November 2018 - ghc-tickets

[GHC] #9173: Better type error messages
by GHC 27 Jan '19

27 Jan '19

#9173: Better type error messages ------------------------------------+------------------------------------- Reporter: simonpj | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.8.2 Keywords: | Operating System: Unknown/Multiple Architecture: Unknown/Multiple | Type of failure: None/Unknown Difficulty: Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | ------------------------------------+------------------------------------- Generating better type-error messages is a hoary chestnut, but Herbert brought my attention to [https://pay.reddit.com/r/haskell/comments/26tcrk/curious_with_a_bit_of_begi… this thread on reddit], which has the following idea. At the moment, from {{{ module Foo where addThree = \x -> x + 3 :: Int y = addThree $ Just 5 }}} we get this error: {{{ Foo.hs:2:20 Couldn't match expected type `Int' with actual type `Maybe a0' In the return type of a call of `Just' In the second argument of `($)', namely `Just 5' In the expression: addThree $ Just 5 }}} Maybe we could generate this instead {{{ Foo.hs:2:20 inferred: "Just 5" has type "Maybe a0" expected: second argument of "($)" must have type "Int" in the expression: addThree $ Just 5 }}} -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9173> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #15621: Error message involving type families points to wrong location
by GHC 26 Jan '19

26 Jan '19

#15621: Error message involving type families points to wrong location -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: 8.6.1 Component: Compiler | Version: 8.4.3 (Type checker) | Keywords: TypeFamilies | 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: -------------------------------------+------------------------------------- Consider the following program: {{{#!hs {-# LANGUAGE TypeFamilies #-} {-# LANGUAGE TypeOperators #-} module Bug where import Data.Type.Equality type family F a f :: () f = let a :: F Int :~: F Int a = Refl b :: F Int :~: F Bool b = Refl in () }}} This doesn't typecheck, which isn't surprising, since `F Int` doesn't equal `F Bool` in the definition of `b`. What //is// surprising is where the error message points to: {{{ $ /opt/ghc/8.4.3/bin/ghc Bug.hs [1 of 1] Compiling Bug ( Bug.hs, Bug.o ) Bug.hs:12:11: error: • Couldn't match type ‘F Int’ with ‘F Bool’ Expected type: F Int :~: F Int Actual type: F Bool :~: F Bool NB: ‘F’ is a non-injective type family • In the expression: Refl In an equation for ‘a’: a = Refl In the expression: let a :: F Int :~: F Int a = Refl b :: F Int :~: F Bool .... in () | 12 | a = Refl | ^^^^ }}} This claims that the error message arises from the definition of `a`, which is completely wrong! As evidence, if you comment out `b`, then the program typechecks again. Another interesting facet of this bug is that if you comment out `a`: {{{#!hs {-# LANGUAGE TypeFamilies #-} {-# LANGUAGE TypeOperators #-} module Bug where import Data.Type.Equality type family F a f :: () f = let {- a :: F Int :~: F Int a = Refl -} b :: F Int :~: F Bool b = Refl in () }}} Then the error message will actually point to `b` this time: {{{ $ /opt/ghc/8.4.3/bin/ghc Bug.hs [1 of 1] Compiling Bug ( Bug.hs, Bug.o ) Bug.hs:15:11: error: • Couldn't match type ‘F Int’ with ‘F Bool’ Expected type: F Int :~: F Bool Actual type: F Bool :~: F Bool NB: ‘F’ is a non-injective type family • In the expression: Refl In an equation for ‘b’: b = Refl In the expression: let b :: F Int :~: F Bool b = Refl in () | 15 | b = Refl | ^^^^ }}} The use of type families appears to be important to triggering this bug, since I can't observe this behavior without the use of `F`. This is a regression that was introduced at some point between GHC 8.0.2 and 8.2.2, since 8.2.2 gives the incorrect error message shown above, while 8.0.2 points to the right location: {{{ $ /opt/ghc/8.0.2/bin/ghc Bug.hs [1 of 1] Compiling Bug ( Bug.hs, Bug.o ) Bug.hs:15:11: error: • Couldn't match type ‘F Int’ with ‘F Bool’ Expected type: F Int :~: F Bool Actual type: F Int :~: F Int NB: ‘F’ is a type function, and may not be injective • In the expression: Refl In an equation for ‘b’: b = Refl In the expression: let a :: F Int :~: F Int a = Refl b :: F Int :~: F Bool .... in () }}} -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15621> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #15753: Inconsistent pattern-match warnings when using guards versus case expressions
by GHC 26 Jan '19

26 Jan '19

#15753: Inconsistent pattern-match warnings when using guards versus case expressions -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: 8.8.1 Component: Compiler | Version: 8.6.1 Keywords: | Operating System: Unknown/Multiple PatternMatchWarnings | Architecture: | Type of failure: Incorrect Unknown/Multiple | error/warning at compile-time Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- Consider the following code (apologies for the rather non-minimal example): {{{#!hs {-# LANGUAGE DataKinds #-} {-# LANGUAGE EmptyCase #-} {-# LANGUAGE GADTs #-} {-# LANGUAGE PolyKinds #-} {-# LANGUAGE RankNTypes #-} {-# LANGUAGE ScopedTypeVariables #-} {-# LANGUAGE TypeFamilies #-} {-# LANGUAGE TypeOperators #-} {-# LANGUAGE UndecidableInstances #-} {-# OPTIONS_GHC -Wall -Wno-unticked-promoted-constructors #-} module Bug where import Data.Kind import Data.Type.Bool import Data.Type.Equality ((:~:)(..)) import Data.Void data family Sing :: forall k. k -> Type data instance Sing :: Bool -> Type where SFalse :: Sing False STrue :: Sing True data instance Sing :: forall a. [a] -> Type where SNil :: Sing '[] SCons :: Sing x -> Sing xs -> Sing (x:xs) data instance Sing :: forall a b. (a, b) -> Type where STuple2 :: Sing x -> Sing y -> Sing '(x, y) newtype instance Sing (f :: k1 ~> k2) = SLambda { (@@) :: forall t. Sing t -> Sing (f @@ t) } data TyFun :: Type -> Type -> Type type a ~> b = TyFun a b -> Type infixr 0 ~> type family (f :: k1 ~> k2) @@ (x :: k1) :: k2 infixl 9 @@ newtype Map k v = MkMap [(k, v)] data instance Sing :: forall k v. Map k v -> Type where SMkMap :: Sing x -> Sing (MkMap x) type family MapEmpty :: Map k v where MapEmpty = MkMap '[] type family MapInsertWith (f :: v ~> v ~> v) (new_k :: k) (new_v :: v) (m :: Map k v) :: Map k v where MapInsertWith _ new_k new_v (MkMap '[]) = MkMap '[ '(new_k, new_v)] MapInsertWith f new_k new_v (MkMap ('(old_k,old_v):old_kvs)) = If (old_k == new_k) (MkMap ('(new_k,f @@ new_v @@ old_v):old_kvs)) (Case (MapInsertWith f new_k new_v (MkMap old_kvs)) old_k old_v) type family Case (m :: Map k v) (old_k :: k) (old_v :: v) :: Map k v where Case (MkMap kvs) old_k old_v = MkMap ('(old_k,old_v) : kvs) sMapInsertWith :: forall k v (f :: v ~> v ~> v) (new_k :: k) (new_v :: v) (m :: Map k v). SEq k => Sing f -> Sing new_k -> Sing new_v -> Sing m -> Sing (MapInsertWith f new_k new_v m) sMapInsertWith _ snew_k snew_v (SMkMap SNil) = SMkMap (STuple2 snew_k snew_v `SCons` SNil) sMapInsertWith sf snew_k snew_v (SMkMap (STuple2 sold_k sold_v `SCons` sold_kvs)) = case sold_k %== snew_k of STrue -> SMkMap (STuple2 snew_k (sf @@ snew_v @@ sold_v) `SCons` sold_kvs) SFalse -> case sMapInsertWith sf snew_k snew_v (SMkMap sold_kvs) of SMkMap skvs -> SMkMap (STuple2 sold_k sold_v `SCons` skvs) class PEq a where type (x :: a) == (y :: a) :: Bool class SEq a where (%==) :: forall (x :: a) (y :: a). Sing x -> Sing y -> Sing (x == y) mapInsertWithNonEmpty1 :: forall k v (f :: v ~> v ~> v) (old_k :: k) (old_v :: v) (old_kvs :: [(k,v)]) (new_k :: k) (new_v :: v) (m :: Map k v). SEq k => Sing f -> Sing new_k -> Sing new_v -> Sing m -> m :~: MkMap ('(old_k,old_v) : old_kvs) -> MapInsertWith f new_k new_v m :~: MapEmpty -> Void mapInsertWithNonEmpty1 sf snew_k snew_v (SMkMap sm) Refl Refl | STuple2 sold_k _ `SCons` sold_kvs <- sm , SFalse <- sold_k %== snew_k = case sMapInsertWith sf snew_k snew_v (SMkMap sold_kvs) of {} mapInsertWithNonEmpty2 :: forall k v (f :: v ~> v ~> v) (old_k :: k) (old_v :: v) (old_kvs :: [(k,v)]) (new_k :: k) (new_v :: v) (m :: Map k v). SEq k => Sing f -> Sing new_k -> Sing new_v -> Sing m -> m :~: MkMap ('(old_k,old_v) : old_kvs) -> MapInsertWith f new_k new_v m :~: MapEmpty -> Void mapInsertWithNonEmpty2 sf snew_k snew_v (SMkMap sm) Refl Refl | STuple2 sold_k _ `SCons` sold_kvs <- sm = case sold_k %== snew_k of SFalse -> case sMapInsertWith sf snew_k snew_v (SMkMap sold_kvs) of {} }}} If you compile this with GHC 8.6.1 or later, you'll notice something interesting happening with the pattern-match coverage checker warnings: {{{ $ /opt/ghc/8.6.1/bin/ghci Bug.hs GHCi, version 8.6.1: http://www.haskell.org/ghc/ :? for help Loaded GHCi configuration from /home/rgscott/.ghci [1 of 1] Compiling Bug ( Bug.hs, interpreted ) Bug.hs:78:1: warning: [-Wincomplete-patterns] Pattern match(es) are non-exhaustive In an equation for ‘mapInsertWithNonEmpty1’: Patterns not matched: _ _ _ (SMkMap _) Refl Refl | 78 | mapInsertWithNonEmpty1 sf snew_k snew_v (SMkMap sm) Refl Refl | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^... Ok, one module loaded. }}} `mapInsertWithNonEmpty1` is deemed non-exhaustive, but `mapInsertWithNonEmpty2` is deemed exhaustive. However, the code for the two functions are functionally identical—the only difference is that `mapInsertWithNonEmpty1` uses one more pattern guard than `mapInsertWithNonEmpty2` does. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15753> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #14259: Worker/Wrapper for sum return
by GHC 26 Jan '19

26 Jan '19

#14259: Worker/Wrapper for sum return -------------------------------------+------------------------------------- Reporter: jheek | Owner: (none) Type: feature | Status: new request | Priority: normal | Milestone: Component: Compiler | Version: 8.2.1 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: -------------------------------------+------------------------------------- GHC version 8.2 introduces Unboxed Sum types. It would be great if the worker/wrapper transformation could make use of this new functionality. For clarification I would expect a function like this: {{{#!haskell maybeEven :: Int -> Maybe Int maybeEven n = case even n of True -> Just n False -> Nothing }}} to be transformed into (the core equivalent) of {{{#!haskell maybeEven :: Int -> Maybe Int MaybeEven (I# n#) = case workerMaybeEven n# of (# | x #) -> Just x (# (# #) | #) -> Nothing workerMaybeEven :: Int# -> (# (# #) | Int #) {-# NOINLINE workerMaybeEven #-} workerMaybeEven n# = case even (I# n#) of True -> (# | I# n# #) False -> (# (# #) | #) }}} Currently the core output for the maybeEven worker is: {{{#!haskell Main.$wmaybeEven :: Int# -> Maybe Int Main.$wmaybeEven = \ (ww_s4WH :: Int#) -> case remInt# ww_s4WH 2# of { __DEFAULT -> GHC.Base.Nothing @ Int; 0# -> GHC.Base.Just @ Int (GHC.Types.I# ww_s4WH) } }}} -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14259> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #12364: Demand analysis for sum types
by GHC 25 Jan '19

25 Jan '19

#12364: Demand analysis for sum types -------------------------------------+------------------------------------- Reporter: nomeata | Owner: Type: task | 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: -------------------------------------+------------------------------------- While working on #10613 it crossed my mind that it might be worthwhile to expand the demand type also onto sum types. So instead of {{{ | UProd [ArgUse] -- Product }}} we would have {{{ | UData [[ArgUse]] -- Data type }}} and a function like {{{ fromMaybe :: a -> Maybe a -> b }}} would have a signature of {{{ <1*U><1*U(;1*U)> }}} which indicates that the second argument of `fromMaybe` is evaluated at most once; the first constructor of the result has no arguments, the second (separated by `;`) has one argument which is also used at most once. I could imagine that this gives a lot of useful information with parsers and other code that repeatedly retuns stuff wrapped in a `Maybe` or similar type. Now, sum types are often recursive (`[]`…), and we probably want to be smarter about recursion here. But note that this is not a new problem. If you write {{{ data Stream = Stream Int Stream Stream Stream foo (Stream 0 x y z) = 0 foo (Stream 1 x y z) = foo x foo (Stream 2 x y z) = foo y foo (Stream _ x y z) = foo z }}} you already get huge demand signatures. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12364> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #12599: Add Hadrian build artifacts to gitignore
by GHC 25 Jan '19

25 Jan '19

#12599: Add Hadrian build artifacts to gitignore -------------------------------------+------------------------------------- Reporter: bgamari | Owner: Type: task | 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: -------------------------------------+------------------------------------- Andrey wrote requesting the following, > For top-level `.gitignore`: > {{{ > # ------------------------------------------------------------------------------------- > # Hadrian's stuff. Note, **/stageN/ folders will be relocated to _build/ after > # resolving https://github.com/snowleopard/hadrian/issues/113 > # ------------------------------------------------------------------------------------- > _build/ > hadrian/ > **/stage?/ > }}} > > For `.gitignore` in affected submodules (array, deepseq, directory, filepath, > hoopl, hpc, process, utils/hsc2hs): > > {{{ > # ------------------------------------------------------------------------------------- > # Hadrian's stuff. Note, stageN/ folders will be relocated to top-level _build/ > # after resolving https://github.com/snowleopard/hadrian/issues/113 > # ------------------------------------------------------------------------------------- > stage?/ > }}} -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12599> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #9476: Implement late lambda-lifting
by GHC 25 Jan '19

25 Jan '19

#9476: Implement late lambda-lifting -------------------------------------+------------------------------------- Reporter: simonpj | Owner: Type: feature request | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.8.2 Keywords: | Operating System: Architecture: Unknown/Multiple | Unknown/Multiple Difficulty: Unknown | Type of failure: Blocked By: | None/Unknown Related Tickets: | Test Case: | Blocking: | Differential Revisions: -------------------------------------+------------------------------------- This ticket exists to coordinate work on Nick Frisby's '''late lambda- lifting''' optimisation. * Wiki page: [wiki:LateLamLift]. All the details are here. * Branch in GHC repo: `wip/llf` * Related tickets * #9279: local closure remains in final program * #9374: static argument transformation -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9476> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #15437: Internal error when applying a scoped type variable inside a typed expression quotation
by GHC 24 Jan '19

24 Jan '19

#15437: Internal error when applying a scoped type variable inside a typed expression quotation -------------------------------------+------------------------------------- Reporter: dminuoso | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: 8.6.1 Component: Compiler | Version: 8.4.3 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: -------------------------------------+------------------------------------- {{{#!hs {-# LANGUAGE TemplateHaskell #-} import TestMod f :: Int f = $$(foo) main :: IO () main = main }}} {{{#!hs {-# LANGUAGE ScopedTypeVariables #-} {-# LANGUAGE TemplateHaskell #-} {-# LANGUAGE TypeApplications #-} module TestMod where import Language.Haskell.TH.Syntax (Q, TExp) get :: forall a. Int get = 1 foo :: forall a. Q (TExp Int) foo = [|| get @a ||] }}} {{{ Test.hs:6:8: error: • The exact Name ‘a’ is not in scope Probable cause: you used a unique Template Haskell name (NameU), perhaps via newName, but did not bind it If that's it, then -ddump-splices might be useful • In the result of the splice: $foo To see what the splice expanded to, use -ddump-splices In the Template Haskell splice $$(foo) In the expression: $$(foo) | 6 | f = $$(foo) | ^^^ Test.hs:6:8: error: • GHC internal error: ‘a’ is not in scope during type checking, but it passed the renamer tcl_env of environment: [r3Kl :-> Identifier[f::Int, TopLevelLet [] True], r3PI :-> Identifier[main::IO (), TopLevelLet [r3PI :-> main] True]] • In the type ‘a’ In the expression: get @a In the result of the splice: $foo To see what the splice expanded to, use -ddump-splices | 6 | f = $$(foo) | }}} -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15437> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #13253: Exponential compilation time with RWST & ReaderT stack with `-02`
by GHC 24 Jan '19

24 Jan '19

#13253: Exponential compilation time with RWST & ReaderT stack with `-02` -------------------------------------+------------------------------------- Reporter: phadej | Owner: 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: -------------------------------------+------------------------------------- See the attached file. The compilation time grows exponentially with additional fields in `HuteStruct`. Tried with GHC-8.0.1 and quite recent HEAD (8.2) adding a field doubles the compilation time with `-O2`. - 9: 17s - 10: 34s - 11: 83s on my machine. With `-01` around 3sec, independently of the struct size. The original issue: https://github.com/yesodweb/yesod/issues/1348 -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13253> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #12920: Overzealous unused-top-binds
by GHC 23 Jan '19

23 Jan '19

#12920: Overzealous unused-top-binds -------------------------------------+------------------------------------- Reporter: crockeea | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.0.2-rc1 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: Incorrect Unknown/Multiple | error/warning at compile-time Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- The following example produces three warnings: {{{ module Foo () where type A = Int type B = A foo :: IO () foo = print (bar 3 :: B) bar :: a -> a bar = id }}} `ghci Foo -Wall` produces {{{ GHCi, version 8.0.1.20161117: http://www.haskell.org/ghc/ :? for help [1 of 1] Compiling Foo ( Foo.hs, interpreted ) Foo.hs:5:1: warning: [-Wunused-top-binds] Defined but not used: type constructor or class ‘B’ Foo.hs:8:1: warning: [-Wunused-top-binds] Defined but not used: ‘foo’ Foo.hs:11:1: warning: [-Wunused-top-binds] Defined but not used: ‘bar’ Ok, modules loaded: Foo. }}} The three errors reported are inconsistent because GHC reports that `B` is unused, but somehow feels that `A` *is* used. It seems to me that two possibilities could work: 1. GHC reports 1 error: GHC should only report that `foo` is unused, since `A` is used in `B`, `B` is used in `foo`, and `bar` is used in `foo`. This seems like the best approach to me. 2. GHC reports 4 errors: One might choose to apply the definition of "unused" transitively so that, since `foo` is unused, everything used in `foo` is considered unused (unless "used" elsewhere). In this case, `A` should also be reported as unused, but it currently isn't. This definition would be exceedingly useful for removing dead code, but it also produces very noisy output when there is a lot of dead code. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12920> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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