• Simon Peyton Jones's avatar
    Allow foralls in instance decls · 2cf3cac6
    Simon Peyton Jones authored
    This patch finally makes it possible to have explicit
    foralls in an instance decl
       instance forall (a :: *). Eq a => Eq [a] where ...
    
    This is useful to allow kind signatures or indeed
    explicicit kind for-alls; see Trac #11519
    
    I thought it would be really easy, because an instance
    declaration already contains an actual HsSigType, so all
    the syntactic baggage is there.  But in fact it turned
    out that instance declarations were kind-checked a
    little differently, because the body kind of the forall
    is 'Constraint' rather than '*'.
    
    So I fixed that.  There a slight kludge
    (see Note [Body kind of a HsQualTy], but it's still a
    significant improvement.
    
    I also did the usual other round of refactoring,
    improved a few error messages, tidied up comments etc.
    The only significant aspect of all that was
    
      * Kill mkNakedSpecSigmaTy, mkNakedPhiTy, mkNakedFunTy
        These function names suggest that they do something
        complicated, but acutally they do nothing. So I
        killed them.
    
      * Swap the arg order of mkNamedBinder, just so that it is
        convenient to say 'map (mkNamedBinder Invisible) tvs'
    
      * I had to improve isPredTy, to deal with (illegal)
        types like
           (Eq a => Eq [a]) => blah
        See Note [isPeredTy complications] in Type.hs
    
    Still to come: user manual documentation for the
    instance-decl change.
    2cf3cac6
HsUtils.hs 45.8 KB