A location is a span, because a column cannot draw a squiggle

Loc.t grows an exclusive end, defaulting to the start, so a location nobody
widened is a zero-width span at a point and every existing call site keeps
its old meaning. Only the reader knows where a form ends, so only the reader
fills them in — one helper in the one place that holds both ends, which is
why nothing above Reader had to learn a span exists.

The width assertion is the point of the tests: the field could exist, nothing
could fill it, and every underline would be one character long while the
feature looked finished.

lib/loc.ml

@@ -1,15 +1,51 @@
 (** Source locations. Every form carries one: error messages, the step debugger
     and nREPL's find-definition all need them, and retrofitting locations onto a
-    reader is far worse than carrying them from the start. *)
+    reader is far worse than carrying them from the start.
+
+    A location is a *span*, not a point. The start is what [file:line:col]
+    prints and what every consumer that wants one place uses; the end is what
+    lets an error underline the thing it is about. A column number cannot draw
+    a squiggle and a span can, which is the whole reason the two extra fields
+    are here.
+
+    The end is *exclusive* and defaults to the start, so a location nobody
+    widened is a zero-width span at a point and every old call site keeps its
+    old meaning. Only the reader knows where a form ends, so only the reader
+    fills these in; a location the checker invents for a node with no syntax
+    stays a point. *)
 
 type t = {
-  file : string;
-  line : int;  (* 1-based *)
-  col  : int;  (* 1-based *)
+  file  : string;
+  line  : int;  (* 1-based *)
+  col   : int;  (* 1-based *)
+  eline : int;  (* 1-based, exclusive end *)
+  ecol  : int;
 }
 
-let make file line col = { file; line; col }
-let unknown = { file = "<unknown>"; line = 0; col = 0 }
+let make file line col = { file; line; col; eline = line; ecol = col }
+let unknown = make "<unknown>" 0 0
+
+(** [upto start stop] is [start] widened to end where [stop] begins. A [stop]
+    that is not after [start], or is in another file, leaves it alone: a span
+    that runs backwards would draw nonsense. *)
+let upto (start : t) (stop : t) =
+  if
+    stop.file = start.file
+    && (stop.line > start.line
+        || (stop.line = start.line && stop.col > start.col))
+  then { start with eline = stop.line; ecol = stop.col }
+  else start
+
+(** True when the span covers more than its first line. The underline is drawn
+    on the start line either way — a form that spans twenty lines is pointed
+    at, not boxed — so this is what tells the renderer to run the underline to
+    the end of that line rather than to [ecol]. *)
+let multiline (t : t) = t.eline > t.line
+
+(** How many columns to underline on the start line, or [None] when the span
+    was never widened and there is nothing but a point to draw. *)
+let width (t : t) =
+  if t.eline = t.line && t.ecol > t.col then Some (t.ecol - t.col) else None
 
 let to_string t = Printf.sprintf "%s:%d:%d" t.file t.line t.col
 

lib/reader.ml

@@ -39,6 +39,12 @@ type state = {
 let of_string ~file src = { src; file; pos = 0; line = 1; col = 1 }
 
 let here st = Loc.make st.file st.line st.col
+
+(* A form's location is the span it occupies, so every reader below takes the
+   location it started at and closes it where the cursor now is. Doing it here,
+   in the one place that knows both ends, is why nothing above [Reader] has to
+   know a span exists. *)
+let spanned st loc v = Form.make v (Loc.upto loc (here st))
 let at_end st = st.pos >= String.length st.src
 let peek st = if at_end st then '\000' else st.src.[st.pos]
 let peek2 st =
@@ -99,7 +105,7 @@ let read_string st =
       | c -> advance st; Buffer.add_char buf c; go ()
   in
   go ();
-  Form.make (Form.Str (Buffer.contents buf)) loc
+  spanned st loc (Form.Str (Buffer.contents buf))
 
 (* \space \tab \newline \return \nul, or \<any single char> *)
 let read_byte st =
@@ -119,7 +125,7 @@ let read_byte st =
     | n when String.length n = 1 -> Char.code n.[0]
     | n -> Loc.fail loc "unknown character literal \\%s" n
   in
-  Form.make (Form.Byte code) loc
+  spanned st loc (Form.Byte code)
 
 (* A token that started with a digit, or with '-'/'+' followed by a digit. *)
 let read_number st =
@@ -132,26 +138,27 @@ let read_number st =
   in
   if is_hex then
     match Int64.of_string_opt text with
-    | Some i -> Form.make (Form.Int i) loc
-    | None -> Loc.fail loc "malformed hex literal %s" text
+    | Some i -> spanned st loc (Form.Int i)
+    | None -> Loc.fail (Loc.upto loc (here st)) "malformed hex literal %s" text
   else if String.contains text '.' || String.contains text 'e' then
     match float_of_string_opt text with
-    | Some f -> Form.make (Form.Float f) loc
-    | None -> Loc.fail loc "malformed float literal %s" text
+    | Some f -> spanned st loc (Form.Float f)
+    | None -> Loc.fail (Loc.upto loc (here st)) "malformed float literal %s" text
   else
     match Int64.of_string_opt text with
-    | Some i -> Form.make (Form.Int i) loc
-    | None -> Loc.fail loc "malformed integer literal %s" text
+    | Some i -> spanned st loc (Form.Int i)
+    | None -> Loc.fail (Loc.upto loc (here st)) "malformed integer literal %s" text
 
 let read_symbol_or_keyword st =
   let loc = here st in
   let text = take_while st (fun c -> not (is_delimiter c)) in
   if text = "" then Loc.fail loc "unexpected character %C" (peek st);
   if text.[0] = ':' then begin
-    if String.length text = 1 then Loc.fail loc "empty keyword";
-    Form.make (Form.Kw (String.sub text 1 (String.length text - 1))) loc
+    if String.length text = 1 then
+      Loc.fail (Loc.upto loc (here st)) "empty keyword";
+    spanned st loc (Form.Kw (String.sub text 1 (String.length text - 1)))
   end else
-    Form.make (Form.Sym text) loc
+    spanned st loc (Form.Sym text)
 
 (* ── Forms ─────────────────────────────────────────────────────────── *)
 
@@ -213,7 +220,7 @@ and read_sugar st loc name = advance st; read_wrapped st loc name
 
 and read_wrapped st loc name =
   let inner = read_form st in
-  Form.make (Form.List [ Form.make (Form.Sym name) loc; inner ]) loc
+  spanned st loc (Form.List [ Form.make (Form.Sym name) loc; inner ])
 
 and read_seq st open_c loc =
   advance st;
@@ -232,7 +239,8 @@ and read_seq st open_c loc =
         Loc.fail (here st) "expected %C to close %C, found %C" want open_c c
       else go (read_form st :: acc)
   in
-  Form.make (wrap open_c (go [])) loc
+  let items = go [] in
+  spanned st loc (wrap open_c items)
 
 (** All top-level forms in a source string. *)
 let read_all ~file src =

test/test_flan.ml

@@ -249,6 +249,38 @@ let () =
    | exception Loc.Error (loc, _) ->
      check "unclosed reports opening loc" (loc.line = 1 && loc.col = 1));
 
+  (* ── Spans ─────────────────────────────────────────────────────
+     A location ends where the form ends, which is what an underline needs
+     and what a column number cannot give. Asserted on the width rather than
+     on the end column alone: a span that never got widened is zero wide, and
+     that is the failure mode worth catching — the field would exist, nothing
+     would fill it, and every squiggle would be one character long. *)
+  (match read ~file:"f.flan" "(foo bar)" with
+   | [ l ] ->
+     check "span covers the list" (Loc.width l.loc = Some 9);
+     (match l.Form.v with
+      | Form.List [ head; arg ] ->
+        check "span covers the head symbol" (Loc.width head.loc = Some 3);
+        check "span covers the argument"    (Loc.width arg.loc = Some 3);
+        check "span starts at the symbol"   (arg.loc.col = 6)
+      | _ -> check "span: two elements" false)
+   | _ -> check "span: one form" false);
+
+  (match read ~file:"f.flan" "\"hi\" 42 :kw" with
+   | [ s; n; k ] ->
+     check "span covers a string with its quotes" (Loc.width s.loc = Some 4);
+     check "span covers a number" (Loc.width n.loc = Some 2);
+     check "span covers a keyword with its colon" (Loc.width k.loc = Some 3)
+   | _ -> check "span: three atoms" false);
+
+  (* A form that runs over a line end has no width on its first line, and says
+     so rather than reporting a negative one. *)
+  (match read ~file:"f.flan" "(a\n b)" with
+   | [ l ] ->
+     check "multi-line span is flagged" (Loc.multiline l.loc);
+     check "multi-line span has no single-line width" (Loc.width l.loc = None)
+   | _ -> check "span: one multi-line form" false);
+
   if !failures = 0 then print_endline "reader: all tests passed"
   else begin
     Printf.printf "\n%d failure(s)\n" !failures;