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mutable vars scoped to function closure?
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Why does this code fail?

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    let mutable c = 0
    [1; 2; 3]
    |> Seq.iter (fun n -> 
        c <- c + n
        JavaScript.Console.Log("c = " + c.ToString());
    )
    JavaScript.Console.Log("last c = " + c.ToString());

it produces this output:

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c = 1
c = 3
c = 6
last c = 0
.

I can reproduce this problem. Replacing the mutable with a ref yields the correct behavior:

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    let c = ref 0
    [1; 2; 3]
    |> Seq.iter (fun n -> 
        c := !c + n
        JavaScript.Console.Log("c = " + (!c).ToString());
    )
    JavaScript.Console.Log("last c = " + (!c).ToString());

So something is wrong with the way we deal with mutables through closures. Being able to do that is a new F# 4.0 feature, which is probably why it hasn't been noticed yet.

I submitted this as a ticket, and will look into fixing this soon.

Thanks for the report!

By on 10/14/2015 8:38 AM ()

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