[Git][ghc/ghc][wip/T25647] update note

[Patrick (@soulomoon)]

Patrick pushed to branch wip/T25647 at Glasgow Haskell Compiler / GHC


Commits:
668ca485 by Patrick at 2025-02-09T16:14:37+08:00
update note

- - - - -


2 changed files:

- compiler/GHC/Tc/Utils/TcMType.hs
- compiler/GHC/Tc/Utils/Unify.hs


Changes:

=====================================
compiler/GHC/Tc/Utils/TcMType.hs
=====================================
@@ -695,18 +695,17 @@ the thinking.
 
 {- Note [NoDefTauTv]
 ~~~~~~~~~~~~~~~~~
-A NoDefTauTv behaves like a TauTv, except that when it is not unified
-with anything, it should not be defaulted.
+A NoDefTauTv behaves like a TauTv, except that it should not be defaulted.
+Making it more polymorphic than a TauTv which can be defaulted.
 
 It is used for a anonymous wildcard in a type family, e.g.
   type Dix8 :: RuntimeRep -> Type
   data family Dix8 r
   newtype instance Dix8 _ = Dix8 Int
-We want to keep `_` polymorphic, so it behaves like a named wildcard.
 
-When unified other type, it should be replaced by the other type, since
-the `not unified with anything` assumption is break. It's property no longer
-holds.
+We want to keep `_` polymorphic, so it behaves more like a named wildcard.
+
+NB. Should we default `_` if XNoPolyKind is on?
 -}
 
 {- Note [TyVarTv]


=====================================
compiler/GHC/Tc/Utils/Unify.hs
=====================================
@@ -2575,11 +2575,13 @@ lhsPriority tv
              TyVarTv        -> 1
              ConcreteTv {}  -> 2
              TauTv          -> 3
-             RuntimeUnkTv   -> 4
-             NoDefTauTv     -> 5 -- ^ when unifying with other, prefer to keep the other.
-                                 -- see Note [NoDefTauTv] for more details.
-                                -- NB. it is not clear to me(soulomoon), RuntimeUnkTv should
-                                -- be higher than NoDefTauTv or not.
+             NoDefTauTv     -> 4 -- ^ when unifying with other, prefer to keep the other.
+                                 -- Since NoDefTauTv is more polymorphic than tau,
+                                 --  see Note [NoDefTauTv] for more details.
+
+                                 -- NB. it is not clear to me(soulomoon), RuntimeUnkTv should
+                                 -- be higher than NoDefTauTv or not.
+             RuntimeUnkTv   -> 5
 
 {- Note [Unification preconditions]
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~



View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/commit/668ca48567d0f7d5ba2de90f5919827f30e19c86

-- 
View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/commit/668ca48567d0f7d5ba2de90f5919827f30e19c86
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