Difference in Partial TypeFamily application between 7.8.3 & 7.8.4

[magesh b]

Thanks Richard. Basically I was storing the type family and its arguments
in the data type, so that I can transform its arguments before it gets
applied to stored typefamily (via. another closed type function). Is this
even possible to do by any chance in 7.8.4 & later?

On Tue, Jan 13, 2015 at 8:32 PM, Richard Eisenberg <eir at cis.upenn.edu>
wrote:

> Hello Magesh,
>
> The bug is in 7.8.3, which should never have allowed your `Test1`. Haskell
> type families may not be partially applied -- the type system and type
> inference just don't know how to handle such things. In 7.8.3, the check
> was accidentally turned off, as discussed in #9433, as you found. 7.8.3
> allows you to do a few limited things with partially applied families, but
> you'll get very strange errors if you continue down that road, as GHC
> quickly gets horribly confused.
>
> I'm afraid you'll have to find a different way to express what you want.
>
> Richard
>
> On Jan 13, 2015, at 9:49 AM, magesh b <magesh85 at gmail.com> wrote:
>
> Hi,
>
> {-# LANGUAGE TypeFamilies, ConstraintKinds #-}
>
> import GHC.Exts
>
> type family TyFun a
>
> data DictC (c :: * -> Constraint)
>
> data DictTF (tf :: * -> *)
>
> type Test1 = DictTF TyFun -- Fails here in 7.8.4
>
> type Test2 = DictC Show
>
> When I compile the above code, I'm getting the following error in 7.8.4
> and the same code works in 7.8.3.
>
> Test.hs:11:1:
>     Type synonym ‘TyFun’ should have 1 argument, but has been given none
>     In the type declaration for ‘Test1’
>
> Is this a bug or a expected behavior?
> For reference, I could find two fixes to type family related bug in this
> release.
> https://ghc.haskell.org/trac/ghc/ticket/9433
> https://ghc.haskell.org/trac/ghc/ticket/9316
>
>
> Thanks & Regards,
> Magesh B
>
>
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