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

[Tom Ellis]

That's amazing!  How many times have I been tripped up by that ...  Thanks a
lot, Adam.

On Thu, Feb 22, 2018 at 11:11:18PM +0000, Adam Bergmark wrote:
> This works if you enable NoMonomorphismRestriction.
> 
> 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