diff options
| author | Simon Peyton Jones <simonpj@microsoft.com> | 2018-10-16 12:38:16 +0100 |
|---|---|---|
| committer | Simon Peyton Jones <simonpj@microsoft.com> | 2018-10-24 16:38:55 +0100 |
| commit | 0faf7fd3e6c652575af9993787f07cad86f452f6 (patch) | |
| tree | 029d5a626ac2677305151120cd4a0a4a144c6493 /compiler/deSugar | |
| parent | 321bc1a644a9e4598a4af30d4aeae315f0ff487a (diff) | |
| download | haskell-0faf7fd3e6c652575af9993787f07cad86f452f6.tar.gz | |
Refactor the treatment of predicate types
Trac #15648 showed that GHC was a bit confused about the
difference between the types for
* Predicates
* Coercions
* Evidence (in the typechecker constraint solver)
This patch cleans it up. See especially Type.hs
Note [Types for coercions, predicates, and evidence]
Particular changes
* Coercion types (a ~# b) and (a ~#R b) are not predicate types
(so isPredTy reports False for them) and are not implicitly
instantiated by the type checker. This is a real change, but
it consistently reflects that fact that (~#) and (~R#) really
are different from predicates.
* isCoercionType is renamed to isCoVarType
* During type inference, simplifyInfer, we do /not/ want to infer
a constraint (a ~# b), because that is no longer a predicate type.
So we 'lift' it to (a ~ b). See TcType
Note [Lift equality constaints when quantifying]
* During type inference for pattern synonyms, we need to 'lift'
provided constraints of type (a ~# b) to (a ~ b). See
Note [Equality evidence in pattern synonyms] in PatSyn
* But what about (forall a. Eq a => a ~# b)? Is that a
predicate type? No -- it does not have kind Constraint.
Is it an evidence type? Perhaps, but awkwardly so.
In the end I decided NOT to make it an evidence type,
and to ensure the the type inference engine never
meets it. This made me /simplify/ the code in
TcCanonical.makeSuperClasses; see TcCanonical
Note [Equality superclasses in quantified constraints]
Instead I moved the special treatment for primitive
equality to TcInteract.doTopReactOther. See TcInteract
Note [Looking up primitive equalities in quantified constraints]
Also see Note [Evidence for quantified constraints] in Type.
All this means I can have
isEvVarType ty = isCoVarType ty || isPredTy ty
which is nice.
All in all, rather a lot of work for a small refactoring,
but I think it's a real improvement.
Diffstat (limited to 'compiler/deSugar')
| -rw-r--r-- | compiler/deSugar/DsBinds.hs | 6 |
1 files changed, 3 insertions, 3 deletions
diff --git a/compiler/deSugar/DsBinds.hs b/compiler/deSugar/DsBinds.hs index 421adcaccd..f2ff5dd623 100644 --- a/compiler/deSugar/DsBinds.hs +++ b/compiler/deSugar/DsBinds.hs @@ -888,9 +888,9 @@ decomposeRuleLhs dflags orig_bndrs orig_lhs , text "Orig lhs:" <+> ppr orig_lhs , text "optimised lhs:" <+> ppr lhs2 ]) pp_bndr bndr - | isTyVar bndr = text "type variable" <+> quotes (ppr bndr) - | Just pred <- evVarPred_maybe bndr = text "constraint" <+> quotes (ppr pred) - | otherwise = text "variable" <+> quotes (ppr bndr) + | isTyVar bndr = text "type variable" <+> quotes (ppr bndr) + | isEvVar bndr = text "constraint" <+> quotes (ppr (varType bndr)) + | otherwise = text "variable" <+> quotes (ppr bndr) constructor_msg con = vcat [ text "A constructor," <+> ppr con <> |