2 files changed, 22 insertions(+), 3 deletions(-)

M .ocamlformat
M src/common/err.ml

@@ 1,2 1,2 @@
 profile = default
-version = 0.20.1>
\ No newline at end of file
+version = 0.20.1

@@ 4,7 4,16 @@ type expsn_err = [ `ExpsnAmbiguity of string | `UnknownExpsn of string ]
 type parser_err = [ `IllegalCharacter of loc | `SyntaxError of loc ]
 type t = [ checker_err | expsn_err | parser_err | `NoSuchFile of string ]
 
-(* TODO: overview of concerned line. *)
+let get_line ic n =
+  try
+    let i = ref 1 in
+    while !i < n do
+      let _ = input_line ic in
+      incr i
+    done;
+    input_line ic
+  with End_of_file -> assert false
+
 let rec pp_loc ?hint ~msg (spos, epos) =
   let schar = Lexing.(spos.pos_cnum - spos.pos_bol) in
   let echar = Lexing.(epos.pos_cnum - epos.pos_bol) in


@@ 16,10 25,20 @@ let rec pp_loc ?hint ~msg (spos, epos) =
     Printf.sprintf {|File "%s", line %i, characters %s:|} epos.Lexing.pos_fname
       spos.Lexing.pos_lnum char_pos
   in
+  let overview =
+    let line = get_line (open_in epos.Lexing.pos_fname) spos.Lexing.pos_lnum in
+    let margin =
+      String.make
+        (schar + (spos.Lexing.pos_lnum |> string_of_int |> String.length))
+        ' '
+    in
+    let cursor = if echar <= 0 then "^" else String.make (echar - schar) '^' in
+    Printf.sprintf "%d | %s\n   %s%s" spos.Lexing.pos_lnum line margin cursor
+  in
   let descr =
     match hint with None -> pp_string msg | Some hint -> pp_string msg ~hint
   in
-  Printf.sprintf "%s\n%s" carret descr
+  Printf.sprintf "%s\n%s\n%s" carret overview descr
 
 and pp_string ?hint msg =
   let err = Printf.sprintf "Error: %s" msg in