F# negation when testing file existence

ilse25 Oct

I'm using this code to signal an error if a file is missing.

  if !File.Exists(inputPath) then
      printfn "inputPath doesn't exist %s" inputPath; failwith "stop"

Instead, this error appears. What did I misunderstand?

  error FS0001: This expression was expected to have type
      bool ref    
  but here has type
      bool

The conversation continues 2

Follow the reasoning

smallprint25 Oct Permalink
in F#, ! doesn't mean NOT, it's the reference opperator, so negation requires the not function instead, for example you could write if not <| File.Exists....

F# gives ! a different meaning, and thats defined as follows:

  type 'a ref { Contents : 'a }
  let (!) (cell : ref 'a) = cell.Contents

This fails because you supplied a bool, whereas the operand required by ! is a bool ref.

Replace it with the not function:

  if not(File.Exists(inputPath)) then
      printfn "inputPath doesn't exist %s" inputPath; failwith "stop"