FParsec: backtracking `sepBy`

Consider the following toy grammar and parser:

(* in EBNF:
  ap = "a", { "ba" }
  bp = ap, "bc"
*)
let ap = sepBy1 (pstring "a") (pstring "b")
let bp = ap .>> (pstring "bc")
let test = run bp "abababc"

I get the following output:

Error in Ln: 1 Col: 7
abababc
      ^
Expecting: 'a'

Clearly sepBy1 sees the last b and expects it to lead into another a, failing when it doesn't find one. Is there a variant of sepBy1 which would backtrack over b and make this parse succeed? Is there any reason why I shouldn't use that instead?

FParsec: backtracking `sepBy`

3 Answers