Don't suggest using mutable keyword for parameters

[psfinaki]

This is misleading:

The error is okay I think, we just shouldn't suggest making parameters mutable, this is illegal.

[Happypig375]

Instead it could suggest to redefine the x as mutable with a let mutable x = x.

[psfinaki]

@Happypig375 mhm this would be compilable code indeed, I just wonder if there can really be a valid use case for that - as in, any real world code where you would recommend a developer such an approach instead of e.g. refactoring code to remove mutables :)

[smoothdeveloper]

@psfinaki, it would make sense to have another error message for this case:

Suggestion, but there is probably better way to phrase it.

The x binding is an immutable parameter, consider defining a local mutable binding or adjusting the parameter to byref.

related: #1103

[psfinaki]

Yeah I think I would be cautious with the word "consider" and be more like "if you for some weird reason intended to ... then ..." :) But yeah, that's details, more important it should be just a different diag/message.

[vzarytovskii]

@Happypig375 mhm this would be compilable code indeed, I just wonder if there can really be a valid use case for that - as in, any real world code where you would recommend a developer such an approach instead of e.g. refactoring code to remove mutables :)

Yes, when you want to make defensive copy.

[T-Gro]

Using a ref<> here in case of a parameter would be probable a good suggestion (or at least mentioned in it).
Or shadowing with let mutable x = x.

[voronoipotato]

Suggesting making the parameter mutable is more hostile to beginners than it is helpful to experienced F# devs. We can acknowledge that a new binding would solve the issue as most people who run across this issue are learning F# for the first time, do not need to mutate it, and there may be a good reason their teammates made it immutable in the first place 😃 .

The x binding is an immutable parameter, consider defining a new variable or adjusting the parameter to byref.

This way, it doesn't actively encourage a mutable copy when most beginners learning the language probably would have an easier time with a new immutable variable.