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

23 Feb 2018

      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@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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