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List:       haskell-cafe
Subject:    Re: [Haskell-cafe] Fundeps and overlapping instances
From:       Gábor_Lehel <illissius () gmail ! com>
Date:       2012-06-09 23:16:06
Message-ID: CAPNUp0_7tGq+24Lt8m=dFQujF19YPokduNgaR8oPqxTSiNCcMw () mail ! gmail ! com
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On Tue, Jun 5, 2012 at 2:25 PM, Gábor Lehel <illissius@gmail.com> wrote:
> The encoded version would be:
>
> instance (f a b) => FMap f (HJust a) (HJust b)
>  where type f :<$>: (HJust a) = HJust b
>
> and I think this actually demonstrates a *different* limitation, namely that
>
>> The RHS of an associated type declaration mentions type variable `b'
>>   All such variables must be bound on the LHS
>
> which means that the standard encoding doesn't work for this case.

From a reddit comment[1] by Reiner Pope it turns out you can actually do this:

instance (f a b) => FMap f (HJust a) (HJust b) where
    type f :<$>: (HJust a) = HJust (f :<$>: a)

A bit more awkward to write, but we're back to TFs not having any
expressivity problem in this department.

[1] http://www.reddit.com/r/haskell/comments/ut85i/a_few_typefamilies_nuggets/c4ygefh

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