.pos_in_src.clone()
     }
+    fn inner_statements(&self) -> Vec<&dyn MersStatement> {
+        vec![self.first.as_ref(), self.chained.as_ref()]
+    }
+    fn as_any(&self) -> &dyn std::any::Any {
+        self
+    }
 }
diff --git a/mers_lib/src/program/parsed/custom_type.rs b/mers_lib/src/program/parsed/custom_type.rs
index c54cef8..ff2d8a8 100644
--- a/mers_lib/src/program/parsed/custom_type.rs
+++ b/mers_lib/src/program/parsed/custom_type.rs
@@ -37,6 +37,16 @@ impl MersStatement for CustomType {
     fn source_range(&self) -> SourceRange {
         self.pos_in_src.clone()
     }
+    fn inner_statements(&self) -> Vec<&dyn MersStatement> {
+        if let Err(s) = &self.source {
+            vec![s.as_ref()]
+        } else {
+            vec![]
+        }
+    }
+    fn as_any(&self) -> &dyn std::any::Any {
+        self
+    }
 }
 
 impl Debug for CustomType {
diff --git a/mers_lib/src/program/parsed/function.rs b/mers_lib/src/program/parsed/function.rs
index 6d5ac9c..759960f 100755
--- a/mers_lib/src/program/parsed/function.rs
+++ b/mers_lib/src/program/parsed/function.rs
@@ -29,8 +29,10 @@ impl MersStatement for Function {
         let arg_target = Arc::new(self.arg.compile(info, comp)?);
         comp.is_init = false;
         let run = Arc::new(self.run.compile(info, comp)?);
-        let arg2: Arc<Box<dyn crate::program::run::MersStatement>> = Arc::clone(&arg_target);
-        let run2: Arc<Box<dyn crate::program::run::MersStatement>> = Arc::clone(&run);
+        let arg2 = Arc::clone(&arg_target);
+        let run2 = Arc::clone(&run);
+        let arg3 = Arc::clone(&arg_target);
+        let run3 = Arc::clone(&run);
         Ok(Box::new(program::run::function::Function {
             pos_in_src: self.pos_in_src.clone(),
             func_no_info: data::function::Function {
@@ -44,10 +46,17 @@ impl MersStatement for Function {
                     data::defs::assign(&arg, &arg_target.run(info));
                     run.run(info)
                 }),
+                inner_statements: Some((arg3, run3)),
             },
         }))
     }
     fn source_range(&self) -> SourceRange {
         self.pos_in_src.clone()
     }
+    fn inner_statements(&self) -> Vec<&dyn MersStatement> {
+        vec![self.arg.as_ref(), self.run.as_ref()]
+    }
+    fn as_any(&self) -> &dyn std::any::Any {
+        self
+    }
 }
diff --git a/mers_lib/src/program/parsed/if.rs b/mers_lib/src/program/parsed/if.rs
index 1e37b98..d2757a2 100755
--- a/mers_lib/src/program/parsed/if.rs
+++ b/mers_lib/src/program/parsed/if.rs
@@ -36,4 +36,18 @@ impl MersStatement for If {
     fn source_range(&self) -> SourceRange {
         self.pos_in_src.clone()
     }
+    fn inner_statements(&self) -> Vec<&dyn MersStatement> {
+        if let Some(on_false) = &self.on_false {
+            vec![
+                self.condition.as_ref(),
+                self.on_true.as_ref(),
+                on_false.as_ref(),
+            ]
+        } else {
+            vec![self.condition.as_ref(), self.on_true.as_ref()]
+        }
+    }
+    fn as_any(&self) -> &dyn std::any::Any {
+        self
+    }
 }
diff --git a/mers_lib/src/program/parsed/include_mers.rs b/mers_lib/src/program/parsed/include_mers.rs
index 93f96fe..129fe60 100644
--- a/mers_lib/src/program/parsed/include_mers.rs
+++ b/mers_lib/src/program/parsed/include_mers.rs
@@ -58,6 +58,7 @@ impl MersStatement for IncludeMers {
                     info_check: Arc::new(Mutex::new(info::Info::neverused())),
                     out: Arc::new(move |_, i| compiled.check(&mut i.duplicate(), None)),
                     run: Arc::new(move |_, i| compiled2.run(&mut i.duplicate())),
+                    inner_statements: None,
                 },
             }),
             as_part_of_include: Some(self.inner_src.clone()),
@@ -66,4 +67,10 @@ impl MersStatement for IncludeMers {
     fn source_range(&self) -> SourceRange {
         self.pos_in_src.clone()
     }
+    fn inner_statements(&self) -> Vec<&dyn MersStatement> {
+        vec![]
+    }
+    fn as_any(&self) -> &dyn std::any::Any {
+        self
+    }
 }
diff --git a/mers_lib/src/program/parsed/init_to.rs b/mers_lib/src/program/parsed/init_to.rs
index e4b2eef..0c8cfbc 100755
--- a/mers_lib/src/program/parsed/init_to.rs
+++ b/mers_lib/src/program/parsed/init_to.rs
@@ -37,4 +37,10 @@ impl MersStatement for InitTo {
     fn source_range(&self) -> SourceRange {
         self.pos_in_src.clone()
     }
+    fn inner_statements(&self) -> Vec<&dyn MersStatement> {
+        vec![self.source.as_ref(), self.target.as_ref()]
+    }
+    fn as_any(&self) -> &dyn std::any::Any {
+        self
+    }
 }
diff --git a/mers_lib/src/program/parsed/mod.rs b/mers_lib/src/program/parsed/mod.rs
index d8a839b..56630f1 100755
--- a/mers_lib/src/program/parsed/mod.rs
+++ b/mers_lib/src/program/parsed/mod.rs
@@ -1,4 +1,8 @@
-use std::{collections::HashMap, fmt::Debug};
+use std::{
+    collections::HashMap,
+    fmt::Debug,
+    sync::{Arc, Mutex},
+};
 
 use crate::{
     errors::{CheckError, SourceRange},
@@ -44,21 +48,53 @@ pub trait MersStatement: Debug + Send + Sync {
         info: &mut Info,
         comp: CompInfo,
     ) -> Result<Box<dyn super::run::MersStatement>, CheckError> {
+        info.global.depth += 1;
         if self.has_scope() {
             info.create_scope();
         }
         let o = self.compile_custom(info, comp);
+        if info.global.enable_hooks {
+            // Hooks - keep in sync with run/mod.rs/compile() hooks section
+            {
+                // `save_info_at` hook
+                let mut save_info_at = info.global.save_info_at.try_lock().unwrap();
+                if !save_info_at.is_empty() {
+                    let pos_start = self.source_range().start().pos();
+                    let pos_end = self.source_range().end().pos();
+                    let cloned_info = Arc::new(info.clone());
+                    for (save_to, save_at, deepest_statement) in save_info_at.iter_mut() {
+                        if info.global.depth >= *deepest_statement
+                            && pos_start <= *save_at
+                            && *save_at < pos_end
+                        {
+                            if info.global.depth > *deepest_statement {
+                                *deepest_statement = info.global.depth;
+                                save_to.clear();
+                            }
+                            save_to.push((
+                                self.source_range(),
+                                Arc::clone(&cloned_info),
+                                o.as_ref().map(|_| ()).map_err(|e| e.clone()),
+                            ));
+                        }
+                    }
+                }
+            }
+        }
         if self.has_scope() {
             info.end_scope();
         }
+        info.global.depth -= 1;
         o
     }
     fn source_range(&self) -> SourceRange;
+    fn inner_statements(&self) -> Vec<&dyn MersStatement>;
+    fn as_any(&self) -> &dyn std::any::Any;
 }
 
 #[derive(Clone, Copy)]
 pub struct CompInfo {
-    is_init: bool,
+    pub is_init: bool,
 }
 impl Default for CompInfo {
     fn default() -> Self {
@@ -70,13 +106,30 @@ pub type Info = info::Info<Local>;
 
 #[derive(Default, Clone, Debug)]
 pub struct Local {
-    vars: HashMap<String, (usize, usize)>,
-    vars_count: usize,
+    pub vars: HashMap<String, (usize, usize)>,
+    pub vars_count: usize,
+}
+#[derive(Clone, Debug, Default)]
+pub struct LocalGlobalInfo {
+    pub depth: usize,
+    pub enable_hooks: bool,
+    /// ((results, byte_pos_in_src, deepest_statement))
+    /// you only have to set `byte_pos_in_src`. `deepest` is used internally.
+    /// These values should be initialized to `(vec![], _, 0)`, but `0` can be replaced by a minimum statement depth, i.e. `2` to exclude the outer scope (which has depth `1`).
+    pub save_info_at: Arc<
+        Mutex<
+            Vec<(
+                Vec<(SourceRange, Arc<Info>, Result<(), CheckError>)>,
+                usize,
+                usize,
+            )>,
+        >,
+    >,
 }
 impl info::Local for Local {
     type VariableIdentifier = String;
     type VariableData = (usize, usize);
-    type Global = ();
+    type Global = LocalGlobalInfo;
     fn init_var(&mut self, id: Self::VariableIdentifier, value: Self::VariableData) {
         self.vars_count += 1;
         self.vars.insert(id, value);
diff --git a/mers_lib/src/program/parsed/object.rs b/mers_lib/src/program/parsed/object.rs
index 8e4d005..5d717b1 100644
--- a/mers_lib/src/program/parsed/object.rs
+++ b/mers_lib/src/program/parsed/object.rs
@@ -32,4 +32,10 @@ impl MersStatement for Object {
     fn source_range(&self) -> SourceRange {
         self.pos_in_src.clone()
     }
+    fn inner_statements(&self) -> Vec<&dyn MersStatement> {
+        self.elems.iter().map(|(_, s)| s.as_ref()).collect()
+    }
+    fn as_any(&self) -> &dyn std::any::Any {
+        self
+    }
 }
diff --git a/mers_lib/src/program/parsed/tuple.rs b/mers_lib/src/program/parsed/tuple.rs
index 9215e5f..d1e03c8 100755
--- a/mers_lib/src/program/parsed/tuple.rs
+++ b/mers_lib/src/program/parsed/tuple.rs
@@ -32,4 +32,10 @@ impl MersStatement for Tuple {
     fn source_range(&self) -> SourceRange {
         self.pos_in_src.clone()
     }
+    fn inner_statements(&self) -> Vec<&dyn MersStatement> {
+        self.elems.iter().map(|v| v.as_ref()).collect()
+    }
+    fn as_any(&self) -> &dyn std::any::Any {
+        self
+    }
 }
diff --git a/mers_lib/src/program/parsed/value.rs b/mers_lib/src/program/parsed/value.rs
index 7a4788e..a4528f4 100755
--- a/mers_lib/src/program/parsed/value.rs
+++ b/mers_lib/src/program/parsed/value.rs
@@ -29,4 +29,10 @@ impl MersStatement for Value {
     fn source_range(&self) -> SourceRange {
         self.pos_in_src.clone()
     }
+    fn inner_statements(&self) -> Vec<&dyn MersStatement> {
+        vec![]
+    }
+    fn as_any(&self) -> &dyn std::any::Any {
+        self
+    }
 }
diff --git a/mers_lib/src/program/parsed/variable.rs b/mers_lib/src/program/parsed/variable.rs
index ef39bec..f4d388f 100755
--- a/mers_lib/src/program/parsed/variable.rs
+++ b/mers_lib/src/program/parsed/variable.rs
@@ -59,4 +59,10 @@ impl MersStatement for Variable {
     fn source_range(&self) -> SourceRange {
         self.pos_in_src.clone()
     }
+    fn inner_statements(&self) -> Vec<&dyn MersStatement> {
+        vec![]
+    }
+    fn as_any(&self) -> &dyn std::any::Any {
+        self
+    }
 }
diff --git a/mers_lib/src/program/run/as_type.rs b/mers_lib/src/program/run/as_type.rs
index dd2394a..f732224 100644
--- a/mers_lib/src/program/run/as_type.rs
+++ b/mers_lib/src/program/run/as_type.rs
@@ -75,4 +75,10 @@ impl MersStatement for AsType {
     fn source_range(&self) -> SourceRange {
         self.pos_in_src.clone()
     }
+    fn inner_statements(&self) -> Vec<&dyn MersStatement> {
+        vec![self.statement.as_ref()]
+    }
+    fn as_any(&self) -> &dyn std::any::Any {
+        self
+    }
 }
diff --git a/mers_lib/src/program/run/assign_to.rs b/mers_lib/src/program/run/assign_to.rs
index 7557b16..a3fa8d1 100755
--- a/mers_lib/src/program/run/assign_to.rs
+++ b/mers_lib/src/program/run/assign_to.rs
@@ -83,4 +83,10 @@ impl MersStatement for AssignTo {
     fn source_range(&self) -> SourceRange {
         self.pos_in_src.clone()
     }
+    fn inner_statements(&self) -> Vec<&dyn MersStatement> {
+        vec![self.target.as_ref(), self.source.as_ref()]
+    }
+    fn as_any(&self) -> &dyn std::any::Any {
+        self
+    }
 }
diff --git a/mers_lib/src/program/run/block.rs b/mers_lib/src/program/run/block.rs
index cd55409..c73a43d 100755
--- a/mers_lib/src/program/run/block.rs
+++ b/mers_lib/src/program/run/block.rs
@@ -38,4 +38,10 @@ impl MersStatement for Block {
     fn source_range(&self) -> SourceRange {
         self.pos_in_src.clone()
     }
+    fn inner_statements(&self) -> Vec<&dyn MersStatement> {
+        self.statements.iter().map(|s| s.as_ref()).collect()
+    }
+    fn as_any(&self) -> &dyn std::any::Any {
+        self
+    }
 }
diff --git a/mers_lib/src/program/run/chain.rs b/mers_lib/src/program/run/chain.rs
index 6d307b5..d316473 100755
--- a/mers_lib/src/program/run/chain.rs
+++ b/mers_lib/src/program/run/chain.rs
@@ -101,4 +101,10 @@ impl MersStatement for Chain {
     fn source_range(&self) -> SourceRange {
         self.pos_in_src.clone()
     }
+    fn inner_statements(&self) -> Vec<&dyn MersStatement> {
+        vec![self.first.as_ref(), self.chained.as_ref()]
+    }
+    fn as_any(&self) -> &dyn std::any::Any {
+        self
+    }
 }
diff --git a/mers_lib/src/program/run/custom_type.rs b/mers_lib/src/program/run/custom_type.rs
index e85f8c1..ca122f9 100644
--- a/mers_lib/src/program/run/custom_type.rs
+++ b/mers_lib/src/program/run/custom_type.rs
@@ -54,6 +54,12 @@ impl MersStatement for CustomType {
     fn source_range(&self) -> SourceRange {
         self.pos_in_src.clone()
     }
+    fn inner_statements(&self) -> Vec<&dyn MersStatement> {
+        vec![]
+    }
+    fn as_any(&self) -> &dyn std::any::Any {
+        self
+    }
 }
 
 impl Debug for CustomType {
diff --git a/mers_lib/src/program/run/function.rs b/mers_lib/src/program/run/function.rs
index 8af9b7f..090a9f7 100755
--- a/mers_lib/src/program/run/function.rs
+++ b/mers_lib/src/program/run/function.rs
@@ -34,4 +34,14 @@ impl MersStatement for Function {
     fn source_range(&self) -> SourceRange {
         self.pos_in_src.clone()
     }
+    fn inner_statements(&self) -> Vec<&dyn MersStatement> {
+        if let Some((a, b)) = &self.func_no_info.inner_statements {
+            vec![a.as_ref().as_ref(), b.as_ref().as_ref()]
+        } else {
+            vec![]
+        }
+    }
+    fn as_any(&self) -> &dyn std::any::Any {
+        self
+    }
 }
diff --git a/mers_lib/src/program/run/if.rs b/mers_lib/src/program/run/if.rs
index e2d0994..ee30a31 100755
--- a/mers_lib/src/program/run/if.rs
+++ b/mers_lib/src/program/run/if.rs
@@ -73,4 +73,18 @@ impl MersStatement for If {
     fn source_range(&self) -> SourceRange {
         self.pos_in_src.clone()
     }
+    fn inner_statements(&self) -> Vec<&dyn MersStatement> {
+        if let Some(on_false) = &self.on_false {
+            vec![
+                self.condition.as_ref(),
+                self.on_true.as_ref(),
+                on_false.as_ref(),
+            ]
+        } else {
+            vec![self.condition.as_ref(), self.on_true.as_ref()]
+        }
+    }
+    fn as_any(&self) -> &dyn std::any::Any {
+        self
+    }
 }
diff --git a/mers_lib/src/program/run/mod.rs b/mers_lib/src/program/run/mod.rs
index 69b73b8..bf08c11 100755
--- a/mers_lib/src/program/run/mod.rs
+++ b/mers_lib/src/program/run/mod.rs
@@ -1,7 +1,7 @@
 use std::{
     collections::HashMap,
     fmt::Debug,
-    sync::{Arc, RwLock},
+    sync::{Arc, Mutex, RwLock},
 };
 
 use crate::{
@@ -43,13 +43,50 @@ pub trait MersStatement: Debug + Send + Sync {
     /// if true, local variables etc. will be contained inside their own scope.
     fn has_scope(&self) -> bool;
     fn check(&self, info: &mut CheckInfo, assign: Option<&Type>) -> Result<Type, CheckError> {
+        info.global.depth += 1;
         if self.has_scope() {
             info.create_scope();
         }
         let o = self.check_custom(info, assign);
+        if info.global.enable_hooks {
+            // Hooks - keep in sync with run/mod.rs/compile() hooks section
+            'hook_save_info_at: {
+                // `save_info_at` hook
+                let mut save_info_at = if let Ok(lock) = info.global.save_info_at.try_lock() {
+                    lock
+                } else {
+                    eprintln!(
+                        "[HOOKS/save_info_at] couldn't acquire lock - result may be incomplete"
+                    );
+                    break 'hook_save_info_at;
+                };
+                if !save_info_at.is_empty() {
+                    let pos_start = self.source_range().start().pos();
+                    let pos_end = self.source_range().end().pos();
+                    let cloned_info = Arc::new(info.clone());
+                    for (save_to, save_at, deepest_statement) in save_info_at.iter_mut() {
+                        if info.global.depth >= *deepest_statement
+                            && pos_start <= *save_at
+                            && *save_at < pos_end
+                        {
+                            if info.global.depth > *deepest_statement {
+                                *deepest_statement = info.global.depth;
+                                save_to.clear();
+                            }
+                            save_to.push((
+                                self.source_range(),
+                                Arc::clone(&cloned_info),
+                                o.clone(),
+                            ));
+                        }
+                    }
+                }
+            }
+        }
         if self.has_scope() {
             info.end_scope();
         }
+        info.global.depth -= 1;
         o
     }
     fn run(&self, info: &mut Info) -> Data {
@@ -63,6 +100,8 @@ pub trait MersStatement: Debug + Send + Sync {
         o
     }
     fn source_range(&self) -> SourceRange;
+    fn inner_statements(&self) -> Vec<&dyn MersStatement>;
+    fn as_any(&self) -> &dyn std::any::Any;
 }
 
 pub type Info = info::Info<Local>;
@@ -70,11 +109,11 @@ pub type CheckInfo = info::Info<CheckLocal>;
 
 #[derive(Default, Clone, Debug)]
 pub struct Local {
-    vars: Vec<Arc<RwLock<Data>>>,
+    pub vars: Vec<Arc<RwLock<Data>>>,
 }
 #[derive(Default, Clone)]
 pub struct CheckLocal {
-    vars: Vec<Type>,
+    pub vars: Vec<Type>,
     pub types: HashMap<
         String,
         Result<
@@ -83,6 +122,23 @@ pub struct CheckLocal {
         >,
     >,
 }
+#[derive(Clone, Debug, Default)]
+pub struct CheckLocalGlobalInfo {
+    pub depth: usize,
+    pub enable_hooks: bool,
+    /// ((results, byte_pos_in_src, deepest_statement))
+    /// you only have to set `byte_pos_in_src`. `deepest` is used internally.
+    /// These values should be initialized to `(vec![], _, 0)`, but `0` can be replaced by a minimum statement depth, i.e. `2` to exclude the outer scope (which has depth `1`).
+    pub save_info_at: Arc<
+        Mutex<
+            Vec<(
+                Vec<(SourceRange, Arc<CheckInfo>, Result<Type, CheckError>)>,
+                usize,
+                usize,
+            )>,
+        >,
+    >,
+}
 impl Debug for CheckLocal {
     fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
         write!(f, "CheckLocal {:?}, {:?}", self.vars, self.types.keys())
@@ -124,7 +180,7 @@ impl info::Local for Local {
 impl info::Local for CheckLocal {
     type VariableIdentifier = usize;
     type VariableData = Type;
-    type Global = ();
+    type Global = CheckLocalGlobalInfo;
     fn init_var(&mut self, id: Self::VariableIdentifier, value: Self::VariableData) {
         while self.vars.len() <= id {
             self.vars.push(Type::empty());
diff --git a/mers_lib/src/program/run/object.rs b/mers_lib/src/program/run/object.rs
index a54a712..dad4eab 100644
--- a/mers_lib/src/program/run/object.rs
+++ b/mers_lib/src/program/run/object.rs
@@ -133,4 +133,10 @@ impl MersStatement for Object {
     fn source_range(&self) -> SourceRange {
         self.pos_in_src.clone()
     }
+    fn inner_statements(&self) -> Vec<&dyn MersStatement> {
+        self.elems.iter().map(|(_, s)| s.as_ref()).collect()
+    }
+    fn as_any(&self) -> &dyn std::any::Any {
+        self
+    }
 }
diff --git a/mers_lib/src/program/run/tuple.rs b/mers_lib/src/program/run/tuple.rs
index 93158cc..048b547 100755
--- a/mers_lib/src/program/run/tuple.rs
+++ b/mers_lib/src/program/run/tuple.rs
@@ -93,4 +93,10 @@ impl MersStatement for Tuple {
     fn source_range(&self) -> SourceRange {
         self.pos_in_src.clone()
     }
+    fn inner_statements(&self) -> Vec<&dyn MersStatement> {
+        self.elems.iter().map(|s| s.as_ref()).collect()
+    }
+    fn as_any(&self) -> &dyn std::any::Any {
+        self
+    }
 }
diff --git a/mers_lib/src/program/run/value.rs b/mers_lib/src/program/run/value.rs
index 8ca74a4..166e809 100755
--- a/mers_lib/src/program/run/value.rs
+++ b/mers_lib/src/program/run/value.rs
@@ -31,4 +31,10 @@ impl MersStatement for Value {
     fn source_range(&self) -> SourceRange {
         self.pos_in_src.clone()
     }
+    fn inner_statements(&self) -> Vec<&dyn MersStatement> {
+        vec![]
+    }
+    fn as_any(&self) -> &dyn std::any::Any {
+        self
+    }
 }
diff --git a/mers_lib/src/program/run/variable.rs b/mers_lib/src/program/run/variable.rs
index 5755315..38fba55 100755
--- a/mers_lib/src/program/run/variable.rs
+++ b/mers_lib/src/program/run/variable.rs
@@ -79,4 +79,10 @@ impl MersStatement for Variable {
     fn source_range(&self) -> SourceRange {
         self.pos_in_src.clone()
     }
+    fn inner_statements(&self) -> Vec<&dyn MersStatement> {
+        vec![]
+    }
+    fn as_any(&self) -> &dyn std::any::Any {
+        self
+    }
 }