add string_if function (#2)

* add string_if function

* update CHANGES.md

CHANGES.md

@@ -1,3 +1,7 @@
+v1.0.2 2020-07-30 UK
+--------------------
+- Add string_if parser.
+
 v1.0.1 2020-07-30 UK
 --------------------
 - Add sexp_of_error.

lib/reparse.ml

@@ -133,6 +133,15 @@ let string s state =
       else msgf state "%d: string \"%s\" not found" state.offset s
   | None -> msgf state "%d: got EOF while parsing string \"%s\"" state.offset s
 
+let string_if s state =
+  let len = String.length s in
+  match substring len state with
+  | Some s2 ->
+      if s = s2 then
+        R.map (fun (state, ()) -> (state, Some s)) (advance len state)
+      else R.ok (state, None)
+  | None -> R.ok (state, None)
+
 let rec skip_while f state =
   match satisfy f state with
   | Ok (state, _) -> skip_while f state

lib/reparse.mli

@@ -48,7 +48,7 @@ val char : char -> (char, [> error ]) t
     Otherwise.*)
 
 val char_if : (char -> bool) -> (char option, [> error ]) t
-(** [char_if f] accepts and returns a [Some c] if [f c] is true. Otherwise it
+(** [char_if f] accepts and returns [Some c] if [f c] is true. Otherwise it
     returns [None]. Always succeeds. *)
 
 val satisfy : (char -> bool) -> (char, [> error ]) t
@@ -72,6 +72,10 @@ val peek_string : int -> (string option, [> error ]) t
 val string : string -> (string, [> error ]) t
 (** [string s] accepts [s] exactly and returns it. *)
 
+val string_if : string -> (string option, [> error ]) t
+(** [string_if s] accepts and returns [Some s] if [s] matches input. Otherwise
+    it returns false. Always succeeds. *)
+
 val skip_while : (char -> bool) -> (unit, [> error ]) t
 (** [skip_while f] keeps accepting [c] if [f c] is [true]. [c] is discarded.
     Always succeeds. *)