On Wednesday, February 15, 2017 at 8:49:13 PM UTC, Rupert Smith wrote:
>
> What I don't understand is why this fails to type check:
>
> mapWhenWithPosition : (WithPosition b -> a) -> State -> Maybe a
> mapWhenWithPosition func state =
> case state of
> Aware rect ->
> 105: Just <| func rect
>
> Active rect ->
> Just <| func rect
>
> Inactive rect ->
> Just <| func rect
>
> _ ->
> Nothing
>
> This would seem to make extensible records a lot less useful than they
> could be. Is this one of those cases where an existential qualifier would
> be needed to specify the type of the function? Or is this in fact a case
> that could be typed in Elm without problem and that the type checker should
> accept?
>
Ok, I think I get it. Expecting this function to type check is a analagous
to expecting this function to type check:
badFunc : (a -> a) -> String -> String
badFunc func val =
func val
This would work if I did:
badFunc identity "test"
or
badFunc toUpper "test"
but not if I have (square : Int -> Int)
badFunc square "test"
Similarly I could have a function that matches the type (WithPosition b ->
a), but this particular function will require more fields than 'rect'.
funcThatBreaksIt : (WithPositon b -> a)
funcThatBreaksIt { rect, and, some, other, fields } ->
whereWillTheValuesForTheseComeFrom and some other fields
There is an implicit forAll qualifer over the type parameters in a typing
expression. The argument to a function passed to another function must be
contra-variant with respect to the 'more general type' relation amongst
types.
So, this is one of those cases where an existential qualifier would be
needed to specify the type of the function. That would break type
inference, so it is excluded.
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