Commit ef26182e authored by Committed by Ben GamariBrowse files
Track the order of user-written tyvars in DataCon
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 Mk...
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