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 ->
> Just <| func rect
>
> Active rect ->
> Just <| func rect
>
> Inactive rect ->
> Just <| func rect
>
> _ ->
> Nothing
>
I can get it to type check by defining the more general function in a
let..in construct:
mapWhenWithPosition : (WithPosition {} -> a) -> State -> Maybe a
mapWhenWithPosition func state =
let
func_ : WithPosition b -> a
func_ { rect } =
func { rect = rect }
in
case state of
Aware position ->
Just <| func_ position
Active position ->
Just <| func_ position
Inactive position ->
Just <| func_ position
_ ->
Nothing
let..in behaves somewhat like the existential qualifier - "let us say that
a more general type exists, by defining its implementation". I still have
to unpack and repack the record but at least now, only once.
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