Possible F# Type Inference Bug In Workaround Form

typeInferenceBug.fsx

// NB. to reproduce bug, delete the type after the colon

module AST =
  type name = string
  type param = string
  type arg = Number of int | Arg of param

  type Part =
  | Cell of string

module Parser

open AST
open FParsec

let test p str =
  match run p str with 
  | Success(result, _, _) -> printfn "Success: %A %O" result (result.GetType())
  | Failure(errorMsg, _, _) -> printfn "Failure: %s" errorMsg

let proc : ref<(((string * string) list) list)> = ref []

let one : Parser<string,unit> = pstring "1" 
let two : Parser<string,unit> = pstring "2" 

let three : Parser<Part,unit> = pstring "2" |>> fun s->Part.Cell(s)

test one "1"

let pNumberWord = one <|> two |>> fun a->Part.Cell(a)

test pNumberWord "2"

The annotations "Parser<string,unit>" on "one" and "two" are needed because of the value restriction - "pstring" is generic, returning "Parser<string,'T>" for any 'T, and you need to pin down an exact non-generic value for 'T when using it to create parser values like this.

The annotation on "three" is needed for the same reason, though the hidden "T" implied by the use of "pstring" is not so easy to spot.

Some of the library specific demos / documentation suggested otherwise. That's good news. Thanks Don! Much appreciated.