September 2016 - ghc-tickets

[GHC] #8238: Implement unloading of shared libraries
by GHC 10 Nov '17

10 Nov '17

#8238: Implement unloading of shared libraries ------------------------------------+------------------------------------- Reporter: simonmar | Owner: Type: task | Status: new Priority: normal | Milestone: 7.10.1 Component: Runtime System | Version: 7.7 Keywords: | Operating System: Unknown/Multiple Architecture: Unknown/Multiple | Type of failure: None/Unknown Difficulty: Unknown | Test Case: Blocked By: | Blocking: Related Tickets: 8039 | ------------------------------------+------------------------------------- In #8039 we added support for unloading static objects from the runtime linker, with the GC detecting when there are no references left. We could add this functionality for shared libraries too, using `dl_iterate_phdr`. @heisenbug's comment from #8039 with the relevant pointers: [http://eli.thegreenplace.net/2011/08/25/load-time-relocation-of-shared- libraries Eli Bendersky's article] suggests to use the [http://linux.die.net/man/3/dl_iterate_phdr dl_iterate_phdr] function for finding information about loaded libraries. Seems to be linux only. There is a [http://stackoverflow.com/questions/10009043/dl-iterate-phdr- equivalent-on-mac workaround on OSX], though, on !StackOverflow. And here is how Böhm-Demers-Weiser's GC implement a [https://github.com/ivmai/bdwgc/blob/master/dyn_load.c#L451 callback function for dl_iterate_phdr]. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8238> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #10514: Generic for existential types
by GHC 09 Nov '17

09 Nov '17

#10514: Generic for existential types -------------------------------------+------------------------------------- Reporter: | Owner: ekmett andreas.abel | Status: new Type: feature | Milestone: request | Version: 7.10.1 Priority: normal | Operating System: Unknown/Multiple Component: Core | Type of failure: None/Unknown Libraries | Blocked By: Keywords: | Related Tickets: Architecture: | Unknown/Multiple | Test Case: | Blocking: | Differential Revisions: | -------------------------------------+------------------------------------- I have some use for Generic for an existential type which is constraint to be Generic. {{{!#hs {-# LANGUAGE DeriveGeneric #-} {-# LANGUAGE ExistentialQuantification #-} {-# LANGUAGE StandaloneDeriving #-} import GHC.Generics data U = forall a. (Generic a) => U a -- deriving (Generic) -- Can't make a derived instance of ‘Generic U’: -- Constructor ‘U’ has existentials or constraints in its type -- Possible fix: use a standalone deriving declaration instead -- deriving instance Generic U -- Can't make a derived instance of ‘Generic U’: -- U must be a vanilla data constructor -- In the stand-alone deriving instance for ‘Generic U’ data D1Ser data C1_0Ser instance Generic U where type Rep U = D D1Ser (C1 C1_0Ser (S1 NoSelector (Rep a))) -- Not in scope: type variable ‘a’ -- How to bring the existential type `a' into scope? }}} -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10514> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #11267: Can't parse type with kind ascription with GHCi's :kind command
by GHC 04 Nov '17

04 Nov '17

#11267: Can't parse type with kind ascription with GHCi's :kind command -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: GHCi | Version: 7.10.3 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 playing around with GHCi's `:kind` command and noticed that trying to ascribe a type with a kind doesn't parse: {{{ $ inplace/bin/ghc-stage2 --interactive GHCi, version 7.11.20151219: http://www.haskell.org/ghc/ :? for help λ> :set -XTypeInType λ> :kind 'True :: Bool <interactive>:1:7: error: parse error on input ‘::’ }}} This seems like something that should be able to parse, given that `:type True :: Bool` parses just fine. Is there a reason it couldn't? -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11267> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #12199: GHC is oblivious to injectivity when a type family is used in a GADT type
by GHC 04 Nov '17

04 Nov '17

#12199: GHC is oblivious to injectivity when a type family is used in a GADT type -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.0.1 (Type checker) | Keywords: TypeFamilies, | Operating System: Unknown/Multiple Injective | Architecture: | Type of failure: GHC rejects Unknown/Multiple | valid program Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- [https://www.reddit.com/r/haskell/comments/4oeidw/type_family_dependencies_a… A reddit post] points out a limitation of injective type families: {{{#!hs {-# LANGUAGE GADTs #-} {-# LANGUAGE KindSignatures #-} {-# LANGUAGE TypeFamilyDependencies #-} type family Foo a = b | b -> a where Foo Int = Bool data Bar :: * -> * where Bar :: a -> Bar (Foo a) oink :: Bar Bool -> Int oink (Bar x) = x -- type error }}} Compiling this fails with: {{{ Could not deduce: a ~ Int from the context: Bool ~ Foo a }}} Which seems odd, given that `Bool ~ Foo a` should imply `a ~ Int` by injectivity. Even if you change the type signature of `oink` to `Bar (Foo Int) -> Int`, it fails with: {{{ Could not deduce: a ~ Int from the context: Foo Int ~ Foo a }}} which makes it even more apparent that it isn't aware of the fact that `Foo` is injective. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12199> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #10577: Use empty cases where appropriate when deriving instances for empty types
by GHC 02 Nov '17

02 Nov '17

#10577: Use empty cases where appropriate when deriving instances for empty types -------------------------------------+------------------------------------- Reporter: rwbarton | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.11 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Blocked By: Test Case: | Related Tickets: Blocking: | Differential Revisions: | -------------------------------------+------------------------------------- In some sense the correct instance for {{{ data X deriving instance Eq X }}} is not (what GHC currently produces) {{{ instance Eq X where (==) = error "Void ==" }}} but rather {{{ instance Eq X where a == b = case a of {} -- using the EmptyCase extension }}} See comments starting at ticket:7401#comment:28 for justification. The list of classes that GHC can currently derive is * Eq, Ord, Enum, Bounded, Show, Read (Haskell 2010 Report) * Functor, Foldable, Traversable, Typeable, Generic, Data, "any class" (`-XDerive*`) Deriving Enum and Bounded is not currently allowed for empty data types. The `showList` method of Show and the whole Read instance are easy and already implemented correctly. All the remaining methods of Haskell 2010 derivable type classes take at least one argument of the instance head type `a`. I propose that all these methods be defined as an empty case on the first such argument. Similarly in Functor, Foldable and Traversable, each method has a single argument of the form `t _` where `t :: * -> *` is the instance head type, and the method should be defined as an empty case on that argument. In all these cases so far, the derived methods for a non-empty type would, at the outermost level, be a (non-empty) case on that first argument, or the equivalent (series of pattern matches, one for each constructor), so the use of an empty case is justified for an empty type. Typeable does not care about the values or even the kind of the type at all, so it's not relevant here. Generic and especially Data are above my pay grade, but generally I expect that methods which are normally defined by case analysis on an argument of the instance head type should be defined by an empty case, and methods that (sometimes) produce a value of the instance head type should do whatever they normally do when unable to produce such a value (like `readsPrec` returning an empty list, or `read` calling `error` (if `read` were actually a method of `Read`)). `DeriveAnyClass` doesn't generate any new code, so like Typeable it's not relevant. None of this behavior is specified by the Haskell 2010 Report, which disallows deriving any class for a type with no constructors; but see #7401. So, we are entitled to do what we think is best here. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10577> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #10871: Implement "buildable" interface files which can be directly compiled without source
by GHC 02 Nov '17

02 Nov '17

#10871: Implement "buildable" interface files which can be directly compiled without source -------------------------------------+------------------------------------- Reporter: ezyang | Owner: ezyang Type: feature | Status: new request | Priority: normal | Milestone: Component: Compiler | Version: 7.11 Keywords: backpack | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Revisions: | -------------------------------------+------------------------------------- A fat interface file is like a normal hi file, but with extra information so that GHC can *recompile* the associated module without having to parse, rename and typecheck the source code again. The proposed command-line interface is something like (subject to bikeshedding): {{{ ghc -c A.hs -fno-code -fwrite-buildable-interface # produces a .hi-build file, then ghc -c A.hi-build # completes building, as if ghc -c A.hs had been called }}} The primary motivation for this is to support partially compiling indefinite packages, which cannot be compiled to object code due to the fact that some of the dependencies haven't been provided yet. However, there are some other cases where this might be useful: 1. Supercompilation requires having the source of all bindings, buildable interfaces can make this information available, even beyond what storing inlinings might provide 2. A buildable interface file can be used to build variants of a source, e.g. profiling and optimization, even "on the fly" by GHC if necessary. And maybe more we haven't thought of yet. The primary function you need to implement is a function from `ModIface`/`ModDetails` to `TcGblEnv`, which can then be fed to the desugarer. I'm looking at what things we need to add to `ModIface` to support this. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10871> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #12537: ghc-openGL build Segmentation Fault for openSUSE ppc64le
by GHC 02 Nov '17

02 Nov '17

#12537: ghc-openGL build Segmentation Fault for openSUSE ppc64le ---------------------------------+------------------------------------- Reporter: michelmno | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.10.3 Keywords: | Operating System: Unknown/Multiple Architecture: powerpc64 | Type of failure: None/Unknown Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: ---------------------------------+------------------------------------- as detailed in attached log below the initial failure is a reported Segmentation Fault and if adding debuginfo rpms then reported failure is a ghc: panic for mkFastStringWith -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12537> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #8171: Extending ExtendedDefaultRules
by GHC 02 Nov '17

02 Nov '17

#8171: Extending ExtendedDefaultRules ------------------------------------+------------------------------------- Reporter: ekmett | Owner: Type: feature request | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.6.3 Keywords: | Operating System: Unknown/Multiple Architecture: Unknown/Multiple | Type of failure: None/Unknown Difficulty: Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | ------------------------------------+------------------------------------- The current `ExtendedDefaultRules` is set up in a way that largely prevents it from being used for scenarios beyond what they get used for in GHCi today. It would be great if there was an extra class that could be included in the list of acceptable classes for Rule 3. {{{ class Defaulting a }}} The proposed change would be that Rule 3 under EDR can be be relaxed to: At least one of the classes Ci is numeric, or is `Show`, `Eq`, `Ord` or `Defaulting`. I went with `Defaulting` rather than `Default`, because `Default` is a common class in use across the ecosystem from the `data-default` package, and I wanted to avoid gratuitous fallout. This would permit projects like Shae Erisson's `ghclive` from last year that need to use defaulting to not have to shoe-horn unnecessary `Show` constraints onto functions. Even better would be for that class to be polykinded! That opens up possibilities for expert users to get defaulting to fire on arguments of other kinds, which is impossible under the current EDR. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8171> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #9249: Link to "latest" user's guide
by GHC 01 Nov '17

01 Nov '17

#9249: Link to "latest" user's guide ------------------------------------+------------------------------------- Reporter: goldfire | Owner: Type: bug | Status: new Priority: low | Milestone: Component: Documentation | 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: | ------------------------------------+------------------------------------- When I search (say, on Google) for a GHC-related bit of documentation, I variously get referred to old manuals. There doesn't seem to a good way for us, the GHC devs, to avoid this. But, perhaps we can include some information in the header of the pages to allow visitors to link to the latest manual, or perhaps to several versions. This header should also include the version number of the GHC that the page is documenting; currently, this info is only in the URL. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9249> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #12150: Compile time performance degradation on code that uses undefined/error with CallStacks
by GHC 01 Nov '17

01 Nov '17

#12150: Compile time performance degradation on code that uses undefined/error with CallStacks -------------------------------------+------------------------------------- Reporter: thomie | 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 | performance bug Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- GHC 8 has a lot of trouble compiling the following program: {{{#!hs module Serialize where data Result a = Success a | Error String {- 80 guards ghc-7.10.3 -O : 0.3s ghc-8.0.1 -O : 1.8s -} instance Functor Result where {-# INLINE fmap #-} fmap | bool = f | bool = f | bool = f | bool = f | bool = f | bool = f | bool = f | bool = f | bool = f | bool = f | bool = f | bool = f | bool = f | bool = f | bool = f | bool = f | bool = f | bool = f | bool = f | bool = f | bool = f | bool = f | bool = f | bool = f | bool = f | bool = f | bool = f | bool = f | bool = f | bool = f | bool = f | bool = f | bool = f | bool = f | bool = f | bool = f | bool = f | bool = f | bool = f | bool = f | bool = f | bool = f | bool = f | bool = f | bool = f | bool = f | bool = f | bool = f | bool = f | bool = f | bool = f | bool = f | bool = f | bool = f | bool = f | bool = f | bool = f | bool = f | bool = f | bool = f | bool = f | bool = f | bool = f | bool = f | bool = f | bool = f | bool = f | bool = f | bool = f | bool = f | bool = f | bool = f | bool = f | bool = f | bool = f | bool = f | bool = f | bool = f | bool = f | bool = f where bool = undefined f = undefined }}} Here are some timing results, depending on the number of `| bool = f` clauses: {{{ * ghc-8.0.1 N clauses : time (s) 10 : 0.61 20 : 0.78 40 : 1.03 80 : 1.64 160 : 2.83 320 : 5.16 640 : 10.37 1280 : 21.16 * ghc-7.10.3 N clauses : time (s) 10 : 0.33 20 : 0.29 40 : 0.34 80 : 0.30 160 : 0.32 320 : 0.35 640 : 0.48 1280 : 0.80 }}} I think this compile time difference is caused by the `CallStack` changes introduced in GHC 8.0. When I use a version of `undefined` that doesn't have a CallStack, there is no difference in compile time when using GHC 7.10 or GHC 8.0. This is my implementation of `undefined` without `CallStack`: {{{ import GHC.Exception (errorCallException) import GHC.Prim (raise#) import Prelude (Char) error :: [Char] -> a error s = raise# (errorCallException s) undefined :: a undefined = error "undefined without callstack" }}} This is the quick and dirty Python script I used to generate those timing results (ghc version is hardcoded): {{{#!py import os import tempfile import time import subprocess def src(n): return ''' module Test where data Result a = Success a | Error String instance Functor Result where {{-# INLINE fmap #-}} fmap {0} where bool = undefined f = undefined '''.format('\n | bool = f' * n) tempfile = tempfile.mktemp('.hs') print('tempfile = {0}'.format(tempfile)) print('N clauses : time (s)') for i in range(8): n = 10 * 2 ** i with open(tempfile, 'w') as f: f.write(src(n)) f.flush() t0 = time.time() subprocess.call(['ghc-8.0.1', '-v0', '-O', tempfile]) t1 = time.time() print(str(n).ljust(10) + ': %.2f' % (t1 - t0)) os.remove(tempfile) }}} -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12150> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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