December 2016 - ghc-tickets

[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] #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] #10064: Add support for "foo"## literals to MagicHash
by GHC 24 Jul '17

24 Jul '17

#10064: Add support for "foo"## literals to MagicHash -------------------------------------+------------------------------------- Reporter: chadaustin | Owner: Type: feature | Status: new request | Milestone: Priority: normal | Version: 7.8.4 Component: Compiler | Operating System: Unknown/Multiple Keywords: | Type of failure: None/Unknown Architecture: | Blocked By: Unknown/Multiple | Related Tickets: Test Case: | Blocking: | Differential Revisions: | -------------------------------------+------------------------------------- I'm working on a high-performance library for building up buffers of data. Appending ByteStrings works pretty well, but, for bytestring literals, it's faster to avoid the CAF (and it's associated tag check + indirect jump) entirely and append a zero-terminate MagicHash string literal via Addr#. Then I discovered that it's 30% faster if you can avoid the strlen with an explicit length. See this benchmark: https://github.com/chadaustin /buffer- builder/blob/ac662962d6d3c21f206ab8e8121323b9f18677fa/bench/Bench.hs#L42 That API is not very pleasant, of course. :) Thus, here is a feature request: It would be great if "foo"## produced (# Int#, Addr# #), where the Int# is the length of the buffer in bytes. The compiler could enforce that the literal only contains 8-bit values. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10064> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #9453: The example for GHC Generics is kinda broken
by GHC 24 Jul '17

24 Jul '17

#9453: The example for GHC Generics is kinda broken -------------------------------------+------------------------------------- Reporter: Feuerbach | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Documentation | 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: -------------------------------------+------------------------------------- From http://www.reddit.com/r/haskell/comments/2douzn/problem_with_popular_ghcgen…: The popular GHC.Generics serialization example at http://www.haskell.org/ghc/docs/latest/html/users_guide/generic- programming.html#idp25226064 illustrates how to serialize a sum datatype (a :+: b), with a 0 bit representing a and a 1 bit representing b. However, consider a :+: b :+: c. If the compiler treats this as (a :+: b) :+: c, then a is 00, b is 01, c is 1. If it's a :+: (b :+: c), then a is 0, b is 10, c is 11. The compiler's decision, even though (as I understand it) consistent for any given compile, could differ later. The manual, at http://hackage.haskell.org/package/generic- deriving-1.6.3/docs/Generics-Deriving-Base.html#g:9 states not to depend on the ordering, which the example does. I think this is a valid concern, and the manual should be at least updated to discuss this problem. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9453> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #11375: Type aliases twice as slow to compile as closed type families.
by GHC 23 Jul '17

23 Jul '17

#11375: Type aliases twice as slow to compile as closed type families. -------------------------------------+------------------------------------- Reporter: danilo2 | Owner: Type: bug | Status: new Priority: high | Milestone: Component: Compiler | Version: 8.1 (Type checker) | Keywords: | Operating System: MacOS X Architecture: aarch64 | Type of failure: Compile-time | performance bug Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- Hello guys! This bug is awkard at last to me. So I notiiced today, that using type aliases in my code takes twice as long to copile than using closed type families with just single member. So to be most precise - when I replace the following line: {{{ type XQ1 m a = Targets (Match m a) }}} with {{{ type family XQ1 m a where XQ1 m a = Targets (Match m a) }}} the compilation time drops from 15s to 7s (this type alias is heavily used across the application). I think it could be somehow related to the famous #8095 bug, but I can of course be wrong here. Unfortunetally the codebase is pretty big and its not so easy to make a minimal example, but I'll try to work on it during the upcoming weekend (please notify me if that would not be necessary). And some statistics from `-dshow-passes`: When using type alias: {{{ *** Parser: *** Renamer/typechecker: *** Desugar: Result size of Desugar (after optimization) = {terms: 10,303, types: 66,949, coercions: 16,383,834} *** Simplifier: Result size of Simplifier iteration=1 = {terms: 10,722, types: 75,391, coercions: 16,507,338} Result size of Simplifier iteration=2 = {terms: 10,550, types: 74,437, coercions: 16,504,770} Result size of Simplifier iteration=3 = {terms: 10,543, types: 74,347, coercions: 16,504,734} Result size of Simplifier = {terms: 10,543, types: 74,347, coercions: 16,504,630} *** Tidy Core: Result size of Tidy Core = {terms: 10,846, types: 75,522, coercions: 16,504,729} *** CorePrep: Result size of CorePrep = {terms: 14,823, types: 96,430, coercions: 16,504,729} *** ByteCodeGen: Upsweep completely successful. *** Deleting temp files: Warning: deleting non-existent /var/folders/y9/km24zv3d4yl711mwv85vwljc0000gp/T/ghc64965_0/ghc_95.c Warning: deleting non-existent /var/folders/y9/km24zv3d4yl711mwv85vwljc0000gp/T/ghc64965_0/ghc_94.o Ok, modules loaded: Prologue, Type.Bool, Type.Container, Type.List, Type.Set, Data.Cata, Type.Promotion, Data.Bits.Mask, Control.Monad.State.Dependent, Control.Monad.Poly, Control.Applicative.Poly, Type.Regex, Type.Either, Data.Reprx, Data.Impossible, Type.Cache.TH, Main, Control.Lens.Utils, Control.Lens.Wrapped.Utils, Data.Text.CodeBuilder, Data.Text.CodeBuilder.Tok, Data.Text.CodeBuilder.Builder, Data.Text.CodeBuilder.Doc, Data.Binary.Instances.Missing, Data.Default.Instances.Missing, Data.String.Class, Data.Text.Class, Data.Convert, Data.Layer, Data.Container.Class, Data.Container.List, Data.Functor.Utils, Type.Operators, Type.Show, Type.Wrapped, Data.Container.Opts, Data.Container.Poly, Data.Convert.Base, Data.Convert.Instances, Data.Convert.Instances.Map, Data.Convert.Instances.Num, Data.Convert.Instances.Text, Data.Convert.Instances.TH, Data.Convert.Bound, Data.Bits.Base. *** Parser: *** Desugar: *** Simplify: *** CorePrep: *** ByteCodeGen: ... }}} and with type families: {{{ *** Parser: *** Renamer/typechecker: *** Desugar: Result size of Desugar (after optimization) = {terms: 310, types: 867, coercions: 78} *** Simplifier: Result size of Simplifier iteration=1 = {terms: 301, types: 878, coercions: 296} Result size of Simplifier = {terms: 301, types: 878, coercions: 293} *** Tidy Core: Result size of Tidy Core = {terms: 337, types: 1,002, coercions: 293} *** CorePrep: Result size of CorePrep = {terms: 475, types: 1,305, coercions: 293} *** ByteCodeGen: *** Deleting temp files: Warning: deleting non-existent /var/folders/y9/km24zv3d4yl711mwv85vwljc0000gp/T/ghc64863_0/ghc_93.c Warning: deleting non-existent /var/folders/y9/km24zv3d4yl711mwv85vwljc0000gp/T/ghc64863_0/ghc_92.o compile: input file /var/folders/y9/km24zv3d4yl711mwv85vwljc0000gp/T/ghc64863_0/ghc_1.hscpp *** Checking old interface for Main: [45 of 45] Compiling Main ( /Users/wdanilo/dev/hs/public/variants/Main.hs, interpreted ) *** Parser: *** Renamer/typechecker: *** Desugar: Result size of Desugar (after optimization) = {terms: 10,303, types: 67,609, coercions: 162,249} *** Simplifier: Result size of Simplifier iteration=1 = {terms: 10,722, types: 75,721, coercions: 323,027} Result size of Simplifier iteration=2 = {terms: 10,550, types: 74,767, coercions: 320,473} Result size of Simplifier iteration=3 = {terms: 10,543, types: 74,677, coercions: 320,437} Result size of Simplifier = {terms: 10,543, types: 74,677, coercions: 320,333} *** Tidy Core: Result size of Tidy Core = {terms: 10,846, types: 75,852, coercions: 320,432} *** CorePrep: Result size of CorePrep = {terms: 14,823, types: 96,760, coercions: 320,432} *** ByteCodeGen: Upsweep completely successful. *** Deleting temp files: Warning: deleting non-existent /var/folders/y9/km24zv3d4yl711mwv85vwljc0000gp/T/ghc64863_0/ghc_95.c Warning: deleting non-existent /var/folders/y9/km24zv3d4yl711mwv85vwljc0000gp/T/ghc64863_0/ghc_94.o Ok, modules loaded: Prologue, Type.Bool, Type.Container, Type.List, Type.Set, Data.Cata, Type.Promotion, Data.Bits.Mask, Control.Monad.State.Dependent, Control.Monad.Poly, Control.Applicative.Poly, Type.Regex, Type.Either, Data.Reprx, Data.Impossible, Type.Cache.TH, Main, Control.Lens.Utils, Control.Lens.Wrapped.Utils, Data.Text.CodeBuilder, Data.Text.CodeBuilder.Tok, Data.Text.CodeBuilder.Builder, Data.Text.CodeBuilder.Doc, Data.Binary.Instances.Missing, Data.Default.Instances.Missing, Data.String.Class, Data.Text.Class, Data.Convert, Data.Layer, Data.Container.Class, Data.Container.List, Data.Functor.Utils, Type.Operators, Type.Show, Type.Wrapped, Data.Container.Opts, Data.Container.Poly, Data.Convert.Base, Data.Convert.Instances, Data.Convert.Instances.Map, Data.Convert.Instances.Num, Data.Convert.Instances.Text, Data.Convert.Instances.TH, Data.Convert.Bound, Data.Bits.Base. *** Parser: *** Desugar: *** Simplify: *** CorePrep: *** ByteCodeGen: ... }}} As we can see the type aliases produce much more coertions: 16,383,834 vs 78 using type families. This is really interesting thing and it seems related to the linked bug above. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11375> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #11095: -O0 -g slows GHC down on list literals (compared to -O0 without -g)
by GHC 19 Jul '17

19 Jul '17

#11095: -O0 -g slows GHC down on list literals (compared to -O0 without -g) -------------------------------------+------------------------------------- Reporter: slyfox | 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: -------------------------------------+------------------------------------- Tried to buld syb-0.6 testsuite with '-O0 -g' flags today and noticed compiler hung for a couple of hours. Distilled example from syb-0.6 testsuite (exact example attached): {{{#!hs {-# LANGUAGE NoImplicitPrelude #-} {-# OPTIONS_GHC -O0 #-} module Bug (bug) where {-# NOINLINE f #-} f :: a -> a f v = v bug :: [()] bug = f v v :: [()] v = [(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),() ,(),(),(),(),() ... --- a lot of these ] }}} {{{ $ time ghc -c -fforce-recomp -O0 Bug.hs real 0m0.333s user 0m0.288s sys 0m0.042s $ time ghc -c -fforce-recomp -O0 -g Bug.hs real 0m40.241s user 0m40.021s sys 0m0.070s }}} For some reason '''-g''' manages slow things down at a large factor. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11095> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #8903: Add dead store elimination
by GHC 18 Jul '17

18 Jul '17

#8903: Add dead store elimination ------------------------------------+------------------------------------- Reporter: tibbe | Owner: Type: feature request | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.9 Keywords: | Operating System: Unknown/Multiple Architecture: Unknown/Multiple | Type of failure: None/Unknown Difficulty: Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | ------------------------------------+------------------------------------- We could use some dead store elimination in the code generator. Here's some Cmm that has redundant stores to the same locations: {{{ // thawArray#: I64[Hp - 168] = I64[PicBaseReg + stg_MUT_ARR_PTRS_DIRTY_info@GOTPCREL]; I64[Hp - 160] = 16; I64[Hp - 152] = 17; _c2nT::I64 = Hp - 168; call MO_Memcpy(_c2nT::I64 + 24, _s2cx::P64 + 24, 128, 8); // writeArray#: P64[(_c2nT::I64 + 24) + (_s2cy::I64 << 3)] = _s2cE::P64; I64[_c2nT::I64] = I64[PicBaseReg + stg_MUT_ARR_PTRS_DIRTY_info@GOTPCREL]; I8[(_c2nT::I64 + 24) + ((I64[_c2nT::I64 + 8] << 3) + (_s2cy::I64 >> 7))] = 1 :: W8; // unsafeFreeze# I64[_c2nT::I64] = I64[PicBaseReg + stg_MUT_ARR_PTRS_FROZEN0_info@GOTPCREL]; }}} There are three stores to the same location (`I64[_c2nT::I64]`). (There's also one much less obvious double store to another location, which will probably be much harder to address: the store to `P64[(_c2nT::I64 + 24) + (_s2cy::I64 << 3)]` overwrites a word previous written by the `MO_Memcpy`. Getting to that one will be hard as the memcoy callish !MachOp only gets expanded in the backend.) -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8903> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #9630: compile-time performance regression (probably due to Generics)
by GHC 18 Jul '17

18 Jul '17

#9630: compile-time performance regression (probably due to Generics) -------------------------------------+------------------------------------- Reporter: hvr | Owner: simonpj Type: bug | Status: new Priority: normal | Milestone: 7.10.1 Component: Compiler | Version: 7.9 Keywords: | Operating System: Architecture: Unknown/Multiple | Unknown/Multiple Difficulty: Unknown | Type of failure: Compile- Blocked By: | time performance bug Related Tickets: #9583 | Test Case: | Blocking: | Differential Revisions: -------------------------------------+------------------------------------- This is a continuation of #9583 Compiling latest Cabal from Git's `Language.Haskell.Extension` module shows a significant performance regression with respect to GHC 7.8.3: {{{ $ uname -a Linux duo 3.17.0-031700rc6-generic #201409211935 SMP Sun Sep 21 23:37:11 UTC 2014 x86_64 x86_64 x86_64 GNU/Linux $ git describe cabal-install_merged-1912-g7161e4c ######################################################################################## $ /opt/ghc/7.8.3/bin/ghc --version The Glorious Glasgow Haskell Compilation System, version 7.8.3 $ /opt/ghc/7.8.3/bin/ghc -Rghc-timing -O -c Language/Haskell/Extension.hs <<ghc: 5228751760 bytes, 543 GCs, 34135677/114940864 avg/max bytes residency (14 samples), 273M in use, 0.00 INIT (0.03 elapsed), 4.96 MUT (5.28 elapsed), 3.26 GC (3.47 elapsed) :ghc>> ######################################################################################## $ /opt/ghc/head/bin/ghc --version The Glorious Glasgow Haskell Compilation System, version 7.9.20140923 $ /opt/ghc/head/bin/ghc -Rghc-timing -O -c Language/Haskell/Extension.hs <<ghc: 125917854120 bytes, 3181 GCs, 349136895/833496072 avg/max bytes residency (30 samples), 2096M in use, 0.001 INIT (0.001 elapsed), 133.003 MUT (142.730 elapsed), 73.241 GC (78.234 elapsed) :ghc>> }}} -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9630> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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