Merge pull request #227 from goldfirere/T199-T220-improve-reification…

…-wrt-saks

Improve local reification and desugaring with respect to standalone kind signatures

CHANGES.md

@@ -31,6 +31,61 @@ Version 1.18 [????.??.??]
   `\(type a) (x :: a) -> x :: a`) and invisible type patterns (e.g.,
   `\ @a (x :: a) -> x :: a`).
 * Add a `Quote` instance for `DsM`.
+* Add `mapDTVName` and `mapDTVKind` functions, which allow mapping over the
+  `Name` and `DKind` of a `DTyVarBndr`, respectively.
+* Export `substTyVarBndr` from `Language.Haskell.TH.Desugar.Subst`.
+* Add a `Language.Haskell.TH.Desugar.Subst.Capturing` module. This exposes
+  mostly the same API as `Language.Haskell.TH.Desugar.Subst`, except that the
+  substitution functions in `Language.Haskell.TH.Desugar.Subst.Capturing` do
+  not avoid capture when subtituting into a @forall@ type. As a result, these
+  substitution functions are pure rather than monadic.
+* Add `dMatchUpSAKWithDecl`, a function that matches up type variable binders
+  from a standalone kind signature to the corresponding type variable binders
+  in the type-level declaration's header:
+
+  * The type signature for `dMatchUpSAKWithDecl` returns
+    `[DTyVarBndr ForAllTyFlag]`, where `ForAllTyFlag` is a new data type that
+    generalizes both `Specificity` and `BndrVis`.
+  * Add `dtvbForAllTyFlagsToSpecs` and `dtvbForAllTyFlagsToBndrVis` functions,
+    which allow converting the results of calling `dMatchUpSAKWithDecl` to
+    `[DTyVarBndrSpec]` or `[DTyVarBndrVis]`, respectively.
+  * Also add `matchUpSAKWithDecl`, `tvbForAllTyFlagsToSpecs`, and
+    `tvbForAllTyFlagsToBndrVis` functions, which work over `TyVarBndr` instead
+    of `DTyVarBndr`.
+* Local reifying the type of a data constructor or class method now yields type
+  signatures with more precise type variable information, as `th-desugar` now
+  incorporates information from the standalone kind signature (if any) for the
+  parent data type or class, respectively. For instance, consider the following
+  data type declaration:
+
+  ```hs
+  type P :: forall {k}. k -> Type
+  data P (a :: k) = MkP
+  ```
+
+  In previous versions of `th-desugar`, locally reifying `MkP` would yield the
+  following type:
+
+  ```hs
+  MkP :: forall k (a :: k). P a
+  ```
+
+  This was subtly wrong, as `k` is marked as specified (i.e., eligible for
+  visible type application), not inferred. In `th-desugar-1.18`, however, the
+  locally reified type will mark `k` as inferred, as expected:
+
+  ```hs
+  MkP :: forall {k} (a :: k). P a
+  ```
+
+  Similarly, desugaring `MkP` from Template Haskell to `th-desugar` results
+  in a data constructor with the expected type above.
+  * As a result of these changes, the type of `dsCon` has changed slightly:
+
+    ```diff
+    -dsCon :: DsMonad q => [DTyVarBndrUnit] -> DType -> Con -> q [DCon]
+    +dsCon :: DsMonad q => [DTyVarBndrSpec] -> DType -> Con -> q [DCon]
+    ```
 
 Version 1.17 [2024.05.12]
 -------------------------

Language/Haskell/TH/Desugar.hs

@@ -107,7 +107,11 @@ module Language.Haskell.TH.Desugar (
   unboxedSumNameDegree_maybe, unboxedTupleNameDegree_maybe,
   isTypeKindName, typeKindName, bindIP,
   mkExtraDKindBinders, dTyVarBndrToDType, changeDTVFlags,
+  mapDTVName, mapDTVKind,
   toposortTyVarsOf, toposortKindVarsOfTvbs,
+  ForAllTyFlag(..),
+  tvbForAllTyFlagsToSpecs, tvbForAllTyFlagsToBndrVis, matchUpSAKWithDecl,
+  dtvbForAllTyFlagsToSpecs, dtvbForAllTyFlagsToBndrVis, dMatchUpSAKWithDecl,
 
   -- ** 'FunArgs' and 'VisFunArg'
   FunArgs(..), ForallTelescope(..), VisFunArg(..),