[Git][ghc/ghc][wip/T23070-unify] Wibble

[Simon Peyton Jones (@simonpj)]

Simon Peyton Jones pushed to branch wip/T23070-unify at Glasgow Haskell Compiler / GHC


Commits:
525f107c by Simon Peyton Jones at 2023-04-28T18:07:46+01:00
Wibble

- - - - -


2 changed files:

- compiler/GHC/Tc/Solver/Monad.hs
- compiler/GHC/Tc/Types/Constraint.hs


Changes:

=====================================
compiler/GHC/Tc/Solver/Monad.hs
=====================================
@@ -1967,11 +1967,14 @@ Note [wrapUnifierTcS]
 When decomposing equalities we often create new wanted constraints for
 (s ~ t).  But what if s=t?  Then it'd be faster to return Refl right away.
 
-Rather than making an equality test (which traverses the structure of
-the type, perhaps fruitlessly), we call uType (via wrapUnifierTcS) to
-traverse the common structure, and bales out when it finds a
-difference by creating a new Wanted constraint.  But where it succeeds
-in finding common structure, it just builds a coercion to reflect it.
+Rather than making an equality test (which traverses the structure of the type,
+perhaps fruitlessly), we call uType (via wrapUnifierTcS) to traverse the common
+structure, and bales out when it finds a difference by creating a new deferred
+Wanted constraint.  But where it succeeds in finding common structure, it just
+builds a coercion to reflect it.
+
+This is all much faster than creating a new constraint, putting it in the
+work list, picking it out, canonicalising it, etc etc.
 
 Note [unifyFunDeps]
 ~~~~~~~~~~~~~~~~~~~
@@ -2003,14 +2006,14 @@ unifyFunDeps ev role do_unifications
 wrapUnifierTcS :: CtEvidence -> Role
                -> (UnifyEnv -> TcM a)  -- Some calls to uType
                -> TcS (a, Bag Ct, [TcTyVar])
--- Return coercions witnessing the equality of the two types,
--- emitting new work equalities where necessary to achieve that
+-- Invokes the do_unifications argument, with a suitable UnifyEnv.
+-- Emit deferred equalities and kick-out from the inert set as a
+-- result of any unifications.
 -- Very good short-cut when the two types are equal, or nearly so
 -- See Note [wrapUnifierTcS]
--- The returned coercion's role matches the input parameter
 --
--- The [TcTyVar] is the list of unification variables
--- that were unified the process.
+-- The [TcTyVar] is the list of unification variables that were
+-- unified the process; the (Bag Ct) are the deferred constraints.
 
 wrapUnifierTcS ev role do_unifications
   = do { (cos, unified, rewriters, cts) <- wrapTcS $


=====================================
compiler/GHC/Tc/Types/Constraint.hs
=====================================
@@ -247,7 +247,7 @@ and that can be used to rewrite other constrtaints. It satisfies these invariant
   * (TyEq:U) An EqCt is not immediately unifiable. If we can unify a:=ty, we
     will not form an EqCt (a ~ ty).
   * (TyEq:CH) rhs does not mention any coercion holes that resulted from fixing up
-    a hetero-kinded equality.  See Note [Equalites with incompatible kinds] in
+    a hetero-kinded equality.  See Note [Equalities with incompatible kinds] in
     GHC.Tc.Solver.Equality
 
 These invariants ensure that the EqCts in inert_eqs constitute a terminating



View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/commit/525f107c99eff62938f216973f5253d87f3dc57b

-- 
View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/commit/525f107c99eff62938f216973f5253d87f3dc57b
You're receiving this email because of your account on gitlab.haskell.org.


-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.haskell.org/pipermail/ghc-commits/attachments/20230428/e65344bf/attachment-0001.html>