One thing slightly non-trivial issue that I've run into on this is that I seem to walk into holes while printing with the IfaceType printer which
#11660: Remove Type pretty-printer in favor of IfaceType -------------------------------------+------------------------------------- Reporter: bgamari | Owner: Type: task | Status: patch Priority: high | Milestone: 8.2.1 Component: Compiler | Version: 7.10.3 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: #12550, #12447, | Differential Rev(s): Phab:D2528 #11786, #11549, #12024, #12697, | #12510 | Wiki Page: | -------------------------------------+------------------------------------- Comment (by simonpj): previously we avoided. For instance, `tcfail123` fails The typechecker is trying to unify a lifted type with an unlifted one. So it produces {{{ [W] ax1 :: TYPE IntRep ~ TYPE LiftedPtrRep }}} which of course it can't solve. But rather than print that obscure message, it tries to print the offending type, which looks like {{{ Int# |> (hole{ax1}) }}} where `hole{ax1}` is the un-proven assertion above. The type printer suppresses the coercion (though you can see it with current GHC using `-fprint-explicit-coercions`). So for this and debugging reasons we do want to be able to print types with coercion holes in them, although they should never end up in interface files. So let's add `IfaceHoleProv` to `IfaceUnivCoProv`; and make `toIfaceType` use it. But serialisation can barf. Clearly this will need a Note, taken from this comment -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11660#comment:15 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler