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Improve error reporting: Wrong cast operator (:?> instead of :>) #1127

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Improve error reporting: Wrong cast operator (:?> instead of :>)#1127

Description

@isaksky
isaksky
opened on Apr 28, 2016

Related to #1103

What

image

Why

This error message is problematic for newcomers, especially ones from C#, because:

  1. IDictionary<obj,obj> is compatible with Dictionary<obj, obj>
  2. C# only has one cast operator, and this works in C#:
public static IDictionary<object, object> TestM() {
    return new Dictionary<object, object>();
}

Possible solution

Before resorting to this error message, check if the left hand side can be statically cast to the type on the right, and if so, use a "consider using the :> operator" error message instead.

Activity

  1. forki commented on May 3, 2016

    @forki
    forki
    on May 3, 2016
    Contributor

    first WIP PR: #1149

  2. isaacabraham commented on May 3, 2016

    @isaacabraham
    isaacabraham
    on May 3, 2016
    Contributor

    @isaksky Just a thought - C# does have two keywords - as (which doesn't exist in F# AFAIK) and (). It's more that in C# the "safe upcast" happens implicitly whereas in F# you need to be explicit - although I can't remember if this is only for co/contravariant conversions.

  3. isaacabraham commented on May 3, 2016

    @isaacabraham
    isaacabraham
    on May 3, 2016
    Contributor

    In fact here's another whole host of example cases...

    type IFoo = abstract member Bar : unit -> unit
type Foo() = interface IFoo with member __.Bar () = ()

let Thing : IFoo = Foo() // fails
let Thing2 : IFoo = Foo() :?> IFoo // fails
let Thing3 : #IFoo = Foo() // warning - and overrides the type signature!!
let Thing3 : IFoo = upcast(Foo()) // works!
let Thing4 : IFoo = downcast(Foo()) // doesn't work!
let Thing5 : IFoo = Foo() :> IFoo // works!

  4. isaacabraham commented on May 3, 2016

    @isaacabraham
    isaacabraham
    on May 3, 2016
    Contributor

    I propose the error message to say: -

    "The conversion from <type A> to <type B> is a compile-time safe upcast, not a downcast. Considering using the :> (upcast) operator instead of the :?> (downcast) operator."

  5. forki commented on May 9, 2016

    @forki
    forki
    on May 9, 2016
    Contributor

    PR in #1149

  6. 5 remaining items

  7. KevinRansom
    closed this as completedin f04f4cdon May 18, 2016
  8. KevinRansom
    added a commit that references this issue on May 18, 2016

    Merge pull request #1149 from forki/downcast

    7ff89c3
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