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

[Tom Ellis]

Oh, it does already seem to mention this, which is great!

On Thu, Feb 22, 2018 at 11:25:29PM +0000, Tom Ellis wrote:
> 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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