Hi Kai,
This was changed with linear types.
In Core, the constructor Just has now the linear type forall a. a %1 -> Maybe a.
For backwards compatibility, every occurrence of Just is eta-expanded
to \(@a # m) (x :: a) -> Just @a x, so that it can be used in a
non-linear context.
In the case of [], this transformation adds an extra type parameter, which
doesn't change much.
The details are in return_data_con and in Note [Linear fields generalization]
in Tc/Gen/Head.hs.
Best,
Krzysztof
On Wed, Nov 18, 2020 at 5:01 PM Kai-Oliver Prott
<stu126...@mail.uni-kiel.de> wrote:
>
> Hi all,
>
> I've tried a development snapshot of GHC's master branch from the
> beginning of this month and I am curious about the reason for a specific
> change. I noticed that a simple program like
>
> idNil :: [a] -> [a]
> idNil [] = []
>
> is internally represented (after type checking) as:
>
> AbsBinds [] []
> {Exports: [idNil <= idNil2]
> Exported types: idNil :: forall a. [a] -> [a]
> Binds: idNil2 [] = (/\(@a2). [] @a2) @a
>
> Can someone point me to a reason why the right side of idNil2 is not
> represented as [] @ a, like it was in GHC 8.10.1? I have not tried it
> with GHC 9.0. It is interesting to have a type lambda that is
> immediately followed by a type application.
>
> Thanks,
>
> Kai
>
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