“The value is not a function and cannot be applied.” error in F#

Question

I was trying to run the following FParsec code, until by some reason it stopped working:

The error I am getting is

"The value is not a function and cannot be applied."

If I comment out the last line of code (test ns "..") it will not yield an error, though. Any thoughts on how to solve this?

---

The source code in text form is the following:

open System
open FParsec

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

type Namespace = { Name : string; Classes : string list; }

let classes : Parser<string list, unit> = 
  many (spaces >>. many1Satisfy isLetter .>> spaces)

let ns =
  pipe2 
    (spaces >>. skipString "namespace" >>. spaces >>. many1Satisfy isLetter)
    (spaces >>. skipString "{" >>. classes .>> skipString "}")
    (fun name classes -> { Name = name; Classes = classes } )

test ns "namespace abc { def ghi }"

Answer 1

Noone could have guessed the answer here. The problem lied with other thing that I had decided to exclude from the post: the very header of my file:

#if INTERACTIVE
    #r @"C:\Users\xyz\Desktop\fparsec-main-default\Build\VS10\bin\Debug\FParsecCS.dll";
    #r @"C:\Users\xyz\Desktop\fparsec-main-default\Build\VS10\bin\Debug\FParsec.dll";
#endif

Replacing the ; by ;; will make all errors disappear:

#if INTERACTIVE
    #r @"C:\Users\xyz\Desktop\fparsec-main-default\Build\VS10\bin\Debug\FParsecCS.dll";;
    #r @"C:\Users\xyz\Desktop\fparsec-main-default\Build\VS10\bin\Debug\FParsec.dll";;
#endif

Answer 2

The red underlines clearly show that the compiler thinks that pipe2 is taking four arguments--you should be able to confirm this by added parentheses around the entire test expression like so: (test ns "namespace abs { def ghi })

I'm not sure why though; try putting parentheses around the pipe2 call:

let ns = 
  (pipe2  
     (spaces >>. skipString "namespace" >>. spaces >>. many1Satisfy isLetter) 
     (spaces >>. skipString "{" >>. classes .>> skipString "}") 
     (fun name classes -> { Name = name; Classes = classes } ))