I propose the following clean-up to the handling of implicit parameters.
Currently they are special in all sorts of ways, notably by having their own
name-cache. See IfaceEnv.allocateIPName, and TysWiredIn.mkIPName. This is
pretty messy.
But consider: this gets very close
class IP (name :: Symbol) ty | name -> ty where
value :: ty
Instead of the constraint (?x::Int) we'd have (IP "x" Int). I'm not actually
suggesting changing the surface syntax, only the internal representation.
This would, at a stroke, get rid of much of the special handling of implicit
parameters. There are still some special rules about generalisation that we
might want to preserve, but by and large everything would be handled with much
less code for implicit parameters.
Any comments?
Iavor, you're an expert on implicit parameters, and you implemented the Symbol
stuff that the idea depends on. Might you even like to implement the above
plan?
Simon
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