#15952: Reduce zonking-related invariants in the type checker -------------------------------------+------------------------------------- Reporter: goldfire | Owner: (none) Type: task | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.6.2 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Description changed by simonpj: Old description:
The type checker currently maintains two invariants, documented in `Note [The well-kinded type invariant]` in TcType and `Note [The tcType invariant]` in TcHsType. These are a pain to maintain, and Simon and I cooked up a plan not to.
Step 1. The tcType invariant is all about `tcInferApps`, at least according to the Note. Previously, we couldn't zonk in `tcInferApps`, because types were knot-tied. But now we can! So, `tcInferApps` could take the type only (not its kind), zonk it, then get the type's (zonked) kind. This might also mean that `tc_infer_hs_type` no longer needs to return a type/kind pair, but could just return a type.
After this is done, we can likely remove all the zonks that are in service to the tcType invariant, findable by a search.
Step 2. The well-kinded type invariant is also about the need to zonk sometimes. But, instead, we could just always zonk on-the-fly. That is, we introduce new functions like `tcTypeKind` and `tcSubstTy` that zonk as they go. To make sure we're calling the right function at the right time, we introduce `newtype TcType = TcType { unTcType :: Type }`. Then, we're forced to call monadic functions on `TcType`s. This will likely lead to code duplication between `Type` and `TcType`, but we might be able to banish `zonkTcType` (and its many friends) entirely. Also gone would be the `naked` functions, which are all about maintaining the well-kinded type invariant. This step would, as a side effect, fix #15799 and #15918, which is about treating `TcType`s differently from `Type`s.
'''Work in progress on `wip/T15952`'''
New description: The type checker currently maintains two invariants, documented in `Note [The well-kinded type invariant]` in TcType and `Note [The tcType invariant]` in TcHsType. These are a pain to maintain, and Simon and I cooked up a plan not to. Step 1. The tcType invariant is all about `tcInferApps`, at least according to the Note. Previously, we couldn't zonk in `tcInferApps`, because types were knot-tied. But now we can! So, `tcInferApps` could take the type only (not its kind), zonk it, then get the type's (zonked) kind. This might also mean that `tc_infer_hs_type` no longer needs to return a type/kind pair, but could just return a type. After this is done, we can likely remove all the zonks that are in service to the tcType invariant, findable by a search. Step 2. The well-kinded type invariant is also about the need to zonk sometimes. But, instead, we could just always zonk on-the-fly. That is, we introduce new functions like `tcTypeKind` and `tcSubstTy` that zonk as they go. To make sure we're calling the right function at the right time, we introduce `newtype TcType = TcType { unTcType :: Type }`. Then, we're forced to call monadic functions on `TcType`s. This will likely lead to code duplication between `Type` and `TcType`, but we might be able to banish `zonkTcType` (and its many friends) entirely. Also gone would be the `naked` functions, which are all about maintaining the well-kinded type invariant. This step would, as a side effect, fix #15799 and #15918, which is about treating `TcType`s differently from `Type`s. '''Work in progress on `wip/T15952`''' '''Merge request at [https://gitlab.haskell.org/ghc/ghc/merge_requests/74]''' -- -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15952#comment:9 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler