• Simon Peyton Jones's avatar
    Refactor free tyvars on LHS of rules · 6ad2b42f
    Simon Peyton Jones authored
    A RULE can have unbound meta-tyvars on the LHS.  Consider
      data T a = C
      foo :: T a -> Int
      foo C = 1
      {-# RULES "myrule"  foo C = 1 #-}
    After type checking the LHS becomes (foo alpha (C alpah)) and we do
    not want to zap the unbound meta-tyvar 'alpha' to Any, because that
    limits the applicability of the rule.  Instead, we want to quantify
    over it!
    Previously there was a rather clunky implementation of this
    quantification, buried in the zonker in TcHsSyn (zonkTvCollecting).
    This patch refactors it so that the zonker just turns the meta-tyvar
    into a skolem, and the desugarer adds the quantification.  See DsBinds
    Note [Free tyvars on rule LHS]. As it happened, the desugarer was
    already doing something similar for dictionaries. See DsBinds
    Note [Free dictionaries on rule LHS]
    No change in functionality, but less cruft.
DsBinds.hs 49.8 KB