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Ryan Scott authored
After typechecking a data constructor's type signature, its type variables are partitioned into two distinct groups: the universally quantified type variables and the existentially quantified type variables. Then, when prompted for the type of the data constructor, GHC gives this: ```lang=haskell MkT :: forall <univs> <exis>. (...) ``` For H98-style datatypes, this is a fine thing to do. But for GADTs, this can sometimes produce undesired results with respect to `TypeApplications`. For instance, consider this datatype: ```lang=haskell data T a where MkT :: forall b a. b -> T a ``` Here, the user clearly intended to have `b` be available for visible type application before `a`. That is, the user would expect `MkT @Int @Char` to be of type `Int -> T Char`, //not// `Char -> T Int`. But alas, up until now that was not how GHC operated—regardless of the order in which the user actually wrote the tyvars, GHC would give `MkT` the type: ```lang=haskell MkT :: forall a b. b -> T a ``` Since `a` is universal and `b` is existential. This makes predicting what order to use for `TypeApplications` quite annoying, as demonstrated in #11721 and #13848. This patch cures the problem by tracking more carefully the order in which a user writes type variables in data constructor type signatures, either explicitly (with a `forall`) or implicitly (without a `forall`, in which case the order is inferred). This is accomplished by adding a new field `dcUserTyVars` to `DataCon`, which is a subset of `dcUnivTyVars` and `dcExTyVars` that is permuted to the order in which the user wrote them. For more details, refer to `Note [DataCon user type variables]` in `DataCon.hs`. An interesting consequence of this design is that more data constructors require wrappers. This is because the workers always expect the first arguments to be the universal tyvars followed by the existential tyvars, so when the user writes the tyvars in a different order, a wrapper type is needed to swizzle the tyvars around to match the order that the worker expects. For more details, refer to `Note [Data con wrappers and GADT syntax]` in `MkId.hs`. Test Plan: ./validate Reviewers: austin, goldfire, bgamari, simonpj Reviewed By: goldfire, simonpj Subscribers: ezyang, goldfire, rwbarton, thomie GHC Trac Issues: #11721, #13848 Differential Revision: https://phabricator.haskell.org/D3687
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