- , is_orph :: Maybe OccName }
-
--- The "rough-match" fields
--- ~~~~~~~~~~~~~~~~~~~~~~~~~
--- The is_cls, is_args fields allow a "rough match" to be done
--- without poking inside the DFunId. Poking the DFunId forces
--- us to suck in all the type constructors etc it involves,
--- which is a total waste of time if it has no chance of matching
--- So the Name, [Maybe Name] fields allow us to say "definitely
--- does not match", based only on the Name.
---
--- In is_tcs,
--- Nothing means that this type arg is a type variable
---
--- (Just n) means that this type arg is a
--- TyConApp with a type constructor of n.
--- This is always a real tycon, never a synonym!
--- (Two different synonyms might match, but two
--- different real tycons can't.)
--- NB: newtypes are not transparent, though!
---
--- The "proper-match" fields
--- ~~~~~~~~~~~~~~~~~~~~~~~~~
--- The is_tvs, is_tys fields are simply cahced values, pulled
--- out (lazily) from the dfun id. They are cached here simply so
--- that we don't need to decompose the DFunId each time we want
--- to match it. The hope is that the fast-match fields mean
--- that we often never poke th proper-match fields
---
--- However, note that:
--- * is_tvs must be a superset of the free vars of is_tys
---
--- * The is_dfun must itself be quantified over exactly is_tvs
--- (This is so that we can use the matching substitution to
--- instantiate the dfun's context.)
---
--- The "orphan" field
--- ~~~~~~~~~~~~~~~~~~
--- An instance is an orphan if its head (after the =>) mentions
--- nothing defined in this module.
---
--- Just n The head mentions n, which is defined in this module
--- This is used for versioning; the instance decl is
--- considered part of the defn of n when computing versions
---
--- Nothing The head mentions nothing defined in this module
---
--- If a module contains any orphans, then its interface file is read
--- regardless, so that its instances are not missed.
---
--- Functional dependencies worsen the situation a bit. Consider
--- class C a b | a -> b
--- In some other module we might have
--- module M where
--- data T = ...
--- instance C Int T where ...
--- This isn't considered an orphan, so we will only read M's interface
--- if something from M is used (e.g. T). So there's a risk we'll
--- miss the improvement from the instance. Workaround: import M.