[Haskell-cafe] No instance - but it could just put one in the context

[Adam Bergmark]

This works if you enable NoMonomorphismRestriction.

Cheers,
Adam


On Thu, 22 Feb 2018 at 23:41 Tom Ellis <
tom-lists-haskell-cafe-2013 at jaguarpaw.co.uk> wrote:

> I'm puzzled by GHC's behaviour in the following program.
>
> 'baz = bar . foo' does not work because there is "no instance for ...".
> But
> if I manually assume those instances in the context all is fine.  Why can
> GHC not infer that context?  Is there any extension or clever trick I can
> use to get this to infer like I want?
>
> Thanks,
>
> Tom
>
>
>
> {-# LANGUAGE MultiParamTypeClasses #-}
> {-# LANGUAGE TypeFamilies #-}
> {-# LANGUAGE FlexibleContexts #-}
>
> class Foo a b
> class Bar a b
> type family Quux a
>
> foo :: ( Foo a b
>        , b ~ Quux a )
>     => a
>     -> Quux a
> foo = undefined
>
> bar :: Bar a b
>     => a
>     -> b
> bar = undefined
>
> -- Doesn't work
> -- No instance for (Bar (Quux a0) c0) arising from a use of ‘bar’
> -- No instance for (Foo a0 (Quux a0)) arising from a use of ‘foo’
> --baz = bar . foo
>
> baz' :: ( Foo a (Quux a)
>         , Bar (Quux a) b )
>      => a
>      -> b
> baz' = bar . foo
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