February 2015 - Glasgow-haskell-users

Discovery of source dependencies without --make
by Lars Hupel 13 Dec '15

13 Dec '15

(cross-posted from [haskell-cafe]) Hi, I've got a problem in our – admittedly complex – build process. We're running an automated grading system for student-submitted homework exercises. The compilation proceeds in stages: 1) Compile a library file, marked as trustworthy 2) Compile student submissions using Safe Haskell, able to access a whitelisted set of packages and the previously compiled library files 3) Compile the test suite 4) Link everything together into an executable The invocation is similar to this: ghc -c -outputdir "$OUT" -XTrustworthy Library.hs ghc -c -outputdir "$OUT" -i"$OUT" -XSafe "$SUBMISSION" ghc -c -outputdir "$OUT" -i"$OUT" Test_Suite.hs ghc -outputdir "$OUT" -i"$OUT" -o "$OUT/runner" The second stage works well under the assumption that there's just one single submitted file. If there's more than one, I need to specify them in topological order wrt their module dependencies. Consider two simple modules: A.hs > module A where > > import B > import Library B.hs > module B where Invoking GHC with "A.hs" "B.hs" will fail: A.hs:3:1: Failed to load interface for ‘B’ My naive attempt at solving that problem was to just insert "--make" into the compiler flags. No luck, because according to [1, §4.7.3], "--make" will only look at source files and will ignore the "Library.hi" file from the previous compilation stage. If I give the path to "Library.hs" as well, GHC insists on recompiling it with changed flags ("-XSafe" instead of "-XTrustworthy"). Reading a bit further, I discovered the '-M' flag, however, not only does it output Makefile-formatted output, it also ignores .hi files. Is there any way to get the dependency discovery of '--make' without the rest? I know I could probably make it work if I made a package out of the first build stage, but I really want to avoid that in order to not increase the build complexity even further. Cheers Lars [1] <https://downloads.haskell.org/~ghc/7.6.3/docs/html/users_guide/separate-com…>

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Record Puns/Wildcards
by Ben Franksen 25 Feb '15

25 Feb '15

I just noted that code like my_config = default_config {..} where name = "my project" description = "some longer text" gives me a syntax error, even if I have NamedFieldPuns and RecordWildCards extensions enabled. It seems that these extensions only work for record constructors and not for updating values. Is there a special reason puns/wildcards are not allowed in record updates? Cheers Ben -- "There are two ways of constructing a software design: One way is to make it so simple that there are obviously no deficiencies and the other way is to make it so complicated that there are no obvious deficiencies. The first method is far more difficult." ― C.A.R. Hoare

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Re: [Haskell-cafe] PowerPC variants (GHC 7.8.3)
by Carter Schonwald 25 Feb '15

25 Feb '15

Hey Jon, I do know that some of the GHC dev team is probably best equipped to answer your question, i'm cc'ing the ghc dev and users lists so they can jump in and help! -Carter On Wed, Feb 25, 2015 at 7:11 AM, Jon Schneider <haskell(a)jschneider.net> wrote: > Good morning, > > We have a product with an MPC8544E we might want to target. Also known as > PowerQUICC and e500v2. The ABI is gnuspe rather than gnueabi. > > I have built a powerpc---ghc cross compiler but the thing stopping "hello > world" is a SIGILL happening in StgCRun.c where the stfd and lfd > instructions are used to stash registers. The e500 has different > instructions but that cannot be dropped in trivially because of offset > encoding Commenting these out fixes "hello world" though this surely only > scratches the surface of what would need doing. > > I notice that whereas ARMv5, v6 and v7 appear to be catered for along with > various knobs and whistles there appears to be no such thing for PowerPC. > > Is anybody else out there working on this area ? > > Jon > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe >

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Re: Proposal: Turn on ScopedTypeVariables by default
by Roman Cheplyaka 24 Feb '15

24 Feb '15

On 24/02/15 14:46, Herbert Valerio Riedel wrote: > On 2015-02-23 at 18:45:20 +0100, David Feuer wrote: >> I know this will be controversial, because it can break (weird) code and >> because it's not Haskell 2010, but hey, you can't make brain salad without >> breaking a few heads. > > Are you suggesting enabling -XScopedTypeVariables for -XHaskell98 and > -XHaskell2010? or rather for the default when neither of those two modes > is explicitly requested? > > Just be warned though: this is somewhat of a trick-question... :-) Maybe in addition to -XHaskell98 and -XHaskell2010 there should also be -XGhcHaskell. ghc already deviates from the standard by default (e.g. -XNondecreasingIndentation). OTOH we probably don't want end up with -fglasgow-exts once again. Roman

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Proposal: Turn on ScopedTypeVariables by default
by David Feuer 23 Feb '15

23 Feb '15

I know this will be controversial, because it can break (weird) code and because it's not Haskell 2010, but hey, you can't make brain salad without breaking a few heads. ScopedTypeVariables is just awesome for two fundamental reasons: 1. It lets you write type signatures for more things. 2. It lets you write more precise type signatures for many things. As a consequence of those two, 3. It helps you get much better error messages from the type checker. And for all that, 4. It's really easy to use. What do other people think?

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ANNOUNCE: GHC 7.10.1 Release Candidate 2
by Austin Seipp 21 Feb '15

21 Feb '15

We are pleased to announce the second release candidate for GHC 7.10.1: https://downloads.haskell.org/~ghc/7.10.1-rc2/ This includes the source tarball and bindists for 64bit/32bit Linux and Windows. Binary builds for other platforms will be available shortly. (CentOS 6.5 binaries are not available at this time like they were for 7.8.x). These binaries and tarballs have an accompanying SHA256SUMS file signed by my GPG key id (0x3B58D86F). We plan to make the 7.10.1 release sometime in February of 2015. Please test as much as possible; bugs are much cheaper if we find them before the release! -- Regards, Austin Seipp, Haskell Consultant Well-Typed LLP, http://www.well-typed.com/

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type checker plugin success depends on whether an expression is manually inlined
by adam vogt 19 Feb '15

19 Feb '15

Hello list, The following file compiles with my plugin. It makes a data family HList have role representational in a way that I believe is safe: https://github.com/aavogt/HListPlugin/blob/master/ex/Coerce.hs#L19 I expect the highlighted line to be acceptable. However, it seems that the plugin never sees anything from line 19, when I uncomment it. Is there something I can do to make that L19 work? Is this a known or intentional limitation of type checker plugins? Thanks, Adam

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ApplicativeDo
by Thomas Bereknyei 19 Feb '15

19 Feb '15

There is a library for TH: http://hackage.haskell.org/package/applicative-quoters-0.1.0.8 (broken at the moment) There is a proposal at: https://ghc.haskell.org/trac/ghc/wiki/ApplicativeDo Now with AMP, is this worth revisiting? Why or why not? -Tom

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Re: ANNOUNCE: GHC 7.10.1 Release Candidate 2
by Takenobu Tani 17 Feb '15

17 Feb '15

Hi Herbert, Thank you for your directions. I will send the pull-request after checking the file. Thank you, Takenobu 2015-02-17 22:02 GMT+09:00 Herbert Valerio Riedel <hvriedel(a)gmail.com>: > On 2015-02-17 at 13:47:50 +0100, Takenobu Tani wrote: > > I modified System/Process/Internals.hs locally and build on MinGW 32bit. > > Then I was successful to build on 32bit Windows 7. > > > > Shall I write a bug report on trac or any? or ghc7.10.1.rc2 will not > > support 32 bit Windows? > > Please file a pull-request at https://github.com/haskell/process > > I'm somewhat surprised this wasn't noticed before(?) >

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type checker plugin does not affect inferred type signatures
by adam vogt 16 Feb '15

16 Feb '15

Hello, Using ghc-7.10 rc1, I am trying to write a type checker plugin that adds wanted constraint which helps ghc to infer more types. However, it seems that the wanted constraints I add don't get added to the inferred type of the declaration, so that I get a type error like: a.hs:1:1: Warning: Could not deduce (SameLength y x) arising from an application from the context (HLength x ~ HLength y) bound by the inferred type of p :: (HLength x ~ HLength y) => Proxy '(y, x) at a.hs:11:1-69 I think ghc should be able to figure out p :: (SameLength x y, HLength x ~ HLength y) => Proxy '(x,y). The code is self-contained: git clone https://github.com/aavogt/HListPlugin cd HListPlugin/ex make Is this approach supposed to be possible, or am I supposed to rewrite things such that I only produce CtGivenS from the plugin? Regards, Adam

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