April 2018 - ghc-tickets

[GHC] #8089: Implementation of GHC.Event.Poll.poll is broken due to bad coercion
by GHC 16 May '18

16 May '18

#8089: Implementation of GHC.Event.Poll.poll is broken due to bad coercion -------------------------------------------+------------------------------- Reporter: merijn | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: libraries/base | Version: 7.6.3 Keywords: patch | Operating System: Architecture: Unknown/Multiple | Unknown/Multiple Difficulty: Easy (less than 1 hour) | Type of failure: Runtime Blocked By: | crash Related Tickets: | Test Case: | Blocking: -------------------------------------------+------------------------------- Due to an unsafe coercion in GHC.Event.Poll.poll the implementation has a bug that on some platforms (.e.g. linux) results in subtly wrong semantics and on other platforms (e.g. OS X) downright crashes the runtime. The offending line is line 95 in GHC/Event/Poll.hsc: {{{ c_poll ptr (fromIntegral len) (fromIntegral (fromTimeout tout)) }}} `fromTimeout` returns a timeout in seconds as a value of type Int. `c_poll` takes a timeout in seconds as a value of type CInt. If the max value of Int is larger than that of CInt, this coercion results in an overflow, producing a negative value. The faulty semantics are in turn caused by passing this negative value as timeout to `c_poll`. This overflow happens on many 64bit platforms where GHC's Int is 64bit, but CInt is often 32bit. The result of passing a negative value to poll() depends on the platform, but is (on all platforms I checked) different from the desired semantics! On linux any negative timeouts are treated as infinite, this means that a poll() with a timeout that should be finite results in an infinitely blocking poll on linux! On OSX any negative timeout other than -1 (which is treated as infinity) is considered an invalid parameter, returning an error and crashing the runtime. A minimal example reproducing the error (on systems where `fromIntegral (maxBound :: Int) > (fromIntegral (maxBound :: CInt) :: Integer)`): {{{ import Control.Concurrent main = threadDelay maxBound }}} Using 64bit GHC on OSX this produces an immediate runtime crash. The attached patch solves the issue by comparing the Int timeout against the max value of CInt and, if necessary, looping multiple `c_poll` calls (with a timeout equal to `maxBound :: CInt`) if they return `0` (which indicates the timeout has expired with no events occurring). See also: https://github.com/merijn/packages- base/commit/6d8ea02a3f6a2cdb82a9ad786a8c4db780b92091 -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8089> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #12012: Socket operations on Windows check errno instead of calling WSAGetLastError()
by GHC 15 May '18

15 May '18

#12012: Socket operations on Windows check errno instead of calling WSAGetLastError() -------------------------------------+------------------------------------- Reporter: enolan | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Core | Version: Libraries | Keywords: | Operating System: Windows Architecture: | Type of failure: Incorrect result Unknown/Multiple | at runtime Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- Winsock doesn't set errno, but it is checked in `blockingReadRawBufferPtr` and `blockingWriteRawBufferPtr` (both are in `GHC.IO.FD`). I the same thing happens in the non threaded RTS too, but that's in terms of primops and I don't understand it very well. The upshot here is that every error message originating from Winsock is wrong. Nobody noticed since any error used to just crash your program (#12010). Here's some MinGW documentation http://oldwiki.mingw.org/index.php/sockets and something from MSDN https://msdn.microsoft.com/en- us/library/windows/desktop/ms740121%28v=vs.85%29.aspx -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12012> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #14697: Redundant computation in fingerprintDynFlags when compiling many modules
by GHC 15 May '18

15 May '18

#14697: Redundant computation in fingerprintDynFlags when compiling many modules -------------------------------------+------------------------------------- Reporter: niteria | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 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: -------------------------------------+------------------------------------- I profiled a build of a production code base with thousands of modules and computing `fingerprintDynFlags` is `7%` of time and `14%` of allocations. Here's a synthetic test case inspired by what I observed: {{{ SIZE=1000 for i in $(seq -w 1 $SIZE); do echo "module A$i where" > A$i.hs echo "data A$i = A$i" >> A$i.hs done }}} This generates a 1000 modules each with one datatype. Compiling them with: {{{ inplace/bin/ghc-stage2 A*.hs -optP-D__F{1..10000}__ }}} results in `fingerprintDynFlags` being the top cost centre in the profile. AFAICT there's only one module dependent piece that goes into computing `fingerprintDynFlags` and the rest is the same between those 1000 modules. Now, why would I have so many preprocessor flags? This is how the Buck build system currently works. If a Haskell library depends on a C++ library then the GHC invocation gets the C++ library's directory as include path (`-optP-I -optP-I some/library/path`). This can grow quite big. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14697> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #12463: SPECIALIZABLE pragma?
by GHC 15 May '18

15 May '18

#12463: SPECIALIZABLE pragma? -------------------------------------+------------------------------------- Reporter: bgamari | Owner: Type: feature | Status: new request | Priority: low | Milestone: Component: Compiler | Version: 8.0.1 Keywords: Inlining | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- Currently it is common practice for library authors to use the `INLINEABLE` pragma to make it more likely that a polymorphic function should get an unfolding in the module's interface file to ensure that GHC is able to specialize. While in practice this works reasonably well, it's not really saying what we often mean: we don't want to inline, we really just want GHC to behave like each use-site's module has a `SPECIALISE` pragma for each concrete type that the function is used at. For instance, consider, {{{#!hs module ALibrary where aLibraryFunction :: AClass a => a -> a aLibraryFunction = {- some large expression involving methods of AClass -} module SomeUser where import ALibrary aUser :: Int -> Int aUser = {- some large expression involving aLibraryFunction -} }}} Ideally, we would want GHC to take and produce one specialized version of `aLibraryFunction` for every concrete type which it is used at. However, without an `INLINEABLE` function, GHC won't even consider producing an unfolding for `aLibraryFunction` due to its size. If we include an `INLINEABLE` pragma (as most performance-aware authors would do) then we can convince GHC to produce an unfolding, but only at the expense of lowering its inlining cost as well. This is unfortunate since we never wanted GHC to inline; merely to specialize. This is issue especially prevalent in code using MTL-style effects, where we have ubiquitous overloading of very frequently-used functions (e.g. bind). Really what we want in this case is a way of indicating to GHC that a function shouldn't be inlined (use-sites replaced with the body of the function), but rather that GHC should try hard to specialize away particular type variables. This might look like, {{{#!hs aLibraryFunction :: AClass a => a -> a aLibraryFunction = {- some large expression involving methods of AClass -} {-# SPECIALIZE forall a. LibraryFunction a #-} }}} This would request that GHC would keep an inlining around and produce a specialized version of `aLibraryFunction` every time it saw a concrete instantiation of `a`. Moreover, it would try to minim -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12463> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #15067: When Typeable and unboxed sums collide, GHC panics
by GHC 14 May '18

14 May '18

#15067: When Typeable and unboxed sums collide, GHC panics -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: 8.6.1 Component: Compiler | Version: 8.2.2 Keywords: Typeable, | Operating System: Unknown/Multiple UnboxedSums | Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- This program is enough to send GHC into a tizzy: {{{#!hs {-# LANGUAGE UnboxedSums #-} module Bug1 where import Type.Reflection floopadoop :: TypeRep (# Bool | Int #) floopadoop = typeRep }}} {{{ $ ghc Bug.hs [1 of 1] Compiling Bug1 ( Bug.hs, Bug.o ) ghc: panic! (the 'impossible' happened) (GHC version 8.4.1 for x86_64-unknown-linux): tyConRep (#|#) Call stack: CallStack (from HasCallStack): callStackDoc, called at compiler/utils/Outputable.hs:1150:37 in ghc:Outputable pprPanic, called at compiler/deSugar/DsBinds.hs:1314:5 in ghc:DsBinds }}} If you use an unboxed sum data constructor, you can get a different panic: {{{#!hs {-# LANGUAGE PartialTypeSignatures #-} {-# LANGUAGE TemplateHaskell #-} {-# LANGUAGE TypeInType #-} {-# LANGUAGE UnboxedSums #-} module Bug2 where import Language.Haskell.TH import Type.Reflection type Fweemp = $(conT (unboxedSumDataName 1 2)) doopafloop :: _ => TypeRep Fweemp doopafloop = typeRep }}} {{{ $ ghc Bug2.hs [1 of 1] Compiling Bug2 ( Bug2.hs, Bug2.o ) GHC error in desugarer lookup in Bug2: Can't find interface-file declaration for variable $tc'(#_|#) Probable cause: bug in .hi-boot file, or inconsistent .hi file Use -ddump-if-trace to get an idea of which file caused the error ghc: panic! (the 'impossible' happened) (GHC version 8.4.1 for x86_64-unknown-linux): initDs }}} -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15067> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #14068: Turn tail-recursive functions into recursive jointpoints
by GHC 14 May '18

14 May '18

#14068: Turn tail-recursive functions into recursive jointpoints -------------------------------------+------------------------------------- Reporter: nomeata | 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: -------------------------------------+------------------------------------- The function {{{ let f x y = case y of A -> f x' y' B -> e2 C -> e3 in g f }}} is not turned into a recursive join point, because the call to `f` is not in tail call position. But the recursive calls are, and these matter performance-wise! Hence, it would be beneficial to turn this into {{{ let f x y = joinrec $j x y = case x y of A -> $j x' y' B -> e2 C -> e3 in $j x y in g f }}} This has the additional effect that now `f` is no longer recursive and may get inlined. This came up in #13966, and might go well with #14067. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14068> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #10762: On Windows, out-of-codepage characters can cause GHC build to fail
by GHC 14 May '18

14 May '18

#10762: On Windows, out-of-codepage characters can cause GHC build to fail -----------------------------------------+--------------------------------- Reporter: snoyberg | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.10.2 Keywords: | Operating System: Windows Architecture: x86_64 (amd64) | Type of failure: None/Unknown Test Case: | Blocked By: Blocking: | Related Tickets: Differential Revisions: | -----------------------------------------+--------------------------------- You can see where this hit us recently on stack with issues [https://github.com/commercialhaskell/stack/issues/738 738] and [https://github.com/commercialhaskell/stack/issues/734 734]. To demonstrate, I'm attaching a UTF-8 encoded Haskell program with some Hebrew characters, and some warnings. The contents of that file are: {{{#!hs module Main ( main , שלום ) where main :: IO () main = putStrLn שלום שלום = "shalom" }}} If I first set my codepage to 65001 (UTF-8), everything works as expected: {{{ C:\Users\Michael\Desktop>chcp 65001 Active code page: 65001 C:\Users\Michael\Desktop>ghc -fforce-recomp -Wall -ddump-hi -ddump-to-file shalom.hs [1 of 1] Compiling Main ( shalom.hs, shalom.o ) shalom.hs:9:1: Warning: Top-level binding with no type signature: שלום :: [Char] Linking shalom.exe ... }}} However, if I set my codepage to 437 (US), both the warnings sent to the console, and the .hi dump file, cause GHC to exit prematurely: {{{ C:\Users\Michael\Desktop>chcp 437 Active code page: 437 C:\Users\Michael\Desktop>ghc -fforce-recomp -Wall shalom.hs [1 of 1] Compiling Main ( shalom.hs, shalom.o ) shalom.hs:9:1: Warning: Top-level binding with no type signature: <stderr>: commitBuffer: invalid argument (invalid character) }}} {{{ C:\Users\Michael\Desktop>chcp 437 Active code page: 437 C:\Users\Michael\Desktop>ghc -fforce-recomp -ddump-hi -ddump-to-file shalom.hs [1 of 1] Compiling Main ( shalom.hs, shalom.o ) shalom.dump-hi: commitBuffer: invalid argument (invalid character) }}} At the very least, I would argue that -ddump-to-file should always dump to the output files as UTF-8, as this is the most useful for tooling. Beyond that, there are a few options here: * Have all output- including to the console- go out as UTF-8. This may not play terribly nicely with consoles without setting the output codepage. * Provide a command line option or environment variable to specify "output as UTF-8." * More radical: change the default way that all Handles work so that UTF-8 is the default, instead of paying attention to code pages and environment variables. Honestly, this is my preference, but it's a bigger discussion than this one bug. The workaround we've implemented in stack for now is setting the codepage to 65001 for the console while running stack. This is not ideal, since this is essentially a global setting for the entire console. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10762> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #15021: ghc-pkg list crashes on Windows when unicode character is in the path
by GHC 14 May '18

14 May '18

#15021: ghc-pkg list crashes on Windows when unicode character is in the path ----------------------------------------+--------------------------------- Reporter: nh2 | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: 8.6.1 Component: ghc-pkg | Version: 8.2.2 Keywords: | Operating System: Windows Architecture: Unknown/Multiple | Type of failure: None/Unknown Test Case: | Blocked By: Blocking: | Related Tickets: #10762 Differential Rev(s): | Wiki Page: ----------------------------------------+--------------------------------- Below I will attach examples of how `ghc-pkg list` crashes with {{{ <stdout>: commitBuffer: invalid argument (invalid character) }}} when the user name contains a non-ASCII character (in my case `日`). I don't think it's important what the user name is, but more likely whether or not `日` is in some path (probably the path to the package database). The issue is trivially reproducible (I tried on Windows 10, and Server 2016 as deployed by AWS) with cmd.exe and PowerShell. In the below, `chcp 20127` sets the code page to US-ASCII, and `chcp 65001`, sets it to UTF-8. This per-terminal / per-process setting is different from the system locale (as can be set [https://www.java.com/en/download/help/locale.xml this way]). The the behaviour of different combinations of system locale and code page: * English (US) system locale * `chcp 437` (English default): **crash** * `chcp 65001` (UTF-8): no crash * `chcp 20127` (ASCII): **crash** * Japanese system locale * `chcp 932` (Japanese default): no crash * `chcp 65001` (UTF-8): no crash * `chcp 20127` (ASCII): **crash** In none of these situatins should `ghc-pkg list` crash. It breaks all the build tools; from conversatins on the `haskell-jp` Slack, I have learned that Japanese users have learned to avoid unicode in their user names due to this issue. I believe the fix to `ghc-pkg` should be similar to [https://github.com/ghc/ghc/commit/1b56c40578374a15b4a2593895710c68b0e2a717 this fix for `ghc`] (#10762), where the encoding of the stdout `Handle` is set to UTF-8. ---- With English (US) system locale, as copy-pasted from PowerShell: {{{ Windows PowerShell Copyright (C) 2016 Microsoft Corporation. All rights reserved. PS C:\Users\日> chcp Active code page: 437 PS C:\Users\日> C:\Users\日 \AppData\Local\Programs\stack\x86_64-windows\ghc-8.2.2\bin\ghc-pkg.EXE list C:\Users\ghc-pkg.EXE: <stdout>: commitBuffer: invalid argument (invalid character) PS C:\Users\日> chcp 65001 Active code page: 65001 PS C:\Users\日> C:\Users\日 \AppData\Local\Programs\stack\x86_64-windows\ghc-8.2.2\bin\ghc-pkg.EXE list C:\Users\日 \AppData\Local\Programs\stack\x86_64-windows\ghc-8.2.2\lib\package.conf.d Cabal-2.0.1.0 Win32-2.5.4.1 array-0.5.2.0 base-4.10.1.0 binary-0.8.5.1 bytestring-0.10.8.2 containers-0.5.10.2 deepseq-1.4.3.0 directory-1.3.0.2 filepath-1.4.1.2 (ghc-8.2.2) ghc-boot-8.2.2 ghc-boot-th-8.2.2 ghc-compact-0.1.0.0 ghc-prim-0.5.1.1 ghci-8.2.2 haskeline-0.7.4.0 hoopl-3.10.2.2 hpc-0.6.0.3 integer-gmp-1.0.1.0 pretty-1.1.3.3 process-1.6.1.0 rts-1.0 template-haskell-2.12.0.0 time-1.8.0.2 transformers-0.5.2.0 xhtml-3000.2.2 PS C:\Users\日> chcp 20127 Active code page: 20127 PS C:\Users\日> C:\Users\日 \AppData\Local\Programs\stack\x86_64-windows\ghc-8.2.2\bin\ghc-pkg.EXE list C:\Users\ghc-pkg.EXE: <stdout>: commitBuffer: invalid argument (invalid character) }}} With Japanese system locale, as copied from PowerShell: {{{ Windows PowerShell Copyright (C) 2016 Microsoft Corporation. All rights reserved. PS C:\Users\日> chcp Active code page: 932 PS C:\Users\日> C:\Users\日 \AppData\Local\Programs\stack\x86_64-windows\ghc-8.2.2\bin\ghc-pkg.exe list C:\Users\日 \AppData\Local\Programs\stack\x86_64-windows\ghc-8.2.2\lib\package.conf.d Cabal-2.0.1.0 Win32-2.5.4.1 array-0.5.2.0 base-4.10.1.0 binary-0.8.5.1 bytestring-0.10.8.2 containers-0.5.10.2 deepseq-1.4.3.0 directory-1.3.0.2 filepath-1.4.1.2 (ghc-8.2.2) ghc-boot-8.2.2 ghc-boot-th-8.2.2 ghc-compact-0.1.0.0 ghc-prim-0.5.1.1 ghci-8.2.2 haskeline-0.7.4.0 hoopl-3.10.2.2 hpc-0.6.0.3 integer-gmp-1.0.1.0 pretty-1.1.3.3 process-1.6.1.0 rts-1.0 template-haskell-2.12.0.0 time-1.8.0.2 transformers-0.5.2.0 xhtml-3000.2.2 PS C:\Users\日> chcp 65001 Active code page: 65001 PS C:\Users\日> C:\Users\日 \AppData\Local\Programs\stack\x86_64-windows\ghc-8.2.2\bin\ghc-pkg.exe list C:\Users\日 \AppData\Local\Programs\stack\x86_64-windows\ghc-8.2.2\lib\package.conf.d Cabal-2.0.1.0 Win32-2.5.4.1 array-0.5.2.0 base-4.10.1.0 binary-0.8.5.1 bytestring-0.10.8.2 containers-0.5.10.2 deepseq-1.4.3.0 directory-1.3.0.2 filepath-1.4.1.2 (ghc-8.2.2) ghc-boot-8.2.2 ghc-boot-th-8.2.2 ghc-compact-0.1.0.0 ghc-prim-0.5.1.1 ghci-8.2.2 haskeline-0.7.4.0 hoopl-3.10.2.2 hpc-0.6.0.3 integer-gmp-1.0.1.0 pretty-1.1.3.3 process-1.6.1.0 rts-1.0 template-haskell-2.12.0.0 time-1.8.0.2 transformers-0.5.2.0 xhtml-3000.2.2 PS C:\Users\日> chcp 20127 Active code page: 20127 PS C:\Users\日> C:\Users\日 \AppData\Local\Programs\stack\x86_64-windows\ghc-8.2.2\bin\ghc-pkg.exe list C:\Users\ghc-pkg.exe: <stdout>: commitBuffer: invalid argument (invalid character) }}} -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15021> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #14875: -ddump-splices pretty-printing oddities with case statements
by GHC 14 May '18

14 May '18

#14875: -ddump-splices pretty-printing oddities with case statements -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.2.2 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: Debugging Unknown/Multiple | information is incorrect Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- The latest installment in "Ryan finds minor bugs in `-ddump-splices`". Take this program: {{{#!hs {-# LANGUAGE ScopedTypeVariables #-} {-# LANGUAGE TemplateHaskell #-} {-# OPTIONS_GHC -ddump-splices #-} module Bug where $([d| f :: Bool -> Bool f x = case x of (True :: Bool) -> True (False :: Bool) -> False g :: Bool -> Bool g x = (case x of True -> True False -> False) :: Bool |]) }}} Compiling this gives: {{{ $ /opt/ghc/8.2.2/bin/ghci Bug.hs GHCi, version 8.2.2: http://www.haskell.org/ghc/ :? for help Loaded GHCi configuration from /home/rgscott/.ghci [1 of 1] Compiling Bug ( Bug.hs, interpreted ) Bug.hs:(6,3)-(15,6): Splicing declarations [d| f_a1sB :: Bool -> Bool f_a1sB x_a1sD = case x_a1sD of (True :: Bool) -> True (False :: Bool) -> False g_a1sC :: Bool -> Bool g_a1sC x_a1sE = (case x_a1sE of True -> True False -> False) :: Bool |] ======> f_a49Z :: Bool -> Bool f_a49Z x_a4a0 = case x_a4a0 of True :: Bool -> True False :: Bool -> False g_a49Y :: Bool -> Bool g_a49Y x_a4a1 = case x_a4a1 of True -> True False -> False :: Bool }}} Neither the `-ddump-splices` output for `f` nor `g` parse are legal Haskell: * In `f`, GHC fails to parenthesize the pattern signatures `True :: Bool` and `False :: Bool`. * In `g`, GHC fails to parenthesize the `case` expression which has an explicit `Bool` signature. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14875> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #15055: ghci - improve error on hidden package
by GHC 14 May '18

14 May '18

#15055: ghci - improve error on hidden package -------------------------------------+------------------------------------- Reporter: akfp | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: 8.6.1 Component: GHCi | Version: 8.2.2 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: -------------------------------------+------------------------------------- A user boldly types: {{{ > import Data.Foo <no location info>: error: Could not find module ‘Data.Foo’ It is a member of the hidden package ‘package-foo-0.1.0.0@package- foo-0.1.0.0-5Itxx5SAgKEAspB2MHVKqi’. }}} The above error message is rather useless for a newcomer to ghci. After lots of googling, the user figures that `:set -v -package package-foo` seems to solve it. The original error should be: {{{ > import Data.Foo <no location info>: error: Could not find module ‘Data.Foo’ It is a member of the hidden package ‘package-foo-0.1.0.0@package- foo-0.1.0.0-5Itxx5SAgKEAspB2MHVKqi’. Try :set -package package-foo to make it visible. }}} Or something like that. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15055> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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