OCaml parser combinator that compiles to direct recursive descent
add source location spans to errors instead of just position int
author
vm.fail
date
(Jan 16, 2026, 9:51 PM UTC)
commit
2b47e91f
2b47e91f3051e20e681c94cc9f0fc1dc10049cf8
parent
067aeee8
067aeee83bfc6345fd94b7c0122244b3befe9dd1
vm.fail
date
(Jan 16, 2026, 9:51 PM UTC)
commit
2b47e91f
2b47e91f3051e20e681c94cc9f0fc1dc10049cf8
parent
067aeee8
067aeee83bfc6345fd94b7c0122244b3befe9dd1
+23
-12
+18
-10
lib/combin.ml
+18
-10
lib/combin.ml
···
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val show_token : token -> string
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end
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+
type span = { start_pos : int; end_pos : int }
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+
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type error = {
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-
pos : int;
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span : span;
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expected : string list;
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}
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let merge_errors e1 e2 =
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-
if e1.pos > e2.pos then e1
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else if e2.pos > e1.pos then e2
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-
else { pos = e1.pos; expected = e1.expected @ e2.expected }
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if e1.span.end_pos > e2.span.end_pos then e1
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else if e2.span.end_pos > e1.span.end_pos then e2
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else { span = { start_pos = min e1.span.start_pos e2.span.start_pos; end_pos = e1.span.end_pos }; expected = e1.expected @ e2.expected }
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let pure x _input = Ok (x, _input)
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-
let fail msg _input = Error { pos = 0; expected = [msg] }
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let fail msg _input = Error { span = { start_pos = 0; end_pos = 0 }; expected = [msg] }
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module Char_input : sig
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include INPUT with type token = char
···
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let format_error ?(input = "") e =
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let context =
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-
if String.length input > 0 && e.pos < String.length input then
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let start = max 0 (e.pos - 10) in
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let len = min 20 (String.length input - start) in
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if String.length input > 0 && e.span.end_pos < String.length input then
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let start = max 0 (e.span.start_pos - 5) in
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let len = min 30 (String.length input - start) in
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Printf.sprintf " near '%s'" (String.sub input start len)
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else ""
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in
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Printf.sprintf "parse error at position %d%s: expected %s"
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e.pos context (String.concat " or " e.expected)
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let pos_str =
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if e.span.start_pos = e.span.end_pos then
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Printf.sprintf "position %d" e.span.start_pos
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else
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Printf.sprintf "positions %d-%d" e.span.start_pos e.span.end_pos
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in
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Printf.sprintf "parse error at %s%s: expected %s"
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pos_str context (String.concat " or " e.expected)
+5
-2
src/codegen.ml
+5
-2
src/codegen.ml
···
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Printf.sprintf "%s_%d" prefix !counter
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let error_expr ~loc expected =
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[%expr Error { Combin.pos = I.position input; expected = [%e expected] }]
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[%expr Error { Combin.span = { Combin.start_pos = I.position input; end_pos = I.position input }; expected = [%e expected] }]
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let error_expr_span ~loc start_var expected =
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[%expr Error { Combin.span = { Combin.start_pos = [%e start_var]; end_pos = I.position input }; expected = [%e expected] }]
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let expected_list ~loc strs =
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elist ~loc (List.map (estring ~loc) strs)
···
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let saved = input in
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match [%e p_code] with
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| Ok _ as r -> r
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| Error e -> Error { e with Combin.pos = I.position saved }]
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| Error e -> Error { e with Combin.span = { e.Combin.span with Combin.start_pos = I.position saved } }]
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| Cut { loc } ->
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[%expr Ok ((), input)]