FParsec failing on optional parser

Question

I am currently learning the FParsec library, but I have come across an issue. When I want to parse an optional string and continue parsing as normal afterwards, FParsec will return a fatal error on the optional parser, rather than returning None as I expect. The below working code sample illustrates my point:

open System
open FParsec

type AccountEntity = 
    | Default 
    | Entity of string

let pEntity =
    let isEntityFirstChar c = isLetter c
    let isEntityChar c = isLetter c || isDigit c
    (many1Satisfy2L isEntityFirstChar isEntityChar "entity") .>> skipString "/"

let pOptEntity =
     opt pEntity
     |>> (fun optEntity -> 
              match optEntity with 
              | Some entity -> Entity entity 
              | None -> Default)

[<EntryPoint>]
let main argv = 
    printfn "%A" (run pOptEntity "test/account:subaccount") //works
    printfn "%A" (run pOptEntity "account:subaccount") //crashes
    Console.ReadLine() |> ignore
    0 // return an integer exit code

The behavior I would expect is for pOptEntity to return a Default entity when an entity is not provided. However, instead, I get the following error:

Failure:
Error in Ln: 1 Col: 8
account:subaccount
       ^
Expecting: '/'

Shouldn't opt provide the behavior I am describing and continue to parse the account string as normal or am I approaching this in the incorrect manner? I took a look at attempt but, then, I wouldn't be able to provide the default entity behavior like I want.

Your help is much appreciated, thank you.

Answer 1

The opt combinator follows the same rules as <|> ; if you look at the <|> documentation , it mentions that if the first parser fails without changing the parser state , the second parser is attempted. http://www.quanttec.com/fparsec/users-guide/parsing-alternatives.html goes into more detail.

Here, the .>>? combinator is what you want to use in your pEntity parser. Replace the .>> with .>>? and you'll have a pEntity parser that, if it is not followed by a / , will backtrack to the beginning of what it attempted and not consume input. This will allow the opt combinator to function as designed.

PS I tested this, and it worked. Replacing the .>> with .>>? in pEntity and then running your code produced the following output:

Success: Entity "test"
Success: Default