• Simon Peyton Jones's avatar
    Refactor the treatment of predicate types · 0faf7fd3
    Simon Peyton Jones authored
    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.
TcErrors.hs 127 KB