Mutable use bindings permitted by the F# compiler

[eiriktsarpalis]

I just noticed that the F# compiler permits mutable use bindings:

let disp msg = { new System.IDisposable with member __.Dispose () = printfn "%s" msg }

let test () =
 use mutable d = disp "foo"
 d <- disp "bar"

Running test will cause "bar" to be printed. This feels wrong and the sentiment was corroborated by the fact that C# explicitly prevents it:

using (MemoryStream m = new MemoryStream())
{
 m = null;
}

which produces the compiler error:

Error 1 Cannot assign to 'm' because it is a 'using variable'

I checked the F# spec, but couldn't find any explicit mention of mutable use bindings.

Is this a compiler bug then?

[dsyme]

It's by spec but I'd be happy to emit a warning in this case. Could you add an entry to http://fslang.uservoice.com? I'll mark it as approved for a future edition of the language.

[eiriktsarpalis]

Done! http://fslang.uservoice.com/forums/245727-f-language/suggestions/8909788-disable-or-add-a-warning-to-mutable-use-bindings

[dsyme]

Closing this since it's now recoded on uservoice (and approved)

[dsyme]

@eiriktsarpalis An update on this: I believe mutable use bindings may be needed for F# in the case that the "disposable" type is a struct. For example, a struct enumerator of a System.Collections.Generic.List<T>