#14628: Panic (No skolem Info) in GHCi -------------------------------------+------------------------------------- Reporter: AndreasK | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: GHCi | Version: 8.2.2 Resolution: | Keywords: debugger Operating System: Unknown/Multiple | Architecture: Type of failure: Compile-time | Unknown/Multiple crash or panic | Test Case: Blocked By: | Blocking: Related Tickets: #13393 | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by simonpj): I think it's unrelated to the fix for #13393. As you point out, this is debugger-land, and we have these `RuntimeUnk` skolems, which stand for as-yet-unknown types in the debugger. In this case `_result` has type {{{ _result :: StateT Int m_I4K0[rt] () }}} but the debugger can't figure out (by looking at the heap) what this `m_I4K0` type is. So when typechecking an expression involving `_result` we should complain if this `m` gets unified with anything. And it is when you try to evalute {{{ snd $ runStateT _result 0 }}} We get a wanted constraint {{{ [WD] hole{a4SP} :: (m_I4K0[rt] :: (* -> *)) ~# ((,) a_a4SI[tau:1] :: (* -> *)) }}} The trouble is that, in reporting the error, `TcErrors.getSkolemInfo` of course cannot find an enclosing implication constraint binding that `m`. What we should do instead is: * Make `getSkolemInfo` return a `SkolemInfo` rether than an `Implic` * If `getSkolemInfo` gets a `RuntimeUnk`, just return a new data constructor in `SkolemInfo`, perhpas `RuntimeUnkSkol`. * In `TcErrors.pprSkols` print something sensible for a `RuntimeUnkSkol` It'd be most helpful to look at the top-level type envt, and display some of the in-scope Ids that have that variale free in their types. The `relevantBindings` function might be good for finding such bindings. Any volunteers? -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14628#comment:2 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler