April 2017 - ghc-tickets

[GHC] #13430: Can't scope type variables when pattern matching on GADTs
by GHC 27 Jul '17

27 Jul '17

#13430: Can't scope type variables when pattern matching on GADTs -------------------------------------+------------------------------------- Reporter: crockeea | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.0.2 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: GHC rejects Unknown/Multiple | valid program Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- I was very surprised that the following doesn't compile: {{{ {-# LANGUAGE GADTs, ScopedTypeVariables #-} data Foo x where A :: Foo Bool B :: Foo Int foo :: Foo x -> () foo (A :: Foo Bool) = () }}} GHC says that it couldn't match the type `y` with `Bool`. See [this question](http://stackoverflow.com/questions/42824949/type-inference-with- gadts) for a simple motivating example. As someone there pointed out, the situation would be improved with type application (#11350), but this ticket is about bringing the output type in scope with `ScopedTypeVariables`. This most closely reminds me of #1823 (!), but that was resolved many moons ago. This behavior also exists in 7.10.3. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13430> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #12369: data families shouldn't be required to have return kind *, data instances should
by GHC 27 Jul '17

27 Jul '17

#12369: data families shouldn't be required to have return kind *, data instances should -------------------------------------+------------------------------------- Reporter: ekmett | Owner: Type: feature | Status: new request | Priority: normal | Milestone: 8.2.1 Component: Compiler | Version: 8.0.1 (Type checker) | Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- I'd like to be able to define {{{#!hs {-# language PolyKinds, KindSignatures, GADTs, TypeFamilies #-} data family Fix :: (k -> *) -> k newtype instance Fix f = In { out :: f (Fix f) } }}} But currently this is disallowed: {{{ Fix.hs:2:1: error: • Kind signature on data type declaration has non-* return kind (k -> *) -> k • In the data family declaration for ‘Fix’ }}} Ultimately I think the issue here is that data __instances__ should be required to end in kind *, not the families, but this generalization didn't mean anything until we had `PolyKinds`. As an example of a usecase, with the above, I could define a bifunctor fixed point such as {{{#!hs newtype instance Fix f a = In2 { out2 :: f (Fix f a) a } }}} -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12369> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #13610: Unhelpful error messages about lifted and unlifted types
by GHC 27 Jul '17

27 Jul '17

#13610: Unhelpful error messages about lifted and unlifted types -------------------------------------+------------------------------------- Reporter: nomeata | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.1 (Type checker) | Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- I wrote this code: {{{ {-# LANGUAGE MagicHash #-} import GHC.Prim import GHC.Types main = do let primDouble = 0.42## :: Double# let double = 0.42 :: Double IO (\s -> mkWeakNoFinalizer# double () s) }}} and I get this error message: {{{ WeakDouble.hs:8:15: error: • Couldn't match a lifted type with an unlifted type Expected type: (# State# RealWorld, Weak# () #) Actual type: (# State# RealWorld, Weak# () #) • In the expression: mkWeakNoFinalizer# double () s In the first argument of ‘IO’, namely ‘(\ s -> mkWeakNoFinalizer# double () s)’ In a stmt of a 'do' block: IO (\ s -> mkWeakNoFinalizer# double () s) }}} with `-fprint-explicit-kinds`. (Without the flag, it looks the same, but tells me to use this flag…). -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13610> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #11560: panic: isInjectiveTyCon sees a TcTyCon
by GHC 27 Jul '17

27 Jul '17

#11560: panic: isInjectiveTyCon sees a TcTyCon -------------------------------------+------------------------------------- Reporter: rwbarton | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.1 (Type checker) | Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: Compile-time Unknown/Multiple | crash Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- I typed this into ghci and it panicked. Not sure whether the declaration is really valid anyways, but ghci shouldn't panic on it. {{{ rwbarton@morphism:~/ghc-newest$ ./inplace/bin/ghc-stage2 --interactive GHCi, version 8.1.20160201: http://www.haskell.org/ghc/ :? for help Prelude> :set -XTypeFamilies -XTypeInType Prelude> :m +Data.Kind Prelude Data.Kind> type family T (l :: *) (k :: l) :: * where { T * Int = * } ghc-stage2: panic! (the 'impossible' happened) (GHC version 8.1.20160201 for x86_64-unknown-linux): isInjectiveTyCon sees a TcTyCon T Please report this as a GHC bug: http://www.haskell.org/ghc/reportabug }}} -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11560> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #11672: Poor error message
by GHC 27 Jul '17

27 Jul '17

#11672: Poor error message -------------------------------------+------------------------------------- Reporter: adamgundry | Owner: adamgundry Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.0.1-rc2 (Type checker) | Keywords: ErrorMessages | Operating System: Unknown/Multiple Architecture: | Type of failure: Incorrect Unknown/Multiple | warning at compile-time Test Case: | Blocked By: Blocking: | Related Tickets: #11198 Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- [https://mail.haskell.org/pipermail/haskell-cafe/2016-February/123262.html Daniel Díaz recently pointed out] a particularly terrible error message. Here's a reduced example: {{{#!hs {-# LANGUAGE DataKinds #-} {-# LANGUAGE KindSignatures #-} module BadError where import GHC.TypeLits import Data.Proxy f :: Proxy (a :: Symbol) -> Int f _ = f (Proxy :: Proxy (Int -> Bool)) }}} With GHC 8.0 RC2, this leads to the following error: {{{ • Expected kind ‘Proxy ((->) Int Bool)’, but ‘Data.Proxy.Proxy :: Proxy (Int -> Bool)’ has kind ‘Proxy (Int -> Bool)’ • In the first argument of ‘f’, namely ‘(Proxy :: Proxy (Int -> Bool))’ In the expression: f (Proxy :: Proxy (Int -> Bool)) In an equation for ‘f’: f _ = f (Proxy :: Proxy (Int -> Bool)) }}} or with `-fprint-explicit-kinds -fprint-explicit-coercions`: {{{ • Expected kind ‘Proxy Symbol (((->) |> <*>_N -> <*>_N -> U(hole:{aCy}, *, Symbol)_N) Int Bool)’, but ‘(Data.Proxy.Proxy) @ k_aCv @ t_aCw :: Proxy (Int -> Bool)’ has kind ‘Proxy * (Int -> Bool)’ }}} As Iavor, Richard and I discussed, this message has at least three separate problems: * It says `kind` when it should say `type`. * `((->) Int Bool)` is printed instead of `Int -> Bool` (because there is a coercion hiding in the type). * The real error is the insoluble constraint `Symbol ~ *`, which is not reported at all! The first two should be fairly easy to fix. For the third, when reporting insoluble constraints, we should prefer to report those on which no other constraints depend. (In this case, the presence of `hole:{aCy}` in the constraint is an explicit dependency on the other constraint.) I'll try to take a look at this. It is no doubt related to #11198. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11672> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #13530: Horrible error message due to TypeInType
by GHC 27 Jul '17

27 Jul '17

#13530: Horrible error message due to TypeInType -------------------------------------+------------------------------------- Reporter: simonpj | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.0.1 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- Consider this {{{ {-# LANGUAGE MagicHash, UnboxedTuples #-} module Foo where import GHC.Exts g :: Int -> (# Int#, a #) g (I# y) = (# y, undefined #) f :: Int -> (# Int#, Int# #) f x = g x }}} With GHC 8 we get {{{ Foo.hs:11:7: error: • Couldn't match a lifted type with an unlifted type Expected type: (# Int#, Int# #) Actual type: (# Int#, Int# #) }}} What a terrible error message!! It was much better in GHC 7.10: {{{ Foo.hs:11:7: Couldn't match kind ‘*’ with ‘#’ When matching types a0 :: * Int# :: # Expected type: (# Int#, Int# #) Actual type: (# Int#, a0 #) }}} What's going on? The constraint solver sees {{{ [W] alpha::TYPE LiftedRep ~ Int#::TYPE IntRep }}} So it homogenises the kinds, ''and unifies alpha'' (this did not happen in GHC 7.10), thus {{{ alpha := Int# |> TYPE co [W] co :: LiftedRep ~ IntRep }}} Of course the new constraint fails. But since we have unified alpha, when we print out the types are are unifying they both look like `(# Int#, Int# #)` (there's a suppressed cast in the second component). I'm not sure what to do here. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13530> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #12373: Type error but types match
by GHC 27 Jul '17

27 Jul '17

#12373: Type error but types match -------------------------------------+------------------------------------- Reporter: osa1 | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.1 (Type checker) | Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- {{{ > unboxedsums git:(prim_sums_rebase_5) x cat primop_bug.hs {-# LANGUAGE MagicHash, ScopedTypeVariables, UnboxedTuples #-} module Main where import GHC.MVar import GHC.Prim import GHC.Types main :: IO () main = IO (\rw -> newMVar# rw) >> return () > unboxedsums git:(prim_sums_rebase_5) x ghc-stage1 primop_bug.hs -ddump- stg -ddump-cmm -ddump-to-file -fforce-recomp -dumpdir primop_fails -O -fprint-explicit-kinds [1 of 1] Compiling Main ( primop_bug.hs, primop_bug.o ) primop_bug.hs:10:19: error: • Couldn't match a lifted type with an unlifted type Expected type: (# State# RealWorld, MVar# RealWorld a0 #) Actual type: (# State# RealWorld, MVar# RealWorld a0 #) • In the expression: newMVar# rw In the first argument of ‘IO’, namely ‘(\ rw -> newMVar# rw)’ In the first argument of ‘(>>)’, namely ‘IO (\ rw -> newMVar# rw)’ }}} Tried with HEAD, 8.0.1. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12373> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #12801: Misleading error message when deriving Functor
by GHC 25 Jul '17

25 Jul '17

#12801: Misleading error message when deriving Functor -------------------------------------+------------------------------------- Reporter: erikd | Owner: Type: bug | Status: new Priority: normal | Milestone: 8.2.1 Component: Compiler | Version: 8.0.1 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: Poor/confusing Unknown/Multiple | error message Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- Compiling this trivial demo program: {{{ data Container = Container [Wibble Int] deriving (Eq, Show) data Wibble a = Wibble a | Wobble deriving (Eq, Functor, Show) }}} results in the error message: {{{ a.hs:6:13: error: • No instance for (Eq (Wibble Int)) arising from the first field of ‘Container’ (type ‘[Wibble Int]’) Possible fix: use a standalone 'deriving instance' declaration, so you can specify the instance context yourself • When deriving the instance for (Eq Container) a.hs:6:17: error: • No instance for (Show (Wibble Int)) arising from the first field of ‘Container’ (type ‘[Wibble Int]’) Possible fix: use a standalone 'deriving instance' declaration, so you can specify the instance context yourself • When deriving the instance for (Show Container) a.hs:12:17: error: • Can't make a derived instance of ‘Functor Wibble’: You need DeriveFunctor to derive an instance for this class • In the data declaration for ‘Wibble’ }}} The "No instance for (Eq|Show)" are rather misleading, because the *actual* error is that the `{-# LANGUAGE DeriveFunctor #-}` pragma is missing. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12801> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #10684: Error cascade when unrelated class derivation fails
by GHC 25 Jul '17

25 Jul '17

#10684: Error cascade when unrelated class derivation fails -------------------------------------+------------------------------------- Reporter: ezyang | Owner: Type: bug | Status: new Priority: low | Milestone: Component: Compiler | Version: 7.11 (Type checker) | Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Revisions: | -------------------------------------+------------------------------------- Pretty minor bug, but I thought I'd point it out: {{{ module A where import Data.Typeable data A = A deriving (Show, Typeable) data B = B A deriving (Show) }}} I get two errors when I expect one: {{{ A.hs:3:28: Can't make a derived instance of `Typeable A': You need -XDeriveDataTypeable to derive an instance for this class In the data declaration for `A' A.hs:4:24: No instance for (Show A) arising from the 'deriving' clause of a data type declaration Possible fix: add an instance declaration for (Show A) or use a standalone 'deriving instance' declaration, so you can specify the instance context yourself When deriving the instance for (Show B) }}} The failed Typeable derivation also causes the Show derivation to fail, which causes an error later in the file. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10684> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #13168: Ambiguous interface errors in GHCi, even with -XPackageImports
by GHC 24 Jul '17

24 Jul '17

#13168: Ambiguous interface errors in GHCi, even with -XPackageImports -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: GHCi | Version: 8.0.2 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: GHC rejects Unknown/Multiple | valid program Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- Originally reported in https://github.com/haskell/cabal/issues/4253, but I moved it here after being reassured that this was a GHCi bug, not a `Cabal` one. To reproduce this issue, you'll need to install two libraries that export two constructors with the same name. The original issue uses `GLFW` and `GLFW-b` as examples, but for the puposes of test case minimalization, I've prepared a repo here (https://github.com/RyanGlScott/cabal-gh4253) with as small of an example as I can manage. To reproduce the issue, do the following: {{{ $ git clone https://github.com/RyanGlScott/cabal-gh4253 $ cd cabal-gh4253/ $ cabal install package1/ package2/ $ ghci -XPackageImports GHCi, version 8.0.2: http://www.haskell.org/ghc/ :? for help Loaded GHCi configuration from /home/rgscott/.ghci λ> import "package1" DuplicateModuleName λ> Window <interactive>:1:1: error: Ambiguous interface for ‘DuplicateModuleName’: it was found in multiple packages: package1-0.1.0.0 package2-0.1.0.0 }}} This seems to be specific to GHCi, because I cannot find a way to trigger the same issue with `ghc --make`. I've reproduced this with GHC 8.0.2 and HEAD. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13168> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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