#14288: ScopedTypeVariables with nested foralls broken since 8.0.2 -------------------------------------+------------------------------------- Reporter: MikolajKonarski | Owner: RyanGlScott Type: bug | Status: new Priority: normal | Milestone: 8.4.1 Component: Compiler (Type | Version: 8.2.1 checker) | Resolution: | Keywords: Operating System: Linux | Architecture: x86_64 Type of failure: GHC rejects | (amd64) valid program | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by RyanGlScott): Hm. Implementing the renaming part of this patch is a breeze, but modifying the typechecker is proving to be a pain. The source of my headaches can be traced down to this [http://git.haskell.org/ghc.git/blob/0e9681268a38cbc15c9c2b50979624732c9077ce... tcInstType] function: {{{#!hs tcInstType :: ([TyVar] -> TcM (TCvSubst, [TcTyVar])) -- ^ How to instantiate the type variables -> Id -- ^ Type to instantiate -> TcM ([(Name, TcTyVar)], TcThetaType, TcType) -- ^ Result -- (type vars, preds (incl equalities), rho) }}} The problem is that at the moment, the first part of the triple that `tcInstType` returns corresponds to the lexically scoped type variables. But there's an assumption that these variables will all appear consecutively, uninterrupted by a `TcThetaType`. With the proposed changes in this ticket, this assumption will no longer hold true. But I can't just return more type variables in the first part of the triple, because almost all call sites of `tcInstType` reconstruct the instantiated type with something like `forall <tvs>. <theta-type> => <tc- type>`. In other words, cramming more type variables would cause the wrong types to be constructed! So `tcInstType` just seems to be designed the wrong way for what I want to accomplish, but I can't figure out how to redesign it sensibly... -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14288#comment:18 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler