diff --git a/mers_lib/src/data/defs.rs b/mers_lib/src/data/defs.rs
index 895a3b9..0d5da2a 100755
--- a/mers_lib/src/data/defs.rs
+++ b/mers_lib/src/data/defs.rs
@@ -1,15 +1,25 @@
-use crate::program::run::CheckError;
+use super::Data;
 
-use super::{Data, MersType, Type};
-
-pub fn assign(from: Data, target: &Data) {
-    let target = target.get();
+pub fn assign(from: &Data, target: &Data) {
     if let Some(r) = target
+        .get()
         .as_any()
         .downcast_ref::<crate::data::reference::Reference>()
     {
-        *r.0.lock().unwrap().get_mut() = from.get().clone();
+        *r.0.lock().unwrap() = from.clone();
+    } else if let (Some(from), Some(target)) = (
+        from.get()
+            .as_any()
+            .downcast_ref::<crate::data::tuple::Tuple>(),
+        target
+            .get()
+            .as_any()
+            .downcast_ref::<crate::data::tuple::Tuple>(),
+    ) {
+        for (from, target) in from.0.iter().zip(target.0.iter()) {
+            assign(from, target);
+        }
     } else {
-        todo!("assignment to non-reference")
+        unreachable!("invalid assignment")
     }
 }
diff --git a/mers_lib/src/data/function.rs b/mers_lib/src/data/function.rs
index 17fa54f..df08911 100755
--- a/mers_lib/src/data/function.rs
+++ b/mers_lib/src/data/function.rs
@@ -4,10 +4,7 @@ use std::{
     sync::{Arc, Mutex},
 };
 
-use crate::program::{
-    self,
-    run::{CheckError, CheckInfo, Info},
-};
+use crate::program::run::{CheckError, CheckInfo, Info};
 
 use super::{Data, MersData, MersType, Type};
 
diff --git a/mers_lib/src/parsing/mod.rs b/mers_lib/src/parsing/mod.rs
index b3ddb86..623d474 100755
--- a/mers_lib/src/parsing/mod.rs
+++ b/mers_lib/src/parsing/mod.rs
@@ -1,24 +1,85 @@
 use std::sync::Arc;
 
-use crate::program;
+use crate::program::{self, parsed::block::Block};
 
 pub mod errors;
 pub mod statements;
 pub mod types;
 
 pub fn parse(src: &mut Source) -> Result<Box<dyn program::parsed::MersStatement>, ()> {
-    Ok(Box::new(statements::parse_block(src)?))
+    let pos_in_src = src.get_pos();
+    Ok(Box::new(Block {
+        pos_in_src,
+        statements: statements::parse_multiple(src, "")?,
+    }))
 }
 
 pub struct Source {
+    src_raw_len: usize,
     src: String,
+    /// (start, content) of each comment, including start/end (//, \n, /* and */)
+    comments: Vec<(usize, String)>,
     i: usize,
     sections: Vec<SectionMarker>,
 }
 impl Source {
-    pub fn new(src: String) -> Self {
+    pub fn new(source: String) -> Self {
+        let mut src = String::with_capacity(source.len());
+        let mut comment = (0, String::new());
+        let mut comments = Vec::new();
+        let mut chars = source.char_indices().peekable();
+        let mut in_comment = None;
+        loop {
+            if let Some((i, ch)) = chars.next() {
+                match in_comment {
+                    Some(false) => {
+                        comment.1.push(ch);
+                        if ch == '\n' {
+                            in_comment = None;
+                            comments.push((
+                                comment.0,
+                                std::mem::replace(&mut comment.1, String::new()),
+                            ));
+                        }
+                    }
+                    Some(true) => {
+                        comment.1.push(ch);
+                        if ch == '*' && matches!(chars.peek(), Some((_, '/'))) {
+                            chars.next();
+                            comment.1.push('/');
+                            in_comment = None;
+                            comments.push((
+                                comment.0,
+                                std::mem::replace(&mut comment.1, String::new()),
+                            ));
+                        }
+                    }
+                    None => match ch {
+                        '/' if matches!(chars.peek(), Some((_, '/'))) => {
+                            chars.next();
+                            in_comment = Some(false);
+                            comment.0 = i;
+                            comment.1.push('/');
+                            comment.1.push('/');
+                        }
+                        '/' if matches!(chars.peek(), Some((_, '*'))) => {
+                            chars.next();
+                            in_comment = Some(true);
+                            comment.0 = i;
+                            comment.1.push('/');
+                            comment.1.push('*');
+                        }
+                        _ => src.push(ch),
+                    },
+                }
+            } else {
+                break;
+            }
+        }
         Self {
+            src_raw_len: source.len(),
             src,
+            comments,
             i: 0,
             sections: vec![],
         }
@@ -105,6 +166,37 @@ impl Source {
     pub fn sections(&self) -> &Vec<SectionMarker> {
         &self.sections
     }
+
+    pub fn format(&self, insertions: &Vec<(usize, bool, String)>) -> String {
+        let mut o = String::with_capacity(self.src_raw_len);
+        let mut insertions = insertions.iter().peekable();
+        let mut comments = self.comments.iter().peekable();
+        for (i, ch) in self.src.char_indices() {
+            let insert = if let Some((index, pre, _)) = insertions.peek() {
+                if *index <= i {
+                    Some(*pre)
+                } else {
+                    None
+                }
+            } else {
+                None
+            };
+            if let Some((index, comment)) = comments.peek() {
+                if *index == i {
+                    comments.next();
+                    o.push_str(comment);
+                }
+            }
+            if let Some(true) = insert {
+                o.push_str(&insertions.next().unwrap().2);
+            }
+            o.push(ch);
+            if let Some(false) = insert {
+                o.push_str(&insertions.next().unwrap().2);
+            }
+        }
+        o
+    }
 }
 
 impl Drop for Source {
@@ -144,7 +236,7 @@ impl SectionMarker {
     }
 }
 
-#[derive(Clone, Copy)]
+#[derive(Clone, Copy, Debug)]
 pub struct SourcePos(usize);
 impl SourcePos {
     fn diff(&self, rhs: &Self) -> usize {
diff --git a/mers_lib/src/parsing/statements.rs b/mers_lib/src/parsing/statements.rs
index 90cdfca..e2443fc 100755
--- a/mers_lib/src/parsing/statements.rs
+++ b/mers_lib/src/parsing/statements.rs
@@ -1,5 +1,3 @@
-use std::sync::Arc;
-
 use super::Source;
 use crate::{
     data::Data,
@@ -16,32 +14,43 @@ pub fn parse(src: &mut Source) -> Result<Option<Box<dyn program::parsed::MersSta
     src.skip_whitespace();
     match src.peek_word() {
         ":=" => {
+            let pos_in_src = src.get_pos();
             src.next_word();
             first = Box::new(program::parsed::init_to::InitTo {
+                pos_in_src,
                 target: first,
                 source: parse(src)?.expect("todo"),
             });
         }
         "=" => {
+            let pos_in_src = src.get_pos();
             src.next_word();
             first = Box::new(program::parsed::assign_to::AssignTo {
+                pos_in_src,
                 target: first,
                 source: parse(src)?.expect("todo"),
             });
         }
         "->" => {
+            let pos_in_src = src.get_pos();
             src.next_word();
             first = Box::new(program::parsed::function::Function {
+                pos_in_src,
                 arg: first,
                 run: parse(src)?.expect("err: bad eof, fn needs some statement"),
             });
         }
         _ => loop {
+            let pos_in_src = src.get_pos();
             src.skip_whitespace();
             if let Some('.') = src.peek_char() {
                 src.next_char();
                 let chained = parse_no_chain(src)?.expect("err: EOF instead of chain");
-                first = Box::new(program::parsed::chain::Chain { first, chained });
+                first = Box::new(program::parsed::chain::Chain {
+                    pos_in_src,
+                    first,
+                    chained,
+                });
             } else {
                 break;
             }
@@ -52,23 +61,12 @@ pub fn parse(src: &mut Source) -> Result<Option<Box<dyn program::parsed::MersSta
     }
     Ok(Some(first))
 }
-/// Assumes the { has already been parsed
-pub fn parse_block(src: &mut Source) -> Result<program::parsed::block::Block, ()> {
-    Ok(program::parsed::block::Block {
-        statements: parse_multiple(src, '}')?,
-    })
-}
-pub fn parse_tuple(src: &mut Source) -> Result<program::parsed::tuple::Tuple, ()> {
-    Ok(program::parsed::tuple::Tuple {
-        elems: parse_multiple(src, ')')?,
-    })
-}
-pub fn parse_multiple(src: &mut Source, end: char) -> Result<Vec<Box<dyn MersStatement>>, ()> {
+pub fn parse_multiple(src: &mut Source, end: &str) -> Result<Vec<Box<dyn MersStatement>>, ()> {
     src.section_begin("block".to_string());
     let mut statements = vec![];
     loop {
         src.skip_whitespace();
-        if matches!(src.peek_char(), Some(e) if e == end) {
+        if src.peek_char().is_some_and(|ch| end.contains(ch)) {
             src.next_char();
             break;
         } else if let Some(s) = parse(src)? {
@@ -87,15 +85,24 @@ pub fn parse_no_chain(
     src.skip_whitespace();
     match src.peek_char() {
         Some('{') => {
+            let pos_in_src = src.get_pos();
             src.next_char();
-            return Ok(Some(Box::new(parse_block(src)?)));
+            return Ok(Some(Box::new(program::parsed::block::Block {
+                pos_in_src,
+                statements: parse_multiple(src, "}")?,
+            })));
         }
         Some('(') => {
+            let pos_in_src = src.get_pos();
             src.next_char();
-            return Ok(Some(Box::new(parse_tuple(src)?)));
+            return Ok(Some(Box::new(program::parsed::tuple::Tuple {
+                pos_in_src,
+                elems: parse_multiple(src, ")")?,
+            })));
         }
         Some('"') => {
             src.section_begin("string literal".to_string());
+            let pos_in_src = src.get_pos();
             src.next_char();
             let mut s = String::new();
             loop {
@@ -118,16 +125,19 @@ pub fn parse_no_chain(
                     todo!("err: eof in string")
                 }
             }
-            return Ok(Some(Box::new(program::parsed::value::Value(Data::new(
-                crate::data::string::String(s),
-            )))));
+            return Ok(Some(Box::new(program::parsed::value::Value {
+                pos_in_src,
+                data: Data::new(crate::data::string::String(s)),
+            })));
         }
         _ => {}
     }
+    let pos_in_src = src.get_pos();
     Ok(Some(match src.next_word() {
         "if" => {
             src.section_begin("if".to_string());
             Box::new(program::parsed::r#if::If {
+                pos_in_src,
                 condition: parse(src)?.expect("err: EOF instead of condition"),
                 on_true: parse(src)?.expect("err: EOF instead of on_true"),
                 on_false: {
@@ -142,16 +152,14 @@ pub fn parse_no_chain(
                 },
             })
         }
-        "switch" => {
-            src.section_begin("loop".to_string());
-            todo!()
-        }
-        "true" => Box::new(program::parsed::value::Value(Data::new(
-            crate::data::bool::Bool(true),
-        ))),
-        "false" => Box::new(program::parsed::value::Value(Data::new(
-            crate::data::bool::Bool(false),
-        ))),
+        "true" => Box::new(program::parsed::value::Value {
+            pos_in_src,
+            data: Data::new(crate::data::bool::Bool(true)),
+        }),
+        "false" => Box::new(program::parsed::value::Value {
+            pos_in_src,
+            data: Data::new(crate::data::bool::Bool(false)),
+        }),
         "" => return Ok(None),
         o => {
             let o = o.to_string();
@@ -161,28 +169,33 @@ pub fn parse_no_chain(
                     let here = src.get_pos();
                     src.next_char();
                     if let Ok(num) = format!("{o}.{}", src.next_word()).parse() {
-                        Box::new(program::parsed::value::Value(Data::new(
-                            crate::data::float::Float(num),
-                        )))
+                        Box::new(program::parsed::value::Value {
+                            pos_in_src,
+                            data: Data::new(crate::data::float::Float(num)),
+                        })
                     } else {
                         src.set_pos(here);
-                        Box::new(program::parsed::value::Value(Data::new(
-                            crate::data::int::Int(n),
-                        )))
+                        Box::new(program::parsed::value::Value {
+                            pos_in_src,
+                            data: Data::new(crate::data::int::Int(n)),
+                        })
                     }
                 } else {
-                    Box::new(program::parsed::value::Value(Data::new(
-                        crate::data::int::Int(n),
-                    )))
+                    Box::new(program::parsed::value::Value {
+                        pos_in_src,
+                        data: Data::new(crate::data::int::Int(n)),
+                    })
                 }
             } else {
                 if let Some('&') = o.chars().next() {
                     Box::new(program::parsed::variable::Variable {
+                        pos_in_src,
                         is_ref: true,
                         var: o[1..].to_string(),
                     })
                 } else {
                     Box::new(program::parsed::variable::Variable {
+                        pos_in_src,
                         is_ref: false,
                         var: o.to_string(),
                     })
diff --git a/mers_lib/src/program/configs/with_base.rs b/mers_lib/src/program/configs/with_base.rs
index a0eb758..1afd53a 100755
--- a/mers_lib/src/program/configs/with_base.rs
+++ b/mers_lib/src/program/configs/with_base.rs
@@ -1,7 +1,7 @@
 use std::sync::{Arc, Mutex};
 
 use crate::{
-    data::{self, Data, MersType, Type},
+    data::{self, Data, Type},
     program::run::{CheckInfo, Info},
 };
 
@@ -18,7 +18,6 @@ impl Config {
                 info: Arc::new(Info::neverused()),
                 info_check: Arc::new(Mutex::new(CheckInfo::neverused())),
                 out: Arc::new(|a, i| {
-                        dbg!(a);
                     let mut o = Type::empty();
                     for t in &a.types {
                         if let Some(t) = t.as_any().downcast_ref::<data::function::FunctionT>() {
diff --git a/mers_lib/src/program/parsed/assign_to.rs b/mers_lib/src/program/parsed/assign_to.rs
index ed2285b..4013b0e 100755
--- a/mers_lib/src/program/parsed/assign_to.rs
+++ b/mers_lib/src/program/parsed/assign_to.rs
@@ -1,9 +1,10 @@
-use crate::program;
+use crate::{parsing::SourcePos, program};
 
 use super::{CompInfo, MersStatement};
 
 #[derive(Debug)]
 pub struct AssignTo {
+    pub pos_in_src: SourcePos,
     pub target: Box<dyn MersStatement>,
     pub source: Box<dyn MersStatement>,
 }
@@ -18,6 +19,7 @@ impl MersStatement for AssignTo {
         comp: CompInfo,
     ) -> Result<Box<dyn program::run::MersStatement>, String> {
         Ok(Box::new(program::run::assign_to::AssignTo {
+            pos_in_src: self.pos_in_src,
             is_init: false,
             target: self.target.compile(info, comp)?,
             source: self.source.compile(info, comp)?,
diff --git a/mers_lib/src/program/parsed/block.rs b/mers_lib/src/program/parsed/block.rs
index c7407e3..b98a421 100755
--- a/mers_lib/src/program/parsed/block.rs
+++ b/mers_lib/src/program/parsed/block.rs
@@ -1,9 +1,10 @@
-use crate::{info, program};
+use crate::{info, parsing::SourcePos, program};
 
 use super::{CompInfo, MersStatement};
 
 #[derive(Debug)]
 pub struct Block {
+    pub pos_in_src: SourcePos,
     pub statements: Vec<Box<dyn MersStatement>>,
 }
 impl MersStatement for Block {
@@ -16,6 +17,7 @@ impl MersStatement for Block {
         comp: CompInfo,
     ) -> Result<Box<dyn program::run::MersStatement>, String> {
         Ok(Box::new(program::run::block::Block {
+            pos_in_src: self.pos_in_src,
             statements: self
                 .statements
                 .iter()
diff --git a/mers_lib/src/program/parsed/chain.rs b/mers_lib/src/program/parsed/chain.rs
index 4236866..9f5c116 100755
--- a/mers_lib/src/program/parsed/chain.rs
+++ b/mers_lib/src/program/parsed/chain.rs
@@ -1,9 +1,11 @@
+use crate::parsing::SourcePos;
 use crate::{info, program};
 
 use super::{CompInfo, MersStatement};
 
 #[derive(Debug)]
 pub struct Chain {
+    pub pos_in_src: SourcePos,
     pub first: Box<dyn MersStatement>,
     pub chained: Box<dyn MersStatement>,
 }
@@ -17,6 +19,7 @@ impl MersStatement for Chain {
         comp: CompInfo,
     ) -> Result<Box<dyn program::run::MersStatement>, String> {
         Ok(Box::new(program::run::chain::Chain {
+            pos_in_src: self.pos_in_src,
             first: self.first.compile(info, comp)?,
             chained: self.chained.compile(info, comp)?,
         }))
diff --git a/mers_lib/src/program/parsed/function.rs b/mers_lib/src/program/parsed/function.rs
index 841b15c..9276a10 100755
--- a/mers_lib/src/program/parsed/function.rs
+++ b/mers_lib/src/program/parsed/function.rs
@@ -1,3 +1,4 @@
+use crate::parsing::SourcePos;
 use std::sync::{Arc, Mutex};
 
 use crate::{
@@ -9,6 +10,7 @@ use super::{CompInfo, MersStatement};
 
 #[derive(Debug)]
 pub struct Function {
+    pub pos_in_src: SourcePos,
     pub arg: Box<dyn MersStatement>,
     pub run: Box<dyn MersStatement>,
 }
@@ -30,6 +32,7 @@ impl MersStatement for Function {
         let arg2 = Arc::clone(&arg_target);
         let run2 = Arc::clone(&run);
         Ok(Box::new(program::run::function::Function {
+            pos_in_src: self.pos_in_src,
             func_no_info: data::function::Function {
                 info: Arc::new(program::run::Info::neverused()),
                 info_check: Arc::new(Mutex::new(CheckInfo::neverused())),
@@ -38,7 +41,7 @@ impl MersStatement for Function {
                     Ok(run2.check(i, None)?)
                 }),
                 run: Arc::new(move |arg, info| {
-                    data::defs::assign(arg, &arg_target.run(info));
+                    data::defs::assign(&arg, &arg_target.run(info));
                     run.run(info)
                 }),
             },
diff --git a/mers_lib/src/program/parsed/if.rs b/mers_lib/src/program/parsed/if.rs
index 8804349..51751f0 100755
--- a/mers_lib/src/program/parsed/if.rs
+++ b/mers_lib/src/program/parsed/if.rs
@@ -1,9 +1,10 @@
-use crate::program;
+use crate::{parsing::SourcePos, program};
 
 use super::{CompInfo, MersStatement};
 
 #[derive(Debug)]
 pub struct If {
+    pub pos_in_src: SourcePos,
     pub condition: Box<dyn MersStatement>,
     pub on_true: Box<dyn MersStatement>,
     pub on_false: Option<Box<dyn MersStatement>>,
@@ -19,6 +20,7 @@ impl MersStatement for If {
         comp: CompInfo,
     ) -> Result<Box<dyn program::run::MersStatement>, String> {
         Ok(Box::new(program::run::r#if::If {
+            pos_in_src: self.pos_in_src,
             condition: self.condition.compile(info, comp)?,
             on_true: self.on_true.compile(info, comp)?,
             on_false: if let Some(v) = &self.on_false {
diff --git a/mers_lib/src/program/parsed/init_to.rs b/mers_lib/src/program/parsed/init_to.rs
index 34b41e1..256f996 100755
--- a/mers_lib/src/program/parsed/init_to.rs
+++ b/mers_lib/src/program/parsed/init_to.rs
@@ -1,9 +1,11 @@
+use crate::parsing::SourcePos;
 use crate::program;
 
 use super::{CompInfo, MersStatement};
 
 #[derive(Debug)]
 pub struct InitTo {
+    pub pos_in_src: SourcePos,
     pub target: Box<dyn MersStatement>,
     pub source: Box<dyn MersStatement>,
 }
@@ -22,6 +24,7 @@ impl MersStatement for InitTo {
         comp.is_init = false;
         let source = self.source.compile(info, comp)?;
         Ok(Box::new(program::run::assign_to::AssignTo {
+            pos_in_src: self.pos_in_src,
             is_init: true,
             target,
             source,
diff --git a/mers_lib/src/program/parsed/tuple.rs b/mers_lib/src/program/parsed/tuple.rs
index 29b35c6..5a33a72 100755
--- a/mers_lib/src/program/parsed/tuple.rs
+++ b/mers_lib/src/program/parsed/tuple.rs
@@ -1,9 +1,10 @@
-use crate::{info, program};
+use crate::{info, parsing::SourcePos, program};
 
 use super::{CompInfo, MersStatement};
 
 #[derive(Debug)]
 pub struct Tuple {
+    pub pos_in_src: SourcePos,
     pub elems: Vec<Box<dyn MersStatement>>,
 }
 impl MersStatement for Tuple {
@@ -16,6 +17,7 @@ impl MersStatement for Tuple {
         comp: CompInfo,
     ) -> Result<Box<dyn program::run::MersStatement>, String> {
         Ok(Box::new(program::run::tuple::Tuple {
+            pos_in_src: self.pos_in_src,
             elems: self
                 .elems
                 .iter()
diff --git a/mers_lib/src/program/parsed/value.rs b/mers_lib/src/program/parsed/value.rs
index 8e20eaa..f4dbbf9 100755
--- a/mers_lib/src/program/parsed/value.rs
+++ b/mers_lib/src/program/parsed/value.rs
@@ -1,9 +1,13 @@
+use crate::parsing::SourcePos;
 use crate::{data::Data, program};
 
 use super::{CompInfo, MersStatement};
 
 #[derive(Debug)]
-pub struct Value(pub Data);
+pub struct Value {
+    pub pos_in_src: SourcePos,
+    pub data: Data,
+}
 
 impl MersStatement for Value {
     fn has_scope(&self) -> bool {
@@ -15,7 +19,8 @@ impl MersStatement for Value {
         _comp: CompInfo,
     ) -> Result<Box<dyn program::run::MersStatement>, String> {
         Ok(Box::new(program::run::value::Value {
-            val: self.0.clone(),
+            pos_in_src: self.pos_in_src,
+            val: self.data.clone(),
         }))
     }
 }
diff --git a/mers_lib/src/program/parsed/variable.rs b/mers_lib/src/program/parsed/variable.rs
index a94118d..e035d50 100755
--- a/mers_lib/src/program/parsed/variable.rs
+++ b/mers_lib/src/program/parsed/variable.rs
@@ -1,9 +1,10 @@
-use crate::{info::Local, program};
+use crate::{info::Local, parsing::SourcePos, program};
 
 use super::{CompInfo, MersStatement};
 
 #[derive(Debug)]
 pub struct Variable {
+    pub pos_in_src: SourcePos,
     pub is_ref: bool,
     pub var: String,
 }
@@ -27,6 +28,7 @@ impl MersStatement for Variable {
             )
         }
         Ok(Box::new(program::run::variable::Variable {
+            pos_in_src: self.pos_in_src,
             is_init: comp.is_init,
             is_ref: comp.is_init || self.is_ref,
             var: if let Some(v) = info.get_var(&self.var) {
diff --git a/mers_lib/src/program/run/assign_to.rs b/mers_lib/src/program/run/assign_to.rs
index 545ba30..152d146 100755
--- a/mers_lib/src/program/run/assign_to.rs
+++ b/mers_lib/src/program/run/assign_to.rs
@@ -1,9 +1,13 @@
-use crate::data::{self, Type};
+use crate::{
+    data::{self, Type},
+    parsing::SourcePos,
+};
 
 use super::{CheckError, CheckInfo, MersStatement};
 
 #[derive(Debug)]
 pub struct AssignTo {
+    pub pos_in_src: SourcePos,
     pub is_init: bool,
     pub target: Box<dyn MersStatement>,
     pub source: Box<dyn MersStatement>,
@@ -27,10 +31,13 @@ impl MersStatement for AssignTo {
     fn run_custom(&self, info: &mut super::Info) -> crate::data::Data {
         let source = self.source.run(info);
         let target = self.target.run(info);
-        data::defs::assign(source, &target);
+        data::defs::assign(&source, &target);
         target
     }
     fn has_scope(&self) -> bool {
         false
     }
+    fn pos_in_src(&self) -> &SourcePos {
+        &self.pos_in_src
+    }
 }
diff --git a/mers_lib/src/program/run/block.rs b/mers_lib/src/program/run/block.rs
index 10a3212..3ced007 100755
--- a/mers_lib/src/program/run/block.rs
+++ b/mers_lib/src/program/run/block.rs
@@ -1,9 +1,10 @@
-use crate::data::Type;
+use crate::{data::Type, parsing::SourcePos};
 
 use super::{CheckError, MersStatement};
 
 #[derive(Debug)]
 pub struct Block {
+    pub pos_in_src: SourcePos,
     pub statements: Vec<Box<dyn MersStatement>>,
 }
 impl MersStatement for Block {
@@ -31,4 +32,7 @@ impl MersStatement for Block {
     fn has_scope(&self) -> bool {
         true
     }
+    fn pos_in_src(&self) -> &SourcePos {
+        &self.pos_in_src
+    }
 }
diff --git a/mers_lib/src/program/run/chain.rs b/mers_lib/src/program/run/chain.rs
index 1647c2a..224ef15 100755
--- a/mers_lib/src/program/run/chain.rs
+++ b/mers_lib/src/program/run/chain.rs
@@ -1,11 +1,15 @@
 use std::sync::Arc;
 
-use crate::data::{Data, Type};
+use crate::{
+    data::{Data, Type},
+    parsing::SourcePos,
+};
 
 use super::{CheckError, MersStatement};
 
 #[derive(Debug)]
 pub struct Chain {
+    pub pos_in_src: SourcePos,
     pub first: Box<dyn MersStatement>,
     pub chained: Box<dyn MersStatement>,
 }
@@ -54,4 +58,7 @@ impl MersStatement for Chain {
     fn has_scope(&self) -> bool {
         false
     }
+    fn pos_in_src(&self) -> &SourcePos {
+        &self.pos_in_src
+    }
 }
diff --git a/mers_lib/src/program/run/function.rs b/mers_lib/src/program/run/function.rs
index f34ca7e..651e238 100755
--- a/mers_lib/src/program/run/function.rs
+++ b/mers_lib/src/program/run/function.rs
@@ -1,11 +1,15 @@
 use std::sync::Arc;
 
-use crate::data::{self, Data, MersData, Type};
+use crate::{
+    data::{self, Data, MersData, Type},
+    parsing::SourcePos,
+};
 
 use super::{CheckError, MersStatement};
 
 #[derive(Debug)]
 pub struct Function {
+    pub pos_in_src: SourcePos,
     pub func_no_info: data::function::Function,
 }
 
@@ -29,4 +33,7 @@ impl MersStatement for Function {
     fn has_scope(&self) -> bool {
         true
     }
+    fn pos_in_src(&self) -> &SourcePos {
+        &self.pos_in_src
+    }
 }
diff --git a/mers_lib/src/program/run/if.rs b/mers_lib/src/program/run/if.rs
index 113bfe6..a4a275f 100755
--- a/mers_lib/src/program/run/if.rs
+++ b/mers_lib/src/program/run/if.rs
@@ -1,11 +1,15 @@
 use std::sync::Arc;
 
-use crate::data::{self, Data, MersType, Type};
+use crate::{
+    data::{self, Data, MersType, Type},
+    parsing::SourcePos,
+};
 
 use super::{CheckError, MersStatement};
 
 #[derive(Debug)]
 pub struct If {
+    pub pos_in_src: SourcePos,
     pub condition: Box<dyn MersStatement>,
     pub on_true: Box<dyn MersStatement>,
     pub on_false: Option<Box<dyn MersStatement>>,
@@ -55,4 +59,7 @@ impl MersStatement for If {
     fn has_scope(&self) -> bool {
         true
     }
+    fn pos_in_src(&self) -> &SourcePos {
+        &self.pos_in_src
+    }
 }
diff --git a/mers_lib/src/program/run/mod.rs b/mers_lib/src/program/run/mod.rs
index 97521ae..5db0447 100755
--- a/mers_lib/src/program/run/mod.rs
+++ b/mers_lib/src/program/run/mod.rs
@@ -6,6 +6,7 @@ use std::{
 use crate::{
     data::{self, Data, Type},
     info,
+    parsing::SourcePos,
 };
 
 #[cfg(feature = "run")]
@@ -34,6 +35,7 @@ pub trait MersStatement: Debug + Send + Sync {
     fn run_custom(&self, info: &mut Info) -> Data;
     /// if true, local variables etc. will be contained inside their own scope.
     fn has_scope(&self) -> bool;
+    fn pos_in_src(&self) -> &SourcePos;
     fn check(&self, info: &mut CheckInfo, assign: Option<&Type>) -> Result<Type, CheckError> {
         if self.has_scope() {
             info.create_scope();
diff --git a/mers_lib/src/program/run/tuple.rs b/mers_lib/src/program/run/tuple.rs
index ef5bee1..e7ff5ac 100755
--- a/mers_lib/src/program/run/tuple.rs
+++ b/mers_lib/src/program/run/tuple.rs
@@ -1,11 +1,15 @@
 use std::sync::Arc;
 
-use crate::data::{self, tuple::TupleT, Data, Type};
+use crate::{
+    data::{self, tuple::TupleT, Data, Type},
+    parsing::SourcePos,
+};
 
 use super::{CheckError, MersStatement};
 
 #[derive(Debug)]
 pub struct Tuple {
+    pub pos_in_src: SourcePos,
     pub elems: Vec<Box<dyn MersStatement>>,
 }
 impl MersStatement for Tuple {
@@ -31,7 +35,7 @@ impl MersStatement for Tuple {
                     }
                 } else {
                     return Err(CheckError(
-                        "can't init to statement type Tuple with value type {t}, which is part of {init_to} - only tuples can be assigned to tuples".to_string(),
+                        format!("can't init to statement type Tuple with value type {t}, which is part of {init_to} - only tuples can be assigned to tuples"),
                     ));
                 }
             }
@@ -64,4 +68,7 @@ impl MersStatement for Tuple {
     fn has_scope(&self) -> bool {
         false
     }
+    fn pos_in_src(&self) -> &SourcePos {
+        &self.pos_in_src
+    }
 }
diff --git a/mers_lib/src/program/run/value.rs b/mers_lib/src/program/run/value.rs
index d4487d7..21b9cdf 100755
--- a/mers_lib/src/program/run/value.rs
+++ b/mers_lib/src/program/run/value.rs
@@ -1,9 +1,13 @@
-use crate::data::{Data, Type};
+use crate::{
+    data::{Data, Type},
+    parsing::SourcePos,
+};
 
 use super::{CheckError, MersStatement};
 
 #[derive(Debug)]
 pub struct Value {
+    pub pos_in_src: SourcePos,
     pub val: Data,
 }
 
@@ -24,4 +28,7 @@ impl MersStatement for Value {
     fn run_custom(&self, _info: &mut super::Info) -> Data {
         self.val.clone()
     }
+    fn pos_in_src(&self) -> &SourcePos {
+        &self.pos_in_src
+    }
 }
diff --git a/mers_lib/src/program/run/variable.rs b/mers_lib/src/program/run/variable.rs
index 1546fbe..6d69cc2 100755
--- a/mers_lib/src/program/run/variable.rs
+++ b/mers_lib/src/program/run/variable.rs
@@ -1,11 +1,15 @@
 use std::sync::{Arc, Mutex};
 
-use crate::data::{self, Data, Type};
+use crate::{
+    data::{self, Data, Type},
+    parsing::SourcePos,
+};
 
 use super::MersStatement;
 
 #[derive(Debug)]
 pub struct Variable {
+    pub pos_in_src: SourcePos,
     pub is_init: bool,
     pub is_ref: bool,
     pub var: (usize, usize),
@@ -55,4 +59,7 @@ impl MersStatement for Variable {
                 .clone()
         }
     }
+    fn pos_in_src(&self) -> &SourcePos {
+        &self.pos_in_src
+    }
 }