#13610: Unhelpful error messages about lifted and unlifted types -------------------------------------+------------------------------------- Reporter: nomeata | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler (Type | Version: 8.1 checker) | Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by nomeata): Ah, the problem is that `IO`’s type expects the second component of the tuple to be lifted, but `Weak#` is not. So the code is indeed bogus, but the error message is not very helpful. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13610#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler

#13610: Unhelpful error messages about lifted and unlifted types -------------------------------------+------------------------------------- Reporter: nomeata | Owner: (none) Type: bug | Status: new Priority: low | Milestone: Component: Compiler (Type | Version: 8.1 checker) | Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Changes (by nomeata): * priority: normal => low Comment: I am not sure if printing type prefixes would have improved manners. What would it say? Maybe {{{ Expected type: (#,#) VoidRep LiftedRep (State# RealWorld) (Weak# ()) Actual type: (#,#) VoidRep PrimRep (State# RealWorld) (Weak# ()) }}} But the “actual type” is not well-kinded; it cannot really be the “actual type” of anything. So it seems that something went wrong earlier during type inference? I guess something along these lines would have been more helpful: {{{ WeakDouble.hs:8:15: error: • Couldn't match a lifted type with an unlifted type Expected type representation: LiftedRep Actual type representation: PrimRep • In the second element of the unlifted tuple • In the type of the expression: mkWeakNoFinalizer# double () s }}} Anyways; this was not triggered by writing real code, so I’ll lower the priority. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13610#comment:3 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler

#13610: Unhelpful error messages about lifted and unlifted types -------------------------------------+------------------------------------- Reporter: nomeata | Owner: (none) Type: bug | Status: new Priority: low | Milestone: 8.2.1 Component: Compiler (Type | Version: 8.1 checker) | Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Changes (by bgamari): * milestone: => 8.2.1 -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13610#comment:5 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler

#13610: Unhelpful error messages about lifted and unlifted types -------------------------------------+------------------------------------- Reporter: nomeata | Owner: (none) Type: bug | Status: new Priority: low | Milestone: 8.4.1 Component: Compiler (Type | Version: 8.1 checker) | Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by Richard Eisenberg ): In [changeset:"8e15e3d370e9c253ae0dbb330e25b72cb00cdb76/ghc" 8e15e3d3/ghc]: {{{ #!CommitTicketReference repository="ghc" revision="8e15e3d370e9c253ae0dbb330e25b72cb00cdb76" Improve error messages around kind mismatches. Previously, when canonicalizing (or unifying, in uType) a heterogeneous equality, we emitted a kind equality and used the resulting coercion to cast one side of the heterogeneous equality. While sound, this led to terrible error messages. (See the bugs listed below.) The problem is that using the coercion built from the emitted kind equality is a bit like a wanted rewriting a wanted. The solution is to keep heterogeneous equalities as irreducible. See Note [Equalities with incompatible kinds] in TcCanonical. This commit also removes a highly suspicious switch to FM_SubstOnly when flattening in the kinds of a type variable. I have no idea why this was there, other than as a holdover from pre-TypeInType. I've not left a Note because there is simply no reason I can conceive of that the FM_SubstOnly should be there. One challenge with this patch is that the emitted derived equalities might get emitted several times: when a heterogeneous equality is in an implication and then gets floated out from the implication, the Derived is present both in and out of the implication. This causes a duplicate error message. (Test case: typecheck/should_fail/T7368) Solution: track the provenance of Derived constraints and refuse to float out a constraint that has an insoluble Derived. Lastly, this labels one test (dependent/should_fail/RAE_T32a) as expect_broken, because the problem is really #12919. The different handling of constraints in this patch exposes the error. This fixes bugs #11198, #12373, #13530, and #13610. test cases: typecheck/should_fail/{T8262,T8603,tcail122,T12373,T13530,T13610} }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13610#comment:7 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler