March 2017 - ghc-tickets

[GHC] #11706: Increase precedence of statements (if-then-else, case-of, do)
by GHC 03 Feb '18

03 Feb '18

#11706: Increase precedence of statements (if-then-else, case-of, do) -------------------------------------+------------------------------------- Reporter: YoYoYonnY | Owner: Type: feature | Status: new request | Priority: lowest | Milestone: ⊥ Component: Compiler | Version: (Parser) | 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: -------------------------------------+------------------------------------- Warning: Skip to the {{{TL; DR}}}, and maybe the {{{How it works}}} section; You don't want to read this. (I've warned you.) == The problem == We do it all the time: Writing that one dollar sign just before our expressions, just because Haskell can't parse it: {{{#!hs f x = g $ if x then {- ... -} else {- ... -} f x = g $ case x of {- ... -} f x = g $ do {- ... -} }}} However, we programmers are lazy and want to type as few characters as possible. Think about the precious characters we would save if we could write code this way: {{{#!hs f x = g if x then {- ... -} else {- ... -} f x = g case x of {- ... -} f x = g do {- ... -} }}} Wow, our productivity just shot up by a whopping 6%! However, this code looks ugly, noone would ever write this... Right? There has to be some way to catagorize this feature with others people would never use... == Pragmas to the rescue! == The {{{LANGUAGE}}} pragma! Of course! We can add a syntactic extension to enable above syntax! But how should we name it... {{{InlineStatements}}}? {{{InfixStatements}}}? {{{ParenthesizedStatements}}}? {{{IncreasedStatementPrecedence}}}? {{{StatementsWithoutParenthesis}}}? I don't know. I am [1] not the parents who might one day give birth to this feature. I am merely the one who conceptualized it. [1] (probably, unless this request remains unnoticed for another 20 years or so) == How it works == * Implicit parenthesis will be put around ''all'' statements, according to the following rules: 1. Parenthesis will be opened at the start of the statement. 2. Parenthesis will be closed at the end of the statement. The end of the statement is determined by 1. The curly brackets around the statement, or; 2. The indentation of the statement Basically, these are the rule Haskell already uses. Sorry if this is obvious, but some people seemed to get confused, so here's some examples: === (Correct) Examples === Examples are in the form: {{{#!hs implicitParenthesis -- New syntax explicitParenthesis -- Old syntax }}} Call {{{idM :: Monad m => m a -> m a}}} as {{{idM :: IO String -> IO String}}}. {{{#!hs idM do putStrLn "What's my name?" getLine idM (do { putStrLn "What's my name?"; getLine; }) }}} Create function {{{whatsMyName :: Maybe String -> String}}}. {{{#!hs whatsMyName x = id case x of Just name = "My name is " ++ name Nothing = "What's my name?" whatsMyName x = id (case x of { Just name = "My name is " ++ name; Nothing = "What's my name?"; }) }}} Another example using {{{let}}} and {{{if-then-else}}}. {{{#!hs main = putStrLn (++) "I've been tryna work it out... The square root of 69 is 8 something, right? " let eight_something = 8.306623862918075 in if sqrt 69 == eight_something then "Yeah" else "Oh na na" main = putStrLn ((++) "I've been tryna work it out... The square root of 69 is 8 something, right? " (let eight_something = 8.306623862918075 in (if sqrt 69 == eight_something then "Yeah" else "Oh na na")) }}} === Incorrect examples === {{{#!hs f do putStrLn "This won't work..." True f (do { putStrLn "This won't work..."; True; }) }}} {{{#!hs f do putStrLn "This won't work..." True -- Indented by extra space f (do { putStrLn "This won't work..."; True; }) }}} == TL; DR == What there is: {{{#!hs main = when $ do putStrLn "Parenthesis..." main = when True (do putStrLn "Parenthesis...") }}} What I want: {{{#!hs when (doBlocksNeedParenthesis) do putStrLn "This code is invalid." when (doBlocksNeedParenthesis) $ do putStrLn "This code is valid." when (doBlocksNeedParenthesis) (do putStrLn "This code is valid aswell.") when (thisIsImplemented) do putStrLn "These are all equal" when (thisIsImplemented) (do putStrLn "These are all equal") when (thisIsImplemented) $ do putStrLn "These are all equal" }}} -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11706> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #13032: Redundant forcing of Given dictionaries
by GHC 01 Feb '18

01 Feb '18

#13032: Redundant forcing of Given dictionaries -------------------------------------+------------------------------------- Reporter: simonpj | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.0.1 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- If you look at the test case for #13025 you'll see code like {{{ f :: (a~~b) => a -> b -> Bool f d x y = case HEq_sc d of (cobox :: a ~# b) -> True }}} From the Given dictionary `(a~~b)`, the contraint solver generates a given binding, just in case {{{ (cobox :: a~# b) = HEq_sc d }}} But because `cobox` is a ''coercion'' we evaluate this binding strictly, and so the desugarer produces the case expression above. Most redundant Given bindings are let-bound and hence discarded quickly by the occurrence analyser, but GHC doesn't discard cases (termination worries). So the task here is to remove the redundant Given in some way. (Of course, if the equality is actually used, we need to keep it.) NB: See `Note [The equality types story]` in `TysPrim` for the meanagerie of equality types. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13032> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #11195: New pattern-match check can be non-performant
by GHC 01 Feb '18

01 Feb '18

#11195: New pattern-match check can be non-performant -------------------------------------+------------------------------------- Reporter: goldfire | Owner: Type: bug | Status: new Priority: highest | Milestone: Component: Compiler | Version: 7.11 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: -------------------------------------+------------------------------------- In my work in merging Phab:D808 (and originally pointed out by thomie), I ran into a peculiar bug: the new pattern-match check took gobs of memory (~10GB) to check !OptCoercion in the stage-2 compiler. My analysis got only as far as nailing the problem down to `pmTraverse`, which took 80% of the time, and got called roughly a gajillion times. To reproduce, simply compile !OptCoercion (with `-package ghc`) and you'll see your memory usage explode. So that you'll be able to build GHC until this is fixed, I've disable the pattern-match check in that file. There is a good chance that this is caused by an interaction between the pattern-match check and the type=kind code. I'm happy to probe further, but I'd want George's help. NB: This applies only after my patch is merged. Which should be today. For some value of today. Hopefully today's value of today. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11195> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #12665: Make Read instances faster, and make them fail fast
by GHC 01 Feb '18

01 Feb '18

#12665: Make Read instances faster, and make them fail fast -------------------------------------+------------------------------------- Reporter: dfeuer | Owner: Type: feature | Status: new request | Priority: high | Milestone: 8.2.1 Component: Core | Version: 8.0.1 Libraries | Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: Runtime Unknown/Multiple | performance bug Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- At present, the `Read` instances for standard types are generally written ''as though'' `Read` were a typical parser for a programming language. However, it most assuredly is not. Specifically, we currently follow the pattern of lexing and then parsing. Lexing looks at the string and attempts to identify the next token, whatever it may be. But for `Read`, we don't need that. Thanks to the types, we have a very clear sense of what characters we expect to encounter. I've started to sketch out an improvement, which reads `Int` and `Word` around seven times as fast (9.5 times as fast with parentheses and negation), and will fail immediately on something like `read (fix ('a':)) :: Int` rather than going into an infinite loop. To begin with, modify the definition of `paren` thus: {{{#!hs expectP' :: Char -> ReadPrec () expectP' c = lift (expect c) {-# INLINE expectP' #-} paren :: ReadPrec a -> ReadPrec a -- ^ @(paren p)@ parses \"(P0)\" -- where @p@ parses \"P0\" in precedence context zero paren p = do expectP' '(' x <- reset p expectP' ')' return x }}} This allows fast failure when looking for parentheses, so we don't have to scan to the end of the first token (whatever it may be) before concluding that it is not `'('`. Now we can parse `Word` and `Int` very efficiently. I had to specialize earlier than I wanted to convince GHC that I don't want to convert through `Integer`. I'm not sure why the `fromIntegral` rule doesn't fire reliably around here. The code below (temporarily) uses the current definition for base 16, because that's a bit fussy; I'll rewrite it soon. {{{#!hs charDiff :: Char -> Char -> Word charDiff c1 c2 = fromIntegral (ord c1 - ord c2) {-# INLINE charDiff #-} readHexOct :: ReadP Word readHexOct = do _ <- ReadP.char '0' baseId <- lexBaseChar case baseId of Oct -> readBaseP 8 Hex -> L.readHexP {-# INLINE readHexOct #-} data BaseId = Oct | Hex lexBaseChar :: ReadP BaseId lexBaseChar = do { c <- ReadP.get; case c of 'o' -> pure Oct 'O' -> pure Oct 'x' -> pure Hex 'X' -> pure Hex _ -> ReadP.pfail } readWord :: ReadP Word readWord = readNumber (readHexOct ReadP.<++ readBaseP 10) readInt :: ReadP Int readInt = fromIntegral <$> readWord readBaseP :: Integral a => Word -> ReadP a readBaseP !base = do c <- ReadP.get let diff = charDiff c '0' if diff < base then readBaseP' base (fromIntegral diff) else ReadP.pfail {-# INLINE readBaseP #-} readBaseP' :: Integral a => Word -> a -> ReadP a readBaseP' !base !acc0 = ReadP.look >>= go acc0 where go !acc (c:cs) | diff < base = ReadP.get *> go (fromIntegral base * acc + fromIntegral diff) cs where diff = charDiff c '0' go !acc _ = pure acc {-# INLINE readBaseP' #-} readNumber :: Num a => ReadP.ReadP a -> ReadPrec a readNumber p = parens $ do cs <- lift skipSpaces *> look case cs of ('-': _) -> get *> lift (skipSpaces *> (negate <$> p)) _ -> lift $ skipSpaces *> p {-# INLINE readNumber #-} }}} Other `Word` and `Int`-like types can be built on top of that foundation. I haven't yet attempted to deal with other instances, but I think there are probably a lot of opportunities for similar improvements. WARNING: I know very little about parsing, and less about `ReadPrec`. It's conceivable that I've made some semantic errors here, although I don't think I have. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12665> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #10855: GHC rejects code that Haskell 2010 report accepts
by GHC 01 Feb '18

01 Feb '18

#10855: GHC rejects code that Haskell 2010 report accepts -------------------------------------+------------------------------------- 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 Revisions: | -------------------------------------+------------------------------------- According to [https://www.haskell.org/onlinereport/haskell2010/haskellch3.html#x8-220003 The Haskell 2010 Report], the following two modules should compile. But they don't, failing with parse errors: {{{ {-# LANGUAGE Haskell2010 #-} module Bug where bool = - case b of False -> 0; True -> (-1) }}} {{{ {-# LANGUAGE Haskell2010 #-} instance Num (IO a) where negate = id main = - do putStrLn "hi!" }}} Note the unary `-` signs. I'm not sure whether we should complicate the parser to fix these, but we should document the compliance-failure in the manual. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10855> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #10074: Implement the 'Improved LLVM Backend' proposal
by GHC 01 Feb '18

01 Feb '18

#10074: Implement the 'Improved LLVM Backend' proposal -------------------------------------+------------------------------------- Reporter: | Owner: thoughtpolice thoughtpolice | Status: new Type: task | Milestone: 7.12.1 Priority: high | Version: Component: Compiler | Operating System: Unknown/Multiple (LLVM) | Type of failure: None/Unknown Keywords: llvm, | Blocked By: codegen | Related Tickets: Architecture: | Unknown/Multiple | Test Case: | Blocking: | Differential Revisions: Phab:D530 | -------------------------------------+------------------------------------- This is a meta ticket designed to reflect the current implementation status of the 'Improved LLVM Backend' proposal, documented here: https://ghc.haskell.org/trac/ghc/wiki/ImprovedLLVMBackend -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10074> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #12245: Deriving Data at higher kinds
by GHC 01 Feb '18

01 Feb '18

#12245: Deriving Data at higher kinds -------------------------------------+------------------------------------- Reporter: simonpj | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.0.1 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- Here's what [https://mail.haskell.org/pipermail/generics/2016-June/000564.html Lennart Spitzner wanted to do]: {{{ > {-# LANGUAGE StandaloneDeriving #-} > {-# LANGUAGE DeriveDataTypeable #-} > {-# LANGUAGE FlexibleInstances #-} > > import Data.Data ( Data ) > > data Foo f = Foo (f Bool) (f Int) > > deriving instance Data (Foo []) > deriving instance Data (Foo Maybe) }}} Of course you can't derive `Data` for `Foo` because we don't know what `f` is, so Lennart is making multiple instances, one for each instance of `f`. It's a bit clumsy. What we would really like is {{{ deriving instance (forall a. Data => Data (f a)) => Data (Foo f) }}} but we don't have higher order instances yet! So Lennart is manually making two instances. This should work, but he gets {{{ > Main.hs: line 45, column 1: > Multiple declarations of ‘$cr2C’ > Declared at: Main.hs:44:1 > Main.hs:45:1 > Main.hs: line 45, column 1: > Multiple declarations of ‘$tr2D’ > Declared at: Main.hs:44:1 > Main.hs:45:1 }}} -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12245> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #10754: truncate /= double2Int
by GHC 28 Jan '18

28 Jan '18

#10754: truncate /= double2Int -------------------------------------+------------------------------------- Reporter: cblp | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: | Version: 7.10.2 libraries/base | Keywords: truncate, | Operating System: Unknown/Multiple double2Int, rewrite, rule | Architecture: | Type of failure: Incorrect result Unknown/Multiple | at runtime Test Case: | Blocked By: Blocking: | Related Tickets: Differential Revisions: | -------------------------------------+------------------------------------- There is a rule in libraries/base/GHC/Float.hs: {{{#!hs "truncate/Double->Int" truncate = double2Int }}} But it is not true for some values. Particularly, {{{#!hs let infinity = 1/0 :: Double truncate infinity :: Int == 0 double2Int infinity == -9223372036854775808 -- minBound }}} As a result, the value of {{{truncate (1/0 :: Double) :: Int}}} depends on optimization level. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10754> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #12690: Segmentation fault in GHC runtime system under low memory with USE_LARGE_ADDRESS_SPACE
by GHC 27 Jan '18

27 Jan '18

#12690: Segmentation fault in GHC runtime system under low memory with USE_LARGE_ADDRESS_SPACE --------------------------------------+---------------------------------- Reporter: pggiarrusso | Owner: Type: bug | Status: new Priority: high | Milestone: Component: Runtime System | Version: 8.0.1 Keywords: | Operating System: Linux Architecture: x86_64 (amd64) | Type of failure: Runtime crash Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: --------------------------------------+---------------------------------- I have here a ~600MB core dump on GHC 8.0.1 x86_64 linux triggered under low memory (1GB RAM total, no swap, trying to build aeson with optimizations), with a stacktrace blaming the GHC runtime system. Based on code review, it appears that when requesting memory from the OS fails, the error is ignored and GHC segfaults as soon as it writes to the newly "allocated" memory. == Diagnosis == My diagnosis is that osCommitMemory calls the potentially failing `my_mmap` but returns void (hence not propagating failures); it should instead probably use `my_mmap_or_barf` or handle the failure anyhow: https://github.com/ghc/ghc/blob/a6111b8cc14a5dc019e2613f6f634dec4eb57a8a/rt… However, while I'm very convinced that's a bug, I'm only mostly confident that's the culprit, since the segfault happens later; I've just looked for bugs on allocation failures in the code ran shortly before the crash. == Raw data == The core dump is on a VM from the reporter of https://github.com/commercialhaskell/stack/issues/2575, https://github.com/jiakai0419, which graciously gave me access to the VM and bothered reporting in the first place—I'd like to thank her. I'd like to avoid copying the dump here unless strictly needed. == Reproduction == The bug is deterministic on that machine, but it requires a failure in mmap without triggering an OOM, so I'm not 100% sure how easy it is to reproduce but it should be easy on a VM with 1GB RAM and 1 core. Here's the output of `free`: {{{ $ free total used free shared buff/cache available Mem: 1015352 95864 71396 51244 848092 733132 Swap: 0 0 0 }}} Instructions to produce a dump. First, as root: {{{ echo /tmp/core > /proc/sys/kernel/core_pattern }}} that's needed because otherwise the core file is produced in a temporary folder that stack removes. Then (as non-root, root has privileged access to memory), install stack, and run: {{{ ulimit -c unlimited git clone https://github.com/jiakai0419/snowflake.git stack build --verbose }}} Below is some raw data from the core dump. In short, an initializer for newly allocated data segfaults, and gdb tells that the address it's trying to write (bd->start) to can't be accessed, even though it's lower than mblock_high_watermark so it should have been committed (mmap'ed) by getFreshMBlocks (I don't get getReusableMBlocks in detail, but it also calls `osCommitMemory`, so I guess there's the same invariant). The existence of mblock_high_watermark (beyond other reasons) should confirm this is using USE_LARGE_ADDRESS_SPACE. If this diagnosis is correct, the bug exists since USE_LARGE_ADDRESS_SPACE was introduced in https://github.com/ghc/ghc/commit/0d1a8d09f452977aadef7897aa12a8d41c7a4af0, so it's a regression in 8.0.1. {{{ (gdb) bt #0 initMBlock (mblock=0x225c00000) at rts/sm/BlockAlloc.c:676 #1 alloc_mega_group (mblocks=mblocks@entry=1) at rts/sm/BlockAlloc.c:328 #2 0x00007fe650438d98 in allocGroup (n=1) at rts/sm/BlockAlloc.c:379 #3 allocGroup (n=1) at rts/sm/BlockAlloc.c:336 #4 0x00007fe65043ed55 in allocBlock_sync () at rts/sm/GCUtils.c:38 #5 0x00007fe65043ef7d in alloc_todo_block (ws=ws@entry=0x13196b0, size=size@entry=3) at rts/sm/GCUtils.c:330 #6 0x00007fe65043f13a in todo_block_full (size=size@entry=3, ws=0x13196b0) at rts/sm/GCUtils.c:292 #7 0x00007fe65041baed in alloc_for_copy (gen_no=<optimized out>, size=3) at rts/sm/Evac.c:81 #8 copy_tag (tag=2, gen_no=<optimized out>, size=3, src=0x21b353868, info=<optimized out>, p=0x225b13f10) at rts/sm/Evac.c:99 #9 evacuate1 (p=p@entry=0x225b13f10) at rts/sm/Evac.c:596 #10 0x00007fe65041d65c in scavenge_block1 (bd=0x225b004c0) at rts/sm/Scav.c:570 #11 0x00007fe6504415dc in scavenge_find_work () at rts/sm/Scav.c:2040 #12 scavenge_loop1 () at rts/sm/Scav.c:2103 #13 0x00007fe65043cee2 in scavenge_until_all_done () at rts/sm/GC.c:968 #14 0x00007fe65043d75a in GarbageCollect (collect_gen=collect_gen@entry=1, do_heap_census=do_heap_census@entry=rtsFalse, gc_type=gc_type@entry=2, cap=cap@entry=0x7fe650669a80 <MainCapability>) at rts/sm/GC.c:403 #15 0x00007fe65042ff81 in scheduleDoGC (force_major=rtsFalse, task=0x13228a0, pcap=<optimized out>) at rts/Schedule.c:1652 #16 scheduleDoGC (pcap=<optimized out>, task=0x13228a0, force_major=rtsFalse) at rts/Schedule.c:1433 #17 0x00007fe650430c1a in schedule (initialCapability=initialCapability@entry=0x7fe650669a80 <MainCapability>, task=task@entry=0x13228a0) at rts/Schedule.c:551 #18 0x00007fe650431cf8 in scheduleWaitThread (tso=0x20000b388, ret=ret@entry=0x0, pcap=pcap@entry=0x7ffc4ee22c08) at rts/Schedule.c:2361 #19 0x00007fe65042c528 in rts_evalLazyIO (cap=cap@entry=0x7ffc4ee22c08, p=p@entry=0x7895c0, ret=ret@entry=0x0) at rts/RtsAPI.c:500 #20 0x00007fe65042e3a7 in hs_main (argc=96, argv=0x7ffc4ee22dc8, main_closure=0x7895c0, rts_config=...) at rts/RtsMain.c:64 #21 0x0000000000428414 in main () (gdb) info locals bd = 0x225c00100 block = 0x225c04000 <Address 0x225c04000 out of bounds> (gdb) print/x mblock_high_watermark $11 = 0x225d00000 (gdb) print bd->start Cannot access memory at address 0x225c00100 (gdb) print *bd Cannot access memory at address 0x225c00100 }}} -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12690> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #13352: Strange requirement for re-exported duplicate record fields
by GHC 26 Jan '18

26 Jan '18

#13352: Strange requirement for re-exported duplicate record fields -------------------------------------+------------------------------------- Reporter: crockeea | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.0.2 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: -------------------------------------+------------------------------------- In the following example, {{{ module A(A(..)) where data A = A {x::Int} module B(B(..)) where data B = B {x::Bool} {-# LANGUAGE DuplicateRecordFields #-} module C(A(..),B(..)) where import A(A(..)) import B(B(..)) }}} I get an error about `conflicting exports for 'x'`. This doesn't seem right to me: I've got `-XDuplicateRecordFields` exactly where the duplicate occurs. However, I noticed that the if I move the pragma to A.hs, C.hs compiles without error: {{{ {-# LANGUAGE DuplicateRecordFields #-} module A(A(..)) where data A = A {x::Int} module B(B(..)) where data B = B {x::Bool} module C(A(..),B(..)) where import A(A(..)) import B(B(..)) }}} This is bizarre because it is asymmetric (I arbitrarily put the pragma in A.hs, but it need not occur in B.hs), and because no duplicate records exist in module A, but they ''do'' exist in module C, where the pragma isn't needed. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13352> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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