• Simon Peyton Jones's avatar
    Add AnonArgFlag to FunTy · 6cce36f8
    Simon Peyton Jones authored
    The big payload of this patch is:
      Add an AnonArgFlag to the FunTy constructor
      of Type, so that
        (FunTy VisArg   t1 t2) means (t1 -> t2)
        (FunTy InvisArg t1 t2) means (t1 => t2)
    The big payoff is that we have a simple, local test to make
    when decomposing a type, leading to many fewer calls to
    isPredTy. To me the code seems a lot tidier, and probably
    more efficient (isPredTy has to take the kind of the type).
    See Note [Function types] in TyCoRep.
    There are lots of consequences
    * I made FunTy into a record, so that it'll be easier
      when we add a linearity field, something that is coming
      down the road.
    * Lots of code gets touched in a routine way, simply because it
      pattern matches on FunTy.
    * I wanted to make a pattern synonym for (FunTy2 arg res), which
      picks out just the argument and result type from the record. But
      alas the pattern-match overlap checker has a heart attack, and
      either reports false positives, or takes too long.  In the end
      I gave up on pattern synonyms.
      There's some commented-out code in TyCoRep that shows what I
      wanted to do.
    * Much more clarity about predicate types, constraint types
      and (in particular) equality constraints in kinds.  See TyCoRep
      Note [Types for coercions, predicates, and evidence]
      and Note [Constraints in kinds].
      This made me realise that we need an AnonArgFlag on
      AnonTCB in a TyConBinder, something that was really plain
      wrong before. See TyCon Note [AnonTCB InivsArg]
    * When building function types we must know whether we
      need VisArg (mkVisFunTy) or InvisArg (mkInvisFunTy).
      This turned out to be pretty easy in practice.
    * Pretty-printing of types, esp in IfaceType, gets
      tidier, because we were already recording the (->)
      vs (=>) distinction in an ad-hoc way.  Death to
    * mkLamType needs to keep track of whether it is building
      (t1 -> t2) or (t1 => t2).  See Type
      Note [mkLamType: dictionary arguments]
    Other minor stuff
    * Some tidy-up in validity checking involving constraints;
      Trac #16263
HsUtils.hs 56.7 KB