October 2017 - ghc-tickets

[GHC] #12670: Representation polymorphism validity check is too strict
by GHC 22 Dec '17

22 Dec '17

#12670: Representation polymorphism validity check is too strict -------------------------------------+------------------------------------- Reporter: bgamari | Owner: Type: bug | Status: new Priority: normal | Milestone: 8.2.1 Component: Compiler | Version: 8.0.1 (Type checker) | Keywords: typeable | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- As of 465c6c5d15f8fb54afb78408f3a79e75e74d2cd4 GHC rejects this program, {{{#!hs {-# LANGUAGE GADTs #-} {-# LANGUAGE TypeInType #-} {-# LANGUAGE RankNTypes #-} module Hi where import GHC.Exts data TypeRep (a :: k) where TrFun :: forall (r1 :: RuntimeRep) (r2 :: RuntimeRep) (a :: TYPE r1) (b :: TYPE r2). TypeRep a -> TypeRep b -> TypeRep (a -> b) }}} Claiming that it has inappropriate polymorphism over `RuntimeRep`, {{{ $ ghc hi2.hs [1 of 1] Compiling Hi ( hi2.hs, hi2.o ) hi2.hs:7:5: error: • A representation-polymorphic type is not allowed here: Type: a Kind: TYPE r1 • In the definition of data constructor ‘TrFun’ In the data type declaration for ‘TypeRep’ }}} I suspect this check is unnecessarily strict since we never need to store a value of type `a` to represent `TypeRep a` at runtime. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12670> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #14288: ScopedTypeVariables with nexted foralls broken since 8.0.2
by GHC 20 Dec '17

20 Dec '17

#14288: ScopedTypeVariables with nexted foralls broken since 8.0.2 -------------------------------------+------------------------------------- Reporter: | Owner: (none) MikolajKonarski | Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.2.1 (Type checker) | Keywords: | Operating System: Linux Architecture: x86_64 | Type of failure: GHC rejects (amd64) | valid program Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- The following example fails in 8.2.1 and 8.0.2. Works fine in 7.10.3. If that's intended (that only the first forall has extended scope), I didn't find any mention of that in GHC manual. The two commented out variants work fine in all. {{{#!hs {-# LANGUAGE ScopedTypeVariables #-} main :: IO () main = do -- let f :: forall a ref. ref () -> () -- let f :: forall ref. forall a. ref () -> () let f :: forall a. forall ref. ref () -> () f x = let r :: ref () r = x in () return $ f (Just ()) }}} The error (the same in 8.2.1 and 8.0.2) is: {{{#!hs $ ghc --make forall.hs [1 of 1] Compiling Main ( forall.hs, forall.o ) forall.hs:8:21: error: • Couldn't match type ‘ref’ with ‘ref1’ ‘ref’ is a rigid type variable bound by the type signature for: f :: forall a (ref :: * -> *). ref () -> () at forall.hs:6:29 ‘ref1’ is a rigid type variable bound by the type signature for: r :: forall (ref1 :: * -> *). ref1 () at forall.hs:7:22 Expected type: ref1 () Actual type: ref () • In the expression: x In an equation for ‘r’: r = x In the expression: let r :: ref () r = x in () • Relevant bindings include r :: ref1 () (bound at forall.hs:8:17) x :: ref () (bound at forall.hs:7:9) f :: ref () -> () (bound at forall.hs:7:7) }}} -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14288> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #12512: UnboxedTupleRep-related panic involving typeclasses
by GHC 20 Dec '17

20 Dec '17

#12512: UnboxedTupleRep-related panic involving typeclasses -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.0.1 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: Compile-time Unknown/Multiple | crash Test Case: | Blocked By: Blocking: | Related Tickets: #11723 Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- You can trigger a GHC panic by abusing `UnboxedTupleRep` in just the right way: {{{#!hs {-# LANGUAGE DataKinds #-} {-# LANGUAGE KindSignatures #-} {-# LANGUAGE StandaloneDeriving #-} {-# LANGUAGE UnboxedTuples #-} import GHC.Exts class Wat (a :: TYPE 'UnboxedTupleRep) deriving instance Wat (# a, b #) }}} {{{ $ /opt/ghc/8.0.1/bin/ghc Bug.hs [1 of 1] Compiling Bug ( Bug.hs, Bug.o ) ghc: panic! (the 'impossible' happened) (GHC version 8.0.1 for x86_64-unknown-linux): newTyConEtadArity (#,#) Please report this as a GHC bug: http://www.haskell.org/ghc/reportabug }}} This is probably a leak that #11723 didn't plug, as it only considered datatypes, not typeclasses. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12512> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #9244: Compiler could warn about type variable shadowing, and hint about ScopedTypeVariables
by GHC 18 Dec '17

18 Dec '17

#9244: Compiler could warn about type variable shadowing, and hint about ScopedTypeVariables -------------------------+------------------------------------------------- Reporter: | Owner: stusmith | Status: new Type: | Milestone: feature request | Version: 7.6.3 Priority: | Operating System: Unknown/Multiple normal | Type of failure: Incorrect warning at Component: | compile-time Compiler | Test Case: Keywords: | Blocking: Architecture: | Unknown/Multiple | Difficulty: | Unknown | Blocked By: | Related Tickets: | -------------------------+------------------------------------------------- GHC already warns about variable shadowing: {{{ timesTwoPlusOne x = timesTwo x + 1 where timesTwo x = x * 2 Warning: This binding for `x' shadows the existing binding bound at <location> }}} However the similar warning doesn't happen for type variables. {{{ tryMaybe :: IO a -> IO (Maybe a) tryMaybe action = do result <- (try action) :: IO (Either SomeException a) return $ case result of Left _ -> Nothing Right v -> Just v Couldn't match type `a' with `a1' `a' is a rigid type variable bound by the type signature for tryMaybe :: IO a -> IO (Maybe a) at types.hs:<line>:13 `a1' is a rigid type variable bound by an expression type signature: IO (Either SomeException a1) at types.hs:<line>:15 Expected type: IO a1 Actual type: IO a ... }}} Here, I thought that the 'a' in the function's type declaration was the same 'a' in the expression type declaration. However in Haskell 98, they are completely different variables. Suggestion: if a type variable is renamed by the compiler due to a clash with another type variable, issue a warning that the second shadows the first, and give a hint about using -XScopedTypeVariables and forall. Alternative suggestion: if an error is displayed, where the error contains a renamed type variable, issue a hint that the second shadows the first, and give a hint about using -XScopedTypeVariables and forall. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9244> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #13627: IndexError: pop from empty list
by GHC 16 Dec '17

16 Dec '17

#13627: IndexError: pop from empty list -------------------------------------+------------------------------------- Reporter: Lemming | 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: -------------------------------------+------------------------------------- ==== How to Reproduce ==== While doing a GET operation on `/attachment/ticket/13626/`, Trac issued an internal error. ''(please provide additional details here)'' Request parameters: {{{ {u'action': u'new', 'path': u'13626/', 'realm': u'ticket'} }}} User agent: `#USER_AGENT#` ==== System Information ==== ''Systeminformation nicht verfügbar'' ==== Enabled Plugins ==== ''Plugininformation nicht verfügbar'' ==== Interface Customization ==== ''Interface customization information not available'' ==== Python Traceback ==== {{{ Traceback (most recent call last): File "build/bdist.linux-x86_64/egg/trac/web/main.py", line 623, in _dispatch_request dispatcher.dispatch(req) File "build/bdist.linux-x86_64/egg/trac/web/main.py", line 259, in dispatch iterable=chrome.use_chunked_encoding) File "build/bdist.linux-x86_64/egg/trac/web/chrome.py", line 1164, in render_template encoding='utf-8') File "/usr/local/lib/python2.7/dist-packages/genshi/core.py", line 184, in render return encode(generator, method=method, encoding=encoding, out=out) File "/usr/local/lib/python2.7/dist-packages/genshi/output.py", line 58, in encode for chunk in iterator: File "/usr/local/lib/python2.7/dist-packages/genshi/output.py", line 350, in __call__ for kind, data, pos in stream: File "/usr/local/lib/python2.7/dist-packages/genshi/output.py", line 829, in __call__ for kind, data, pos in stream: File "/usr/local/lib/python2.7/dist-packages/genshi/output.py", line 669, in __call__ for kind, data, pos in stream: File "/usr/local/lib/python2.7/dist-packages/genshi/output.py", line 774, in __call__ for kind, data, pos in chain(stream, [(None, None, None)]): File "/usr/local/lib/python2.7/dist-packages/genshi/output.py", line 594, in __call__ for ev in stream: File "/usr/local/lib/python2.7/dist-packages/genshi/core.py", line 289, in _ensure for event in stream: File "/usr/local/lib/python2.7/dist-packages/genshi/core.py", line 289, in _ensure for event in stream: File "build/bdist.linux-x86_64/egg/trac/web/chrome.py", line 1431, in _strip_accesskeys for kind, data, pos in stream: File "/usr/local/lib/python2.7/dist-packages/genshi/core.py", line 289, in _ensure for event in stream: File "build/bdist.linux-x86_64/egg/trac/web/chrome.py", line 1420, in _generate for kind, data, pos in stream: File "/usr/local/lib/python2.7/dist-packages/genshi/core.py", line 289, in _ensure for event in stream: File "/usr/local/lib/python2.7/dist-packages/genshi/core.py", line 289, in _ensure for event in stream: File "/usr/local/lib/python2.7/dist- packages/genshi/filters/transform.py", line 706, in _unmark for mark, event in stream: File "/usr/local/lib/python2.7/dist- packages/genshi/filters/transform.py", line 1101, in __call__ for mark, event in stream: File "/usr/local/lib/python2.7/dist- packages/genshi/filters/transform.py", line 118, in __iter__ event = self.stream.next() File "/usr/local/lib/python2.7/dist- packages/genshi/filters/transform.py", line 734, in __call__ for mark, event in stream: File "/usr/local/lib/python2.7/dist- packages/genshi/filters/transform.py", line 702, in _mark for event in stream: File "/usr/local/lib/python2.7/dist-packages/genshi/template/base.py", line 618, in _include for event in stream: File "/usr/local/lib/python2.7/dist-packages/genshi/template/markup.py", line 378, in _match ctxt, start=idx + 1, **vars): File "/usr/local/lib/python2.7/dist-packages/genshi/template/markup.py", line 378, in _match ctxt, start=idx + 1, **vars): File "/usr/local/lib/python2.7/dist-packages/genshi/template/markup.py", line 362, in _match content = list(content) File "/usr/local/lib/python2.7/dist-packages/genshi/template/base.py", line 618, in _include for event in stream: File "/usr/local/lib/python2.7/dist-packages/genshi/template/markup.py", line 326, in _match for event in stream: File "/usr/local/lib/python2.7/dist-packages/genshi/template/markup.py", line 315, in _strip event = next() File "/usr/local/lib/python2.7/dist-packages/genshi/template/base.py", line 558, in _flatten for kind, data, pos in stream: File "/usr/local/lib/python2.7/dist-packages/genshi/core.py", line 289, in _ensure for event in stream: File "/usr/local/lib/python2.7/dist-packages/genshi/path.py", line 588, in _generate subevent = next() File "/usr/local/lib/python2.7/dist-packages/genshi/template/base.py", line 618, in _include for event in stream: File "/usr/local/lib/python2.7/dist-packages/genshi/template/markup.py", line 315, in _strip event = next() File "/usr/local/lib/python2.7/dist-packages/genshi/template/base.py", line 558, in _flatten for kind, data, pos in stream: File "/usr/local/lib/python2.7/dist-packages/genshi/core.py", line 289, in _ensure for event in stream: File "/usr/local/lib/python2.7/dist-packages/genshi/path.py", line 588, in _generate subevent = next() File "/usr/local/lib/python2.7/dist-packages/genshi/template/base.py", line 618, in _include for event in stream: File "/usr/local/lib/python2.7/dist-packages/genshi/template/markup.py", line 315, in _strip event = next() File "/usr/local/lib/python2.7/dist-packages/genshi/template/base.py", line 558, in _flatten for kind, data, pos in stream: File "/usr/local/lib/python2.7/dist-packages/genshi/filters/i18n.py", line 178, in _generate for event in msgbuf.translate(gettext(msgbuf.format())): File "/usr/local/lib/python2.7/dist-packages/genshi/filters/i18n.py", line 1051, in translate events = self.events[order].pop(0) IndexError: pop from empty list }}} -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13627> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #13418: IndexError: pop from empty list
by GHC 16 Dec '17

16 Dec '17

#13418: IndexError: pop from empty list -------------------------------------+------------------------------------- Reporter: ralf | 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: -------------------------------------+------------------------------------- ==== How to Reproduce ==== While doing a GET operation on `/attachment/ticket/13417/`, Trac issued an internal error. ''(please provide additional details here)'' Request parameters: {{{ {u'action': u'new', 'path': u'13417/', 'realm': u'ticket'} }}} User agent: `Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/56.0.2924.76 Safari/537.36` ==== System Information ==== ''Systeminformation nicht verfügbar'' ==== Enabled Plugins ==== ''Plugininformation nicht verfügbar'' ==== Interface Customization ==== ''Interface customization information not available'' ==== Python Traceback ==== {{{ Traceback (most recent call last): File "/usr/local/lib/python2.7/dist-packages/trac/web/main.py", line 613, in _dispatch_request dispatcher.dispatch(req) File "/usr/local/lib/python2.7/dist-packages/trac/web/main.py", line 273, in dispatch iterable=chrome.use_chunked_encoding) File "/usr/local/lib/python2.7/dist-packages/trac/web/chrome.py", line 1159, in render_template encoding='utf-8') File "/usr/local/lib/python2.7/dist-packages/genshi/core.py", line 184, in render return encode(generator, method=method, encoding=encoding, out=out) File "/usr/local/lib/python2.7/dist-packages/genshi/output.py", line 58, in encode for chunk in iterator: File "/usr/local/lib/python2.7/dist-packages/genshi/output.py", line 350, in __call__ for kind, data, pos in stream: File "/usr/local/lib/python2.7/dist-packages/genshi/output.py", line 829, in __call__ for kind, data, pos in stream: File "/usr/local/lib/python2.7/dist-packages/genshi/output.py", line 669, in __call__ for kind, data, pos in stream: File "/usr/local/lib/python2.7/dist-packages/genshi/output.py", line 774, in __call__ for kind, data, pos in chain(stream, [(None, None, None)]): File "/usr/local/lib/python2.7/dist-packages/genshi/output.py", line 594, in __call__ for ev in stream: File "/usr/local/lib/python2.7/dist-packages/genshi/core.py", line 289, in _ensure for event in stream: File "/usr/local/lib/python2.7/dist-packages/genshi/core.py", line 289, in _ensure for event in stream: File "/usr/local/lib/python2.7/dist-packages/trac/web/chrome.py", line 1426, in _strip_accesskeys for kind, data, pos in stream: File "/usr/local/lib/python2.7/dist-packages/genshi/core.py", line 289, in _ensure for event in stream: File "/usr/local/lib/python2.7/dist-packages/trac/web/chrome.py", line 1415, in _generate for kind, data, pos in stream: File "/usr/local/lib/python2.7/dist-packages/genshi/core.py", line 289, in _ensure for event in stream: File "/usr/local/lib/python2.7/dist-packages/genshi/core.py", line 289, in _ensure for event in stream: File "/usr/local/lib/python2.7/dist- packages/genshi/filters/transform.py", line 706, in _unmark for mark, event in stream: File "/usr/local/lib/python2.7/dist- packages/genshi/filters/transform.py", line 1101, in __call__ for mark, event in stream: File "/usr/local/lib/python2.7/dist- packages/genshi/filters/transform.py", line 118, in __iter__ event = self.stream.next() File "/usr/local/lib/python2.7/dist- packages/genshi/filters/transform.py", line 734, in __call__ for mark, event in stream: File "/usr/local/lib/python2.7/dist- packages/genshi/filters/transform.py", line 702, in _mark for event in stream: File "/usr/local/lib/python2.7/dist-packages/genshi/template/base.py", line 618, in _include for event in stream: File "/usr/local/lib/python2.7/dist-packages/genshi/template/markup.py", line 378, in _match ctxt, start=idx + 1, **vars): File "/usr/local/lib/python2.7/dist-packages/genshi/template/markup.py", line 378, in _match ctxt, start=idx + 1, **vars): File "/usr/local/lib/python2.7/dist-packages/genshi/template/markup.py", line 362, in _match content = list(content) File "/usr/local/lib/python2.7/dist-packages/genshi/template/base.py", line 618, in _include for event in stream: File "/usr/local/lib/python2.7/dist-packages/genshi/template/markup.py", line 326, in _match for event in stream: File "/usr/local/lib/python2.7/dist-packages/genshi/template/markup.py", line 315, in _strip event = next() File "/usr/local/lib/python2.7/dist-packages/genshi/template/base.py", line 558, in _flatten for kind, data, pos in stream: File "/usr/local/lib/python2.7/dist-packages/genshi/core.py", line 289, in _ensure for event in stream: File "/usr/local/lib/python2.7/dist-packages/genshi/path.py", line 588, in _generate subevent = next() File "/usr/local/lib/python2.7/dist-packages/genshi/template/base.py", line 618, in _include for event in stream: File "/usr/local/lib/python2.7/dist-packages/genshi/template/markup.py", line 315, in _strip event = next() File "/usr/local/lib/python2.7/dist-packages/genshi/template/base.py", line 558, in _flatten for kind, data, pos in stream: File "/usr/local/lib/python2.7/dist-packages/genshi/core.py", line 289, in _ensure for event in stream: File "/usr/local/lib/python2.7/dist-packages/genshi/path.py", line 588, in _generate subevent = next() File "/usr/local/lib/python2.7/dist-packages/genshi/template/base.py", line 618, in _include for event in stream: File "/usr/local/lib/python2.7/dist-packages/genshi/template/markup.py", line 315, in _strip event = next() File "/usr/local/lib/python2.7/dist-packages/genshi/template/base.py", line 558, in _flatten for kind, data, pos in stream: File "/usr/local/lib/python2.7/dist-packages/genshi/filters/i18n.py", line 178, in _generate for event in msgbuf.translate(gettext(msgbuf.format())): File "/usr/local/lib/python2.7/dist-packages/genshi/filters/i18n.py", line 1051, in translate events = self.events[order].pop(0) IndexError: pop from empty list }}} -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13418> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #8272: testing if SpLim=$rbp and Sp=$rsp changed performance at all
by GHC 15 Dec '17

15 Dec '17

#8272: testing if SpLim=$rbp and Sp=$rsp changed performance at all ----------------------------------------------+---------------------------- Reporter: carter | Owner: carter Type: task | Status: new Priority: normal | Milestone: 7.10.1 Component: Compiler | Version: 7.7 Keywords: callingConvention | Operating System: Architecture: x86_64 (amd64) | Unknown/Multiple Difficulty: Project (more than a week) | Type of failure: Blocked By: | None/Unknown Related Tickets: | Test Case: | Blocking: ----------------------------------------------+---------------------------- testing if SpLim=$rbp and Sp=$rsp changed performance at all would need a stack check but then push could be used to spill to the stack Idea via Nathan Howell. At the very least, the x86 PUSH instruction has a more succinct encoding than MOV. worth hacking out to see if this can measurable shift ghc perf on nofib or not. this would be part of a larger effort to explore ways to improve GHC's calling convention for modern hardware -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8272> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #11203: Kind inference with SigTvs is wrong
by GHC 15 Dec '17

15 Dec '17

#11203: Kind inference with SigTvs is wrong -------------------------------------+------------------------------------- Reporter: goldfire | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.10.2 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 {{{#!hs data SameKind :: k -> k -> * data Q (a :: k1) (b :: k2) c = MkQ (SameKind a b) }}} This code should be rejected. Yet it is accepted. The problem is that, when kind-checking a datatype declaration without a CUSK, GHC uses `SigTv`s for user-written kind variables. `SigTv`s are allowed to unify with other `SigTv`s, leading to incorrect behavior here. The motivating scenario is this: {{{#!hs data T (a :: k1) x = MkT (S a ()) data S (b :: k2) y = MkS (T b ()) }}} This program should be accepted. Neither type has a CUSK and therefore is not generalized before kind-checking the group. GHC must then unify `k1` and `k2`. If they were skolems, this unification would fail and this pair of definitions would be rejected. This ticket is identical in spirit to #9201, but that example has a CUSK and thus works. The bug can happen only when there is no CUSK. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11203> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #14135: PatternSynonyms regression in GHC HEAD (expectJust mkOneConFull)
by GHC 14 Dec '17

14 Dec '17

#14135: PatternSynonyms regression in GHC HEAD (expectJust mkOneConFull) -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: (none) Type: bug | Status: new Priority: highest | Milestone: 8.4.1 Component: Compiler | Version: 8.2.1 Keywords: | Operating System: Unknown/Multiple PatternSynonyms | Architecture: | Type of failure: Compile-time Unknown/Multiple | crash or panic Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- This code works in GHC 8.2: {{{#!hs {-# LANGUAGE PatternSynonyms #-} module Bug where data Foo a = Foo1 a | Foo2 a pattern MyFoo2 :: Int -> Foo Int pattern MyFoo2 i = Foo2 i {-# COMPLETE Foo1, MyFoo2 #-} f :: Foo a -> a f (Foo1 x) = x }}} But it throws a compile-time exception on GHC HEAD: {{{ $ ghc3/inplace/bin/ghc-stage2 Bug.hs [1 of 1] Compiling Bug ( Bug.hs, Bug.o ) ghc-stage2: panic! (the 'impossible' happened) (GHC version 8.3.20170815 for x86_64-unknown-linux): expectJust mkOneConFull CallStack (from HasCallStack): error, called at compiler/utils/Maybes.hs:53:27 in ghc:Maybes expectJust, called at compiler/deSugar/Check.hs:1128:37 in ghc:Check }}} -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14135> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #12372: bug: documentation for Control.Monad.guard not useful after AMP
by GHC 14 Dec '17

14 Dec '17

#12372: bug: documentation for Control.Monad.guard not useful after AMP -------------------------------------+------------------------------------- Reporter: ntc2 | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: | Version: 8.0.1 libraries/base | Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: Documentation Unknown/Multiple | bug Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- Since the AMP refactor, the documentation for `Control.Monad.guard` [1] is no longer useful for beginners. It simply gives the definition of `guard`, but in prose: {{{ guard b is pure () if b is True, and empty if b is False. }}} (and better to just use Haskell instead of prose here, no?) To use `guard` in a `MonadPlus`, you now need to know that `Alternative` is a super class of `MonadPlus`, and that `mzero = zero`. The documentation [2] for `MonadPlus` doens't mention `mzero = zero` in the default definition -- you must look at the source for that -- and the docs for `guard` don't mention `MonadPlus`. The documentation for `Control.Monad.guard` should suggest use with `MonadPlus`, and give an example (compare with the very helpful example for `Control.Monad.when`). A non-monadic example would also be useful. [1] https://hackage.haskell.org/package/base-4.9.0.0/docs/Control- Monad.html#v:guard [2] https://hackage.haskell.org/package/base-4.9.0.0/docs/Control- Monad.html#t:MonadPlus -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12372> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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