#12549: Panic on ":t datatypeName" ---------------------------------+-------------------------------------- Reporter: johnleo | Owner: johnleo Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.1 Resolution: | Keywords: Operating System: MacOS X | Architecture: x86_64 (amd64) Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | ---------------------------------+-------------------------------------- Comment (by johnleo): Thank you, that is indeed very helpful. I agree that comment in `newMetaTyVars` is very likely incorrect and the source of bugs. However I believe the panic that occurs in this case is actually a different problem. First note that the actual substitutions of the kinds that results in `∀ (ba ∷ kj) (cb ∷ kk). al ba cb` happens earlier, on the line {{{ ; (wrap2, rho2) <- deeplyInstantiate orig (substTyUnchecked subst rho) }}} of `deeplyInstantiate`. Note that the substitution was already unchecked there, likely due to the problem you mention (as well as perhaps other problems). The panic in the line {{{ kind = substTy subst (tyVarKind tv) }}} that happens later seems unrelated to the substitution invariant. At this point the kinds have already been substituted, so it is trying to do the substitution `kk -> kk`, which in fact satisfies the invariant. The code line above is very suspicious to me--I'm not sure why any substitution for the kind is being attempted. Why not just `kind = tyVarKind tv`? If all kinds must be specified before the types that use them (and I assume that is checked somewhere earlier) then the kind substitution must have already been happened. The problem seems to be that `subst` here is the substitution for the type variable, but we pass in the kind variable as the second argument to `substTy` which confuses `checkValidSubst`. This can be seen from the values printed out by the assert (here with the full names). {{{ in_scope InScope {k_a12j b_a12p} tenv [a11a :-> b_a12p[tau:2]] cenv [] tys [k_a12k[tau:2]] cos [] needInScope [a12k :-> k_a12k[tau:2]] }}} The substitution being attempted is `kk -> kk` but `tenv` contains the `ba -> bp` substitution. So in the lines {{{ needInScope = (tyCoVarsOfTypes tys `unionVarSet` tyCoVarsOfCos cos) `delListFromUFM_Directly` substDomain }}} of `checkValidSubst` the `substDomain` (of `tenv`) is getting removed from `needInScope` but it is the wrong domain--it should be `kk`, not `ba`. Anyway it seems there seem to be a lot of problems with this code, and it's certainly worth going through and cleaning up, but again I'd appreciate some perspective to make sure I'm not breaking more than I fix. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12549#comment:18 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler