diff --git a/merslsp/Cargo.toml b/merslsp/Cargo.toml
new file mode 100644
index 0000000..4eef121
--- /dev/null
+++ b/merslsp/Cargo.toml
@@ -0,0 +1,10 @@
+[package]
+name = "merslsp"
+version = "0.1.0"
+edition = "2021"
+
+[dependencies]
+mers_lib = "0.8.0"
+lspower = "1.5.0"
+tokio = { version = "1.36.0", features = ["full"] }
+line-span = "0.1.5"
diff --git a/merslsp/src/main.rs b/merslsp/src/main.rs
new file mode 100644
index 0000000..7b2f61e
--- /dev/null
+++ b/merslsp/src/main.rs
@@ -0,0 +1,305 @@
+use std::collections::HashMap;
+use std::path::PathBuf;
+use std::sync::{Arc, Mutex};
+
+use line_span::LineSpans;
+use lspower::jsonrpc;
+use lspower::lsp::*;
+use lspower::{Client, LanguageServer, LspService, Server};
+use mers_lib::data::Type;
+use mers_lib::errors::CheckError;
+use mers_lib::parsing::SourceFrom;
+use mers_lib::prelude_extend_config::Config;
+
+#[derive(Debug)]
+struct Backend {
+    client: Client,
+    documents: Mutex<HashMap<Url, TextDocument>>,
+}
+
+#[derive(Debug)]
+struct TextDocument {
+    source: String,
+    file_path: Option<PathBuf>,
+    srca: Option<Arc<mers_lib::prelude_compile::Source>>,
+    parsed: Option<Result<Box<dyn mers_lib::program::parsed::MersStatement>, CheckError>>,
+    compiled: Option<Result<Box<dyn mers_lib::program::run::MersStatement>, CheckError>>,
+    checked: Option<Result<mers_lib::data::Type, CheckError>>,
+}
+impl TextDocument {
+    pub fn changed(&mut self) {
+        self.srca = None;
+        self.parsed = None;
+        self.compiled = None;
+        self.checked = None;
+    }
+    pub fn srca(&mut self) -> &Arc<mers_lib::prelude_compile::Source> {
+        if self.srca.is_none() {
+            self.srca = Some(Arc::new(mers_lib::prelude_compile::Source::new(
+                self.file_path
+                    .clone()
+                    .map(|v| SourceFrom::File(v))
+                    .unwrap_or(SourceFrom::Unspecified),
+                self.source.clone(),
+            )));
+        }
+        self.srca.as_ref().unwrap()
+    }
+    pub fn parsed(
+        &mut self,
+    ) -> &Result<Box<dyn mers_lib::program::parsed::MersStatement>, CheckError> {
+        if self.parsed.is_none() {
+            self.parsed = Some(mers_lib::prelude_compile::parse(
+                &mut mers_lib::prelude_compile::Source::clone(self.srca()),
+                self.srca(),
+            ))
+        }
+        self.parsed.as_ref().unwrap()
+    }
+    pub fn compiled(
+        &mut self,
+    ) -> &Result<Box<dyn mers_lib::program::run::MersStatement>, CheckError> {
+        if self.compiled.is_none() {
+            self.compiled =
+                Some(self.parsed().as_ref().map_err(|e| e.clone()).and_then(|v| {
+                    mers_lib::prelude_compile::compile(&**v, mers_config().infos().0)
+                }));
+        }
+        self.compiled.as_ref().unwrap()
+    }
+    pub fn checked(&mut self) -> &Result<mers_lib::data::Type, CheckError> {
+        if self.checked.is_none() {
+            self.checked = Some(
+                self.compiled()
+                    .as_ref()
+                    .map_err(|e| e.clone())
+                    .and_then(|v| mers_lib::prelude_compile::check(&**v, mers_config().infos().2)),
+            );
+        }
+        self.checked.as_ref().unwrap()
+    }
+}
+
+fn mers_config() -> Config {
+    Config::new().bundle_std()
+}
+
+#[lspower::async_trait]
+impl LanguageServer for Backend {
+    async fn initialize(&self, _: InitializeParams) -> jsonrpc::Result<InitializeResult> {
+        let mut init = InitializeResult::default();
+        init.capabilities.text_document_sync =
+            Some(TextDocumentSyncCapability::Kind(TextDocumentSyncKind::FULL));
+        init.capabilities.completion_provider = Some(CompletionOptions::default());
+        init.capabilities.hover_provider = Some(HoverProviderCapability::Simple(true));
+        Ok(init)
+    }
+
+    async fn initialized(&self, _: InitializedParams) {}
+
+    async fn shutdown(&self) -> jsonrpc::Result<()> {
+        Ok(())
+    }
+
+    async fn did_open(&self, params: DidOpenTextDocumentParams) {
+        let fp = params.text_document.uri.to_file_path();
+        self.documents.lock().unwrap().insert(
+            params.text_document.uri,
+            TextDocument {
+                source: params.text_document.text,
+                file_path: fp.ok(),
+                srca: None,
+                parsed: None,
+                compiled: None,
+                checked: None,
+            },
+        );
+    }
+
+    async fn did_change(&self, params: DidChangeTextDocumentParams) {
+        let mut last_text = None;
+        for change in params.content_changes {
+            if change.range.is_none() {
+                last_text = Some(change.text);
+            }
+        }
+        if let Some(new_text) = last_text {
+            if let Some(d) = self
+                .documents
+                .lock()
+                .unwrap()
+                .get_mut(&params.text_document.uri)
+            {
+                d.source = new_text;
+                d.changed();
+            }
+        }
+    }
+
+    async fn did_close(&self, params: DidCloseTextDocumentParams) {
+        self.documents
+            .lock()
+            .unwrap()
+            .remove(&params.text_document.uri);
+    }
+
+    async fn completion(
+        &self,
+        params: CompletionParams,
+    ) -> jsonrpc::Result<Option<CompletionResponse>> {
+        if let Some(doc) = self
+            .documents
+            .lock()
+            .unwrap()
+            .get_mut(&params.text_document_position.text_document.uri)
+        {
+            let byte_pos = doc_byte_pos(&doc.source, params.text_document_position.position);
+            let byte_pos_in_src = doc.srca().pos_from_og(byte_pos, false);
+            if let Ok(parsed) = doc.parsed() {
+                let (mut i1, _, mut i3) = mers_config().infos();
+                i1.global.enable_hooks = true;
+                let mut save_info_at = i1.global.save_info_at.lock().unwrap();
+                let my_saved_info_index = save_info_at.len();
+                save_info_at.push((vec![], byte_pos_in_src, 0));
+                drop(save_info_at);
+                let variable_types = if let Ok(compiled) =
+                    mers_lib::prelude_compile::compile_mut(&**parsed, &mut i1)
+                {
+                    i3.global.enable_hooks = true;
+                    let mut save_info_at = i3.global.save_info_at.lock().unwrap();
+                    let my_saved_info_index = save_info_at.len();
+                    save_info_at.push((vec![], byte_pos_in_src, 0));
+                    drop(save_info_at);
+                    _ = mers_lib::prelude_compile::check_mut(&*compiled, &mut i3);
+                    let save_info_at = i3.global.save_info_at.lock().unwrap();
+                    let my_saved_info = &save_info_at[my_saved_info_index].0;
+                    let mut variable_types: HashMap<(usize, usize), Type> = HashMap::new();
+                    for (_range, info, _err) in my_saved_info {
+                        for (is, scope) in info.scopes.iter().enumerate() {
+                            for (iv, var) in scope.vars.iter().enumerate() {
+                                let key = (is, iv);
+                                if let Some(t) = variable_types.get_mut(&key) {
+                                    t.add_all(var);
+                                } else {
+                                    variable_types.insert(key, var.clone());
+                                }
+                            }
+                        }
+                    }
+                    drop(save_info_at);
+                    Some(variable_types)
+                } else {
+                    None
+                };
+                let save_info_at = i1.global.save_info_at.lock().unwrap();
+                let result = &save_info_at[my_saved_info_index].0;
+                let mut variables = HashMap::new();
+                for (_range, a, _err) in result {
+                    for (depth, scope) in a.scopes.iter().enumerate() {
+                        for (var, v) in scope.vars.iter() {
+                            variables.insert(var, (depth, *v));
+                        }
+                    }
+                }
+                let mut variables = variables.into_iter().collect::<Vec<_>>();
+                // sort by depth (rev), then name -> local variables first, globals later, within the same scope A->Z
+                variables.sort_unstable_by(|a, b| b.1 .0.cmp(&a.1 .0).then_with(|| a.0.cmp(&b.0)));
+                let items = variables
+                    .into_iter()
+                    .map(|v| {
+                        CompletionItem::new_simple(
+                            v.0.clone(),
+                            if let Some(typ) =
+                                variable_types.as_ref().and_then(|types| types.get(&v.1 .1))
+                            {
+                                format!("var: `{typ}`")
+                            } else {
+                                format!("variable")
+                            },
+                        )
+                    })
+                    .collect();
+                drop(save_info_at);
+                return Ok(Some(CompletionResponse::Array(items)));
+            }
+        }
+        Ok(None)
+    }
+
+    async fn hover(&self, params: HoverParams) -> crate::jsonrpc::Result<Option<Hover>> {
+        let this_doc = params.text_document_position_params.text_document.uri;
+        Ok(
+            if let Some(doc) = self.documents.lock().unwrap().get_mut(&this_doc) {
+                let pos = params.text_document_position_params.position;
+                let byte_pos = doc_byte_pos(&doc.source, pos);
+                Some(match doc.compiled() {
+                    Ok(compiled) => {
+                        let file = Arc::clone(compiled.source_range().in_file());
+                        let pos = file.pos_from_og(byte_pos, false);
+                        let (_, _, mut i3) = mers_config().infos();
+                        i3.global.enable_hooks = true;
+                        let save_info_at = Arc::new(Mutex::new(vec![(vec![], pos, 0)]));
+                        i3.global.save_info_at = Arc::clone(&save_info_at);
+                        _ = mers_lib::prelude_compile::check_mut(&**compiled, &mut i3);
+                        let hook_res = &save_info_at.lock().unwrap()[0];
+                        Hover {
+                            contents: HoverContents::Markup(MarkupContent {
+                                kind: MarkupKind::Markdown,
+                                value: format!(
+                                    "Local Types:{}",
+                                    hook_res
+                                        .0
+                                        .iter()
+                                        .map(|(_, _, t)| match t {
+                                            Ok(t) => format!("\n- {t}"),
+                                            Err(e) => format!("\n- {e}"),
+                                        })
+                                        .collect::<String>()
+                                ),
+                            }),
+                            range: None,
+                        }
+                    }
+                    Err(e) => Hover {
+                        contents: HoverContents::Markup(MarkupContent {
+                            kind: MarkupKind::PlainText,
+                            value: format!("{e}"),
+                        }),
+                        range: None,
+                    },
+                })
+            } else {
+                None
+            },
+        )
+    }
+}
+
+#[tokio::main]
+async fn main() {
+    let stdin = tokio::io::stdin();
+    let stdout = tokio::io::stdout();
+
+    let (service, messages) = LspService::new(|client| Backend {
+        client,
+        documents: Default::default(),
+    });
+    Server::new(stdin, stdout)
+        .interleave(messages)
+        .serve(service)
+        .await;
+}
+
+fn doc_byte_pos(src: &str, pos: Position) -> usize {
+    let line_start = src
+        .line_spans()
+        .take(pos.line as _)
+        .map(|l| l.as_str_with_ending().len())
+        .sum::<usize>();
+    let line_pos = src
+        .chars()
+        .take(pos.character as _)
+        .map(|c| c.len_utf8())
+        .sum::<usize>();
+    line_start + line_pos
+}