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 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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