[FIXED] How does one call bool.TryParse in F#

Issue

I need to call bool.TryParse in F# and I'm having a little difficulty with the compiler / Ionide.

Ex.

let value = true
let parsable = bool.TryParse("True", &value)

There error I'm seeing in Ionide for VSCode looks like the following.

val value : bool Full name: value

Assembly: example3

The type 'ByRefKinds.InOut' does not match the type 'ByRefKinds.In'F# Compiler(1)

This is the first example that I've used call by ref keywords and syntax in F# so I'm not sure what I'm doing wrong and the byrefs documentation doesn't seem to be of much help on understanding this particular scenario.

Solution

It turns out that the main thing I was missing in my example was that value wasn't mutable as it would need to be when called in this context.

Working example:

let mutable value = true
let parsable = bool.TryParse("True", &value)

This solution came to me after reading a slightly related GitHub issue related to Span<T> and then also playing around in dotnet fsi which at least gave me the following clue.

  let parsable = bool.TryParse("True", &value);;
  -------------------------------------^^^^^^

stdin(34,38): error FS3230: A value defined in a module must be mutable in order to take its address, e.g. 'let mutable x = ...'

As it turns out however F# also seems to have sntaxtic sugar around the Try-Parse pattern which I began to recall after playing around with this example further. This reduced into the below alternative solution to calling bool.TryParse.

let value = true
let (parsable, _) = bool.TryParse "True"

Alternatively without having to bind parsable and to specify the default value within a single expression the following example may be more elegant.

let value = match bool.TryParse "True" with
| true, value -> value
| false, _ -> true

Perhaps there are pros and cons to the different ways of calling bool.TryParse and other Try-Parse methods but the important thing is that I found some solutions that work and get me past the initial stumbling blocks with regard to lack of F# documentation on the subject.

Answered By - jpierson

Answer Checked By - Clifford M. (PHPFixing Volunteer)