7f47443d
by Simon Peyton Jones at 2026-10-07T00:33:37+01:00
Refactor type and coercion substitution
This MR refactors type and coercion substitution so that they use
TyCoMapper, rather than hand-rolled code. Fundamentally it's a simple
idea, but I had to do quite a bit of refactoring to make it fly.
I got Claude to help me this this refactoring, but I paid careful
attention to every line of code.
Main changes:
* New module GHC.Core.TyCo.Make which defines TyCoMapper, and a bunch of other
simple "smart constructors" for types and coercions. See
GHC.Core.TyCo.Rep Note [The Type-related module hierarchy], esp (TRM1)
* The new TyCoMapper has new fields `tcm_tcapp_ty` and `tcm_tcapp_co` to handle
type-constructor applications. These replace `tcm_tycon`.
* Rewrite GHC.Core.TyCo.Subst to use TyCoMapper
* Rewrite GHC.Core.Type.expandTypeSynonyms to use TyCoMapper.
* Implement `seqType` and `seqCo` with TyCoFolder. To to this I had to add
a `tcf_lit` field to TyCoFolder.
* A new module GHC.Utils.StrictIdentity to define a strict identity monad,
which itself is used for
substTyCoMapper :: TyCoMapper Subst StrictIdentity
Refactoring `seq` functions
* In GHC.Core.Seq, redefine `seqOneShot`, `seqRuleInfo`, and friends to return
`SeqResult`, a new type defined in GHC.Types.Baseic, rather than `()`. We
can define a Monoid instance for `SeqResult` that does the right thing.
* This has knock-on effects across the compiler.
Refactoring ForAllTyFlag
In Note [Comparing visibility] in GHC.Core.TyCo.Compare we read that
"we always compare ForAllTyFlags with `eqForAllVis` or `cmpForAllVis`"
But alas there was an Eq and Ord instance for ForAllTyFlag, which made it
impossible to check this claim. So I made these changes:
* Move the definition of ForAllTyFlag and its related functions from
L.H.S.Specificity (where it did not belong, since L.H.S didn't use it)
into GHC.Types.Var.
* Remove the Eq and Ord instances for ForAllTyFlag
* Deal with the fallout, by using `eqForAllVis` or related functions.
* In interface files we use IfaceType as a key for a de-deduplication map
See Note [Deduplication during iface binary serialisation]
in GHC.Iface.Binary. For this we need /exact/ comparison, so we use a
newtype wrapper for IfaceForAllTyFlag, and give use `cmpForAllTyFlagExactly`
to implement its Ord instance. Tiresome but easy.
* Rename `toIfaceForAllBndrs` to `toIfaceForAllTyBndrs`