October 2014 - ghc-tickets

[GHC] #8723: sdist should not have to build everything
by GHC 04 Jun '15

04 Jun '15

#8723: sdist should not have to build everything ----------------------------------------------+---------------------------- Reporter: nomeata | Owner: Type: task | Status: new Priority: low | Milestone: Component: Build System | Version: 7.6.3 Keywords: | Operating System: Architecture: Unknown/Multiple | Unknown/Multiple Difficulty: Moderate (less than a day) | Type of failure: Blocked By: | None/Unknown Related Tickets: | Test Case: | Blocking: ----------------------------------------------+---------------------------- It would be nice if instead of {{{ $ perl boot $ ./configure $ make $ make sdist }}} it would suffice to run {{{ $ perl boot $ ./configure $ make sdist }}} and if that would not build more than necessary to generate the tarballs. It would save time on the GHC-HEAD-Debian-Package-autobuilder that I am currently working on. I might look into this bug eventually, but if someone else feels like doing it: Even better. (Difficulty estimated conservatively; might well be less than an hour, besides maybe waiting for test runs to finish :-)) Remember to update MakingReleases and Building/StandardTargets if this is implemented. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8723> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #9507: ghc-pkg mode to query by package-key
by GHC 04 Jun '15

04 Jun '15

#9507: ghc-pkg mode to query by package-key -------------------------------------+------------------------------------- Reporter: ezyang | Owner: ezyang Type: feature request | Status: new Priority: normal | Milestone: Component: ghc-pkg | Version: 7.9 Keywords: backpack | Operating System: Architecture: Unknown/Multiple | Unknown/Multiple Difficulty: Unknown | Type of failure: Blocked By: | None/Unknown Related Tickets: | Test Case: | Blocking: | Differential Revisions: -------------------------------------+------------------------------------- Right now, ghc-pkg understands package names and IPIDs. It should support package keys too. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9507> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #9506: Name libraries (dll/so) separately from linker symbols
by GHC 03 Jun '15

03 Jun '15

#9506: Name libraries (dll/so) separately from linker symbols -------------------------------------+------------------------------------- Reporter: ezyang | Owner: ezyang Type: task | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.9 Keywords: backpack | Operating System: Architecture: Unknown/Multiple | Unknown/Multiple Difficulty: Unknown | Type of failure: Blocked By: | None/Unknown Related Tickets: | Test Case: | Blocking: | Differential Revisions: -------------------------------------+------------------------------------- Tracking ticket so I don't forget. Based on feedback on `#ghc`, it sounds like it would be helpful if library names on the file system only had their full package name, e.g. `libHScontainers_HASH.so` as opposed to the abbreviated package name. Additionally, it seems less necessary to provide the abbreviated package name in linker symbols, since the modules often give a pretty good clue (and the package key can be looked up.) So let's do this, and make things better for users. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9506> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #8292: linker_unload test doesn't work on Windows
by GHC 31 May '15

31 May '15

#8292: linker_unload test doesn't work on Windows ------------------------------------+---------------------------------- Reporter: ezyang | Owner: simonmar Type: bug | Status: new Priority: normal | Milestone: Component: Test Suite | Version: 7.7 Keywords: | Operating System: Windows Architecture: Unknown/Multiple | Type of failure: None/Unknown Difficulty: Unknown | Test Case: linker_unload Blocked By: | Blocking: Related Tickets: | ------------------------------------+---------------------------------- I fixed up the major issues but I am getting: {{{ "/c/Users/ezyang/Dev/ghc-dyn/inplace/bin/ghc-stage2.exe" -fforce-recomp -dcore-lint -dcmm-lint -dno-debug-output -no-user-package-db -rtsopts -c Test.hs -v0 # -rtsopts causes a warning "/c/Users/ezyang/Dev/ghc-dyn/inplace/bin/ghc-stage2.exe" -fforce-recomp -dcore-lint -dcmm-lint -dno-debug-output -no-user-package-db linker_unload.c -o linker_unload -no-hs-main -optc-Werror ./linker_unload \Users\ezyang\Dev\ghc-dyn\libraries\base\dist- install\build/libHSbase-4.7.0.0.a \Users\ezyang\Dev\ghc-dyn\libraries\ghc-prim\dist-install\build/libHSghc- prim-0.3.1.0.a \Users \ezyang\Dev\ghc-dyn\libraries\integer-gmp\dist-install\build/libHSinteger- gmp-0.5.1.0.a linker_unload.exe: \Users\ezyang\Dev\ghc-dyn\libraries\base\dist- install\build/libHSbase-4.7.0. 0.a: unknown symbol `_select' }}} -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8292> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #9584: Interface file errors (Iface type variable out of scope: k)
by GHC 26 May '15

26 May '15

#9584: Interface file errors (Iface type variable out of scope: k) -------------------------------------+------------------------------------- Reporter: jonsterling | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.8.3 Keywords: | Operating System: Architecture: Unknown/Multiple | Unknown/Multiple Difficulty: Unknown | Type of failure: Blocked By: | None/Unknown Related Tickets: | Test Case: | Blocking: | Differential Revisions: -------------------------------------+------------------------------------- (Please forgive me if I have not formatted this bug report properly) Anyway, yesterday in a project at work we started having a strange problem where pretty much any change in our code at all would result in the next build failing with this error: {{{ The interface for ‘main:HList’ Declaration for $fRLensk:r_$s$w$crlens: Iface type variable out of scope: k Cannot continue after interface file error }}} I can get around the problem by deleting the dist directory, but this is very unfortunate, since it basically changes my build/test cycle from a few seconds to a minutes because I can't take advantage of the cached builds of things that *haven't* changed. It seems like errors like this have cropped up a few times in GHC, but they've always been fixed so far, so hopefully someone here will be familiar with what is causing this one! Please let me know if there is further information I can provide. The file in question looks basically like this: (it depends on some other stuff in our project, and so it won't build immediately if you paste it onto your machine; let me know if you need me to bundle it up into something self-contained). {{{#!hs -- | A bespoke record/HList type. 'el' interprets fields into types; 'tot' is -- the maximal extension of the record type; and 'rs' is the subset of 'tot' -- contained in the record itself. data Record (el :: k -> *) (tot :: [k]) (rs :: [k]) where Nil :: Record el tot '[] (:*) :: ( ElemTF r tot ?? '("the key", r, "is not permitted in this record, which may only contain", tot) , DistinctTF (r ': rs) ?? '("the key", r, "is already in", rs) ) => el r -> Record el tot rs -> Record el tot (r ': rs) infixr 9 :* -- | Records have lenses for their fields. class ElemTF r rs ~ True => RLens rs r where rlens :: proxy r -> CL.Lens' (Record el tot rs) (el r) instance RLens (r ': rs) r where rlens _ = CL.lens (\(x :* _) -> x) (\(_ :* xs) x -> x :* xs) instance (ElemTF r (s ': rs) ~ True, RLens rs r) => RLens (s ': rs) r where rlens _ = CL.lens (\(_ :* xs) -> xs ^. rlens Proxy) (\(x :* xs) y -> x :* xs & rlens Proxy .~ y) -- | Records with fields in 'K' give rise to a functor from 'Hask^K' to 'Hask'. (<<$>>) :: (forall x. f x -> g x) -> Record f tot rs -> Record g tot rs _ <<$>> Nil = Nil eta <<$>> (x :* xs) = eta x :* (eta <<$>> xs) infixl 8 <<$>> -- | Records can be traversed to pull out some of their effects. rtraverse :: Applicative h => (forall x. f x -> h (g x)) -> Record f tot rs -> h (Record g tot rs) rtraverse _ Nil = pure Nil rtraverse f (x :* xs) = (:*) <$> f x <*> rtraverse f xs -- | As a special case, we can yank out the first layer of effects in a -- composed functor stack. rtraverse1 :: Applicative f => Record (f :. g) tot rs -> f (Record g tot rs) rtraverse1 = rtraverse getCompose -- | Records whose fields are uniform in type may be turned into a list. recordToList :: Record (Const t) tbl rs -> [t] recordToList Nil = [] recordToList (Const x :* xs) = x : recordToList xs instance Show (Record el tot '[]) where show _ = "Nil" instance ( Show (Record el tot rs) , Show (el r) ) => Show (Record el tot (r ': rs)) where show (x :* xs) = "(" ++ show x ++ " :* " ++ show xs ++ ")" data family Sing (a :: k) class SingI a where sing :: Sing a class kparam ~ Any => SingE (kparam :: k) rep | kparam -> rep where fromSing :: Sing (a :: k) -> rep }}} -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9584> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #8122: make binary-dist broken on OS X in HEAD
by GHC 26 May '15

26 May '15

#8122: make binary-dist broken on OS X in HEAD --------------------------------+------------------------------------------ Reporter: danharaj | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Build System | Version: 7.7 Keywords: | Operating System: MacOS X Architecture: x86_64 | Type of failure: Installing GHC failed (amd64) | Test Case: Difficulty: Unknown | Blocking: Blocked By: | Related Tickets: | --------------------------------+------------------------------------------ After a successful configure and make, make binary-dist fails: {{{ ➜ ghc git:(master) make ===--- building phase 0 make -r --no-print-directory -f ghc.mk phase=0 phase_0_builds make[1]: Nothing to be done for `phase_0_builds'. ===--- building phase 1 make -r --no-print-directory -f ghc.mk phase=1 phase_1_builds make[1]: Nothing to be done for `phase_1_builds'. ===--- building final phase make -r --no-print-directory -f ghc.mk phase=final all make[1]: Nothing to be done for `all'. ➜ ghc git:(master) make binary-dist rm -f bindist-list make -r --no-print-directory -f ghc.mk bindist BINDIST=YES "cp" -p ghc/stage1/build/tmp/ inplace/lib/bin/ghc-stage1 cp: ghc/stage1/build/tmp/ is a directory (not copied). make[1]: *** [inplace/lib/bin/ghc-stage1] Error 1 make: *** [binary-dist-prep] Error 2 }}} -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8122> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #9709: Make restarts itself sometimes, and that's OK
by GHC 24 May '15

24 May '15

#9709: Make restarts itself sometimes, and that's OK -------------------------------------+------------------------------------- Reporter: goldfire | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Build System | Version: 7.9 Keywords: | Operating System: Architecture: Unknown/Multiple | Unknown/Multiple Difficulty: Unknown | Type of failure: Blocked By: | None/Unknown Related Tickets: | Test Case: | Blocking: | Differential Revisions: -------------------------------------+------------------------------------- I fairly frequently get the message that `make` has restarted itself {{{ ghc.mk:100: *** Make has restarted itself 2 times; is there a makefile bug? See http://ghc.haskell.org/trac/ghc/wiki/Building/Troubleshooting#Makehasrestar… for details. Stop. }}} but then when I say `make` again, things proceed just fine. In particular, this always (I think) happens after I `make clean` in the `libraries` directory. If it matters, I always use the `-j` flag to `make`, sometimes with 2 cores and sometimes with 4. I'm on Mac OS 10.8. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9709> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #8549: GHCI incorrectly link symbols defined with foreign import ccall
by GHC 23 May '15

23 May '15

#8549: GHCI incorrectly link symbols defined with foreign import ccall -------------------------+------------------------------------------------- Reporter: | Owner: qnikst | Status: new Type: bug | Milestone: Priority: | Version: 7.6.3 normal | Operating System: Unknown/Multiple Component: GHCi | Type of failure: GHCi crash Keywords: | Test Case: Architecture: | https://gist.github.com/qnikst/324a66914b3aba878be5 Unknown/Multiple | Blocking: Difficulty: | Unknown | Blocked By: | Related Tickets: | -------------------------+------------------------------------------------- I have a problem that ghci when loads modules that uses external symbols incorrectly links them, the problem is not reproducible if I'm compiling executable or load modules with :load command. As a result every Pointer value is equivalent to constant 0x0000fffffff225ff. A minimal example is in attached link, it contains a small package and description how to reproduce a bug. Problem is also reproduces on the ghc-HEAD. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8549> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #9219: Parallel build proceeds despite errors
by GHC 23 May '15

23 May '15

#9219: Parallel build proceeds despite errors -------------------------+------------------------------------------------- Reporter: | Owner: jstolarek | Status: new Type: bug | Milestone: Priority: | Version: 7.8.2 normal | Operating System: Unknown/Multiple Component: | Type of failure: Incorrect warning at Driver | compile-time Keywords: | Test Case: Architecture: | Blocking: Unknown/Multiple | Difficulty: | Unknown | Blocked By: | Related Tickets: | -------------------------+------------------------------------------------- A few weeks back during my work on singletons I got the following build log: {{{ Building singletons-1.0... Preprocessing library singletons-1.0... [ 6 of 45] Compiling Data.Singletons.Util ( src/Data/Singletons/Util.hs, dist/build/Data/Singletons/Util.o ) [ 7 of 45] Compiling Data.Singletons.Syntax ( src/Data/Singletons/Syntax.hs, dist/build/Data/Singletons/Syntax.o ) [Data.Singletons.Util changed] [ 8 of 45] Compiling Data.Singletons.Names ( src/Data/Singletons/Names.hs, dist/build/Data/Singletons/Names.o ) [Data.Singletons.Util changed] [ 9 of 45] Compiling Data.Singletons.Promote.Monad ( src/Data/Singletons/Promote/Monad.hs, dist/build/Data/Singletons/Promote/Monad.o ) [Data.Singletons.Util changed] [10 of 45] Compiling Data.Singletons.Single.Monad ( src/Data/Singletons/Single/Monad.hs, dist/build/Data/Singletons/Single/Monad.o ) [Data.Singletons.Promote.Monad changed] [11 of 45] Compiling Data.Singletons.Promote.Eq ( src/Data/Singletons/Promote/Eq.hs, dist/build/Data/Singletons/Promote/Eq.o ) [Data.Singletons.Util changed] [12 of 45] Compiling Data.Singletons.Promote.Ord ( src/Data/Singletons/Promote/Ord.hs, dist/build/Data/Singletons/Promote/Ord.o ) [Data.Singletons.Util changed] [13 of 45] Compiling Data.Singletons.Promote.Bounded ( src/Data/Singletons/Promote/Bounded.hs, dist/build/Data/Singletons/Promote/Bounded.o ) [Data.Singletons.Util changed] [14 of 45] Compiling Data.Singletons.Promote.Type ( src/Data/Singletons/Promote/Type.hs, dist/build/Data/Singletons/Promote/Type.o ) [Data.Singletons.Util changed] [15 of 45] Compiling Data.Singletons.Promote.Defun ( src/Data/Singletons/Promote/Defun.hs, dist/build/Data/Singletons/Promote/Defun.o ) [Data.Singletons.Promote.Monad changed] [16 of 45] Compiling Data.Singletons.Promote ( src/Data/Singletons/Promote.hs, dist/build/Data/Singletons/Promote.o ) [Data.Singletons.Promote.Defun changed] [17 of 45] Compiling Data.Promotion.Prelude.Bounded ( src/Data/Promotion/Prelude/Bounded.hs, dist/build/Data/Promotion/Prelude/Bounded.o ) Loading package ghc-prim ... linking ... done. Loading package integer-gmp ... linking ... done. Loading package base ... linking ... done. Loading package array-0.5.0.0 ... linking ... done. Loading package deepseq-1.3.0.2 ... linking ... done. Loading package containers-0.5.5.1 ... linking ... done. Loading package transformers-0.3.0.0 ... linking ... done. Loading package mtl-2.1.2 ... linking ... done. Loading package syb-0.4.1 ... linking ... done. Loading package pretty-1.1.1.1 ... linking ... done. Loading package template-haskell ... linking ... done. Loading package th-desugar-1.4.0 ... linking ... done. src/Data/Promotion/Prelude/Bounded.hs:34:27: Not in scope: ‘boundedTypes’ src/Data/Promotion/Prelude/Bounded.hs:34:27: GHC stage restriction: ‘boundedTypes’ is used in a top-level splice or annotation, and must be imported, not defined locally In the first argument of ‘promoteBoundedInstances’, namely ‘boundedTypes’ In the expression: promoteBoundedInstances boundedTypes [18 of 45] Compiling Data.Singletons.Single.Type ( src/Data/Singletons/Single/Type.hs, dist/build/Data/Singletons/Single/Type.o ) [Data.Singletons.Single.Monad changed] [19 of 45] Compiling Data.Singletons.Single.Eq ( src/Data/Singletons/Single/Eq.hs, dist/build/Data/Singletons/Single/Eq.o ) [Data.Singletons.Util changed] [20 of 45] Compiling Data.Singletons.Single.Data ( src/Data/Singletons/Single/Data.hs, dist/build/Data/Singletons/Single/Data.o ) [Data.Singletons.Single.Monad changed] [21 of 45] Compiling Data.Singletons.Single ( src/Data/Singletons/Single.hs, dist/build/Data/Singletons/Single.o ) [Data.Singletons.Promote changed] [22 of 45] Compiling Data.Singletons.Prelude.Instances ( src/Data/Singletons/Prelude/Instances.hs, dist/build/Data/Singletons/Prelude/Instances.o ) [Data.Singletons.Single changed] [23 of 45] Compiling Data.Singletons.Prelude.Bool ( src/Data/Singletons/Prelude/Bool.hs, dist/build/Data/Singletons/Prelude/Bool.o ) [Data.Singletons.Prelude.Instances changed] [24 of 45] Compiling Data.Singletons.Prelude.Eq ( src/Data/Singletons/Prelude/Eq.hs, dist/build/Data/Singletons/Prelude/Eq.o ) [Data.Singletons.Prelude.Bool changed] [25 of 45] Compiling Data.Singletons.CustomStar ( src/Data/Singletons/CustomStar.hs, dist/build/Data/Singletons/CustomStar.o ) [Data.Singletons.Prelude.Bool changed] [26 of 45] Compiling Data.Promotion.Prelude.Eq ( src/Data/Promotion/Prelude/Eq.hs, dist/build/Data/Promotion/Prelude/Eq.o ) [Data.Singletons.Prelude.Eq changed] [27 of 45] Compiling Data.Promotion.Prelude.Bool ( src/Data/Promotion/Prelude/Bool.hs, dist/build/Data/Promotion/Prelude/Bool.o ) [Data.Singletons.Prelude.Bool changed] [28 of 45] Compiling Data.Singletons.TypeRepStar ( src/Data/Singletons/TypeRepStar.hs, dist/build/Data/Singletons/TypeRepStar.o ) [Data.Singletons.Prelude.Eq changed] [29 of 45] Compiling Data.Singletons.Prelude.Ord ( src/Data/Singletons/Prelude/Ord.hs, dist/build/Data/Singletons/Prelude/Ord.o ) [Data.Singletons.Prelude.Bool changed] [30 of 45] Compiling Data.Promotion.Prelude.Ord ( src/Data/Promotion/Prelude/Ord.hs, dist/build/Data/Promotion/Prelude/Ord.o ) [Data.Singletons.Prelude.Ord changed] [31 of 45] Compiling Data.Singletons.TypeLits ( src/Data/Singletons/TypeLits.hs, dist/build/Data/Singletons/TypeLits.o ) [Data.Singletons.Prelude.Bool changed] [32 of 45] Compiling Data.Singletons.TH ( src/Data/Singletons/TH.hs, dist/build/Data/Singletons/TH.o ) [Data.Singletons.Prelude.Bool changed] [33 of 45] Compiling Data.Singletons.Prelude.Base ( src/Data/Singletons/Prelude/Base.hs, dist/build/Data/Singletons/Prelude/Base.o ) [Data.Singletons.Prelude.Bool changed] [34 of 45] Compiling Data.Singletons.Prelude.Either ( src/Data/Singletons/Prelude/Either.hs, dist/build/Data/Singletons/Prelude/Either.o ) [Data.Singletons.Prelude.Base changed] [35 of 45] Compiling Data.Promotion.Prelude.Either ( src/Data/Promotion/Prelude/Either.hs, dist/build/Data/Promotion/Prelude/Either.o ) [Data.Singletons.Prelude.Either changed] [36 of 45] Compiling Data.Singletons.Prelude.Tuple ( src/Data/Singletons/Prelude/Tuple.hs, dist/build/Data/Singletons/Prelude/Tuple.o ) [Data.Singletons.Prelude.Instances changed] [37 of 45] Compiling Data.Promotion.Prelude.Tuple ( src/Data/Promotion/Prelude/Tuple.hs, dist/build/Data/Promotion/Prelude/Tuple.o ) [Data.Singletons.Prelude.Tuple changed] [38 of 45] Compiling Data.Promotion.Prelude.Base ( src/Data/Promotion/Prelude/Base.hs, dist/build/Data/Promotion/Prelude/Base.o ) [Data.Singletons.Prelude.Base changed] [39 of 45] Compiling Data.Singletons.Prelude.List ( src/Data/Singletons/Prelude/List.hs, dist/build/Data/Singletons/Prelude/List.o ) [Data.Singletons.Prelude.Base changed] [40 of 45] Compiling Data.Singletons.Prelude.Maybe ( src/Data/Singletons/Prelude/Maybe.hs, dist/build/Data/Singletons/Prelude/Maybe.o )[Data.Singletons.Prelude.Instances changed] [41 of 45] Compiling Data.Singletons.Prelude ( src/Data/Singletons/Prelude.hs, dist/build/Data/Singletons/Prelude.o ) [Data.Singletons.Prelude.Base changed] [42 of 45] Compiling Data.Promotion.Prelude.List ( src/Data/Promotion/Prelude/List.hs, dist/build/Data/Promotion/Prelude/List.o ) [Data.Promotion.Prelude.Ord changed] [43 of 45] Compiling Data.Promotion.Prelude.Maybe ( src/Data/Promotion/Prelude/Maybe.hs, dist/build/Data/Promotion/Prelude/Maybe.o ) [Data.Singletons.Prelude.Maybe changed] [45 of 45] Compiling Data.Promotion.TH ( src/Data/Promotion/TH.hs, dist/build/Data/Promotion/TH.o ) [Data.Singletons.Prelude.Bool changed] }}} Notice how file 18 of 45 fails to build and yet the build does not stop. It seems that while one thread fails the remaining ones pick up other files that can be compiled despite the failure. I'm not really sure if I'm against this design choice, but I'd certainly expect to get information that something went wrong during the build. Here I was confused when I realised that despite seemingly successful compilation my package was not built. Only after examining the build log I saw that one of the modules failed to build. This should be changed to clearly notify the user that compilation was not successful. PS. At first I thought this is cabal-install's fault, but [https://github.com/haskell/cabal/issues/1809 this was ruled out]. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9219> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #8555: Simplify given `Coercible` constraints
by GHC 19 May '15

19 May '15

#8555: Simplify given `Coercible` constraints -------------------------------------+------------------------------------- Reporter: nomeata | Owner: Type: feature request | Status: new Priority: normal | Milestone: Component: Compiler (Type | Version: 7.7 checker) | Operating System: Unknown/Multiple Keywords: | Type of failure: GHC rejects Architecture: Unknown/Multiple | valid program Difficulty: Unknown | Test Case: Blocked By: | Blocking: Related Tickets: #8503 | -------------------------------------+------------------------------------- It would be feasible and possibly useful if {{{ foo :: Coercible [a] [b] => a -> b foo = coerce }}} would work. This involve simplifying `CtGiven`s similar to how given (nominal) equalities are simplified. I’ll defer working on this, as it is not strictly required, it seems. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8555> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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