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

[Tom Ellis]

My next question is, could GHC suggest NoMonomorphismRestriction when it
comes across such code?

(In the same way it suggests FlexibleContexts, etc.)

On Thu, Feb 22, 2018 at 11:16:35PM +0000, Tom Ellis wrote:
> 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
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