November 2004 - Glasgow-haskell-users

Implicit parameter constraints not inferred
by Benjamin Franksen 06 Nov '04

06 Nov '04

To be fair, the ghc docs do not mention constraint inference for implicit parameters. OTOH they do not say that signatures must be given explicitly if I use implicit parameters. However, I always thought that this would be ok: > main = let ?b = True in use_b > --use_b :: (?g::Bool) => IO () > use_b = print ?b It isn't: ghc -fimplicit-params says Unbound implicit parameter (?b :: a) arising from use of implicit parameter `?b' at TestBug.hs:4 In the first argument of `print', namely `?b' In the definition of `use_b': use_b = print ?b It works if I uncomment the signature. I also tried > use_b = print (?b :: Bool) but to no avail. Using ghc-6.2.2, btw. My question: Is this as it should be or is it a bug? Ben

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Building the add-ons in libraries/...
by Peter Simons 06 Nov '04

06 Nov '04

The CVS version of the libraries tree comes with all kinds of neat and dangerously unstable code I'd like to use, particularly the "arrows" subdirectory. Now I wonder: Is there some "build.mk" magic I could perform to tell GHC to build these libraries as part of my normal build? So that I get them (and the documentation) installed automatically when I rebuild GHC? Peter P. S.: And while I am at it ... the build currently fails with this error on my machine: ==fptools== make all -wr; in /usr/local/src/ghc-current/happy/doc ------------------------------------------------------------------------ rm -f -rf happy/ /usr/bin/xsltproc --stringparam base.dir happy/ \ --stringparam use.id.as.filename 1 \ --stringparam html.stylesheet fptools.css \ --stringparam toc.section.depth 3 --stringparam section.autolabel 1 \ --stringparam section.label.includes.component.label 1 \ /html/chunk.xsl happy.xml warning: failed to load external entity "/html/chunk.xsl" cannot parse /html/chunk.xsl make[2]: *** [happy/index.html] Error 4 make[1]: *** [all] Error 1 make[1]: Leaving directory `/usr/local/src/ghc-current/happy' make: *** [/usr/local/src/ghc-current/happy/src/happy-inplace] Error 2 make: *** Deleting file `/usr/local/src/ghc-current/happy/src/happy-inplace' Looks like there is a definition path missing somewhere.

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getUserEntryForName weirdness
by Peter Simons 06 Nov '04

06 Nov '04

Is anyone else seeing this on his system? getUserEntryForName [] >>= print . userName "wasabi" "wasabi" happens to be the last entry in the /etc/passwd file, and that is what I get every time I query for an user that doesn't exist. The source code promises an exception, but I don't get one. Peter

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proposal for ghc-pkg to use a directory of .conf files
by Duncan Coutts 06 Nov '04

06 Nov '04

Hi, It has been common in recent years since the widespread use of package management systems to break up configuration / settings files that are used by several packages into a directory of individual files rather than modifying a global file. The advantage of doing this is that it makes things easier for the package managers. Each individual file can belong to the appropriate package and so instead of having to execute registration/unregistration actions on install/uninstall it's just another file to add/remove. The other advantage is that there is no longer a global file that needs to be modified which means uninstallation is cleaner (most package managers do not remove files that were modified after installation). So the idea is that in addition to a single $HC-LIB/package.conf file you have a directory of package files $HC-LIB/package.conf.d/*.conf Obviously, the package chasing code needs to be tweaked slightly to slurp in a whole directory of .conf files and $HC-pkg would want to be able to use a directory as an installed package source. I had a peak at the code in ghc for dealing with .conf files. I'd guess it'd be best ok just check if the specified package file is a directory or an ordinary file and act accordingly. Ie you could say "-package-conf packages/" and ghc could by default read from $HC-LIB/package.conf and $HC-LIB/package.conf.d/ or something. I can knock up a proof of concept patch if anyone thinks this is a good idea. It should be totally backward compatible, it's ok to use both, but ditro packagers might like to enforce a policy of using a directory of package files for external libraries. Duncan

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Re: [Haskell-cafe] Re: Double -> CDouble, realToFrac doesn't work
by MR K P SCHUPKE 05 Nov '04

05 Nov '04

>My guess is because irrationals can't be represented on a discrete computer Well, call it arbitrary precision floating point then. Having built in Integer support, it does seem odd only having Float/Double/Rational... Keean. ..

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RE: Double -> CDouble, realToFrac doesn't work
by Simon Peyton-Jones 05 Nov '04

05 Nov '04

| Hmmm, this is a little bit of a dark corner in the H98 report, but there are | probably other people on this list who know better than me. :-] The problem | is that 'realToFrac' is defined as 'fromRational . toRational', so the first | question is: How is 'toRational' supposed to handle NaN? One thing coming to | my mind is '0 :% 0', but this is normally a value which can't be constructed. | So the next question is: Would this be allowed? +Infinity and -Infinity could | be represented similarly then ('1 :% 0' and '(-1) :% 0'), and 'fromRational' | could handle these values specially. But I can't believe that this has been | discussed for the first time. SPJ? Malcolm? I don't recall a discussion about this, and a quick search in my mail archive didn't turn up anything except the enclosed from George Russell. I'm pretty ignorant about the dark corners of numerics, but it does seem bad that passing through Rational loses information. Perhaps the Report should specify normalised representations for +Inf, -Inf and NaN as Rationals (1:%0, -1:%0, and 0:%0 seem like plausible candidates). If someone wants to try this out, and send us a patch for the Rational library, we could incorporate it. And so far as the report goes, perhaps the Errata could contain a note identifying the issue, and suggesting a solution. It's a bit late to *specify* a solution unless we are really sure about it. Simon -----Original Message----- From: George Russell [mailto:ger@informatik.uni-bremen.de] Sent: 25 February 2000 10:19 To: glasgow-haskell-users(a)haskell.org Subject: Floating-point nitpicking: floor(Inf) and floor(NaN) floor(Inf) and floor(NaN) do not appear to be defined in Standard Haskell. (They both come down to "properFraction" which is only defined for Ratio.) This differs from (for example) the Standard ML Basis Library, where it is specified that floor(Int) should raise Overflow and floor(NaN) should raise Domain. Hence Hugs and GHC do different things. Hugs returns floor(Inf) = 0 and floor(NaN) = 0 GHC returns floor(Inf) = very very large integer and floor(NaN) = even larger integer. (This is because the GHC implementation of properFraction simply ignores the case of Inf/NaN and treats the artificially high exponent encoded in those floating-point numbers as if it were a real one.) My own opinion is that Standard ML is right here and that floor(x) should raise an exception (In Haskell terms, fail) when x does not correspond to a real number. | -----Original Message----- | From: glasgow-haskell-users-bounces(a)haskell.org [mailto:glasgow-haskell-users- | bounces(a)haskell.org] On Behalf Of Sven Panne | Sent: 08 August 2004 17:02 | To: Hal Daume III | Cc: GHC Users Mailing List; Malcom Wallace | Subject: Re: Double -> CDouble, realToFrac doesn't work | | Hal Daume III wrote: | > [...] | > Prelude Foreign.C> (0 :: CDouble) / 0 | > NaN | > Prelude Foreign.C> (0 :: Double) / 0 | > NaN | > Prelude Foreign.C> realToFrac ((0 :: Double) / 0) :: CDouble | > -Infinity | > | > yikes! the NaN got turned into a -Infinity!!! | > | > aside from manually checking for 'strange' Double/CDouble values and | > wrapping realToFrac, is there a better way? also, does this count as a | > bug? | | Hmmm, this is a little bit of a dark corner in the H98 report, but there are | probably other people on this list who know better than me. :-] The problem | is that 'realToFrac' is defined as 'fromRational . toRational', so the first | question is: How is 'toRational' supposed to handle NaN? One thing coming to | my mind is '0 :% 0', but this is normally a value which can't be constructed. | So the next question is: Would this be allowed? +Infinity and -Infinity could | be represented similarly then ('1 :% 0' and '(-1) :% 0'), and 'fromRational' | could handle these values specially. But I can't believe that this has been | discussed for the first time. SPJ? Malcolm? | | When you compile the stuff above with optimizations on, you get what you've | expected, thanks to RULES which shortcut the route via Rational completely. | | Cheers, | S. | | _______________________________________________ | Glasgow-haskell-users mailing list | Glasgow-haskell-users(a)haskell.org | http://www.haskell.org/mailman/listinfo/glasgow-haskell-users

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happy and OPTIONS pragmas
by Ian Lynagh 04 Nov '04

04 Nov '04

Hi, If I have this at the start of a .x file: { {-# OPTIONS -w #-} module Lexer (lex_tok) where } then the generated .hs file starts: {-# OPTIONS -cpp #-} {-# LINE 2 "Lexer.x" #-} {-# OPTIONS -w #-} module Lexer (lex_tok) where and the warnings are suppressed when I compile my program with ghc -Wall --make However, if a .y file starts: { {-# OPTIONS -w #-} -- Foo {-# OPTIONS -w #-} module Parser (parse) where } then the generated .hs file starts: -- parser produced by Happy Version 1.14 -- Foo {-# OPTIONS -w #-} module Parser (parse) where so the pragma before my comment has been eaten and the 2 comments mean that ghc doesn't see the pragma, so gives me all the warnings anyway. Can happy be changed so my pragma gets through please? Thanks Ian, with happy 1.14

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GHC bug typo...
by Keean Schupke 03 Nov '04

03 Nov '04

obviously I meant: type a :+ b = (a,b) In that last post!

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GHC CVS HEAD bug!
by Keean Schupke 03 Nov '04

03 Nov '04

The CVS HEAD branch of GHC seems to ignore the fixity of type constructors imported from another module... This breaks compiling of code that uses this feature: module1: type a b = a :+ b infixr 1 :+ module2: a :: Int :+ Float :+ Double a = undefined Gives a the wrong type. Keean

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GHCI and readline package
by Wassell, Mark P (GE Energy) 02 Nov '04

02 Nov '04

Hi, I'm having a problem running ghci and loading the readline package on WindowsXP. C:\Software\haskell\ghc622\bin>ghci -package readline ghci -package readline ___ ___ _ / _ \ /\ /\/ __(_) / /_\// /_/ / / | | GHC Interactive, version 6.2.2, for Haskell 98. / /_\\/ __ / /___| | http://www.haskell.org/ghc/ \____/\/ /_/\____/|_| Type :? for help. Loading package base ... linking ... done. Loading package readline ... linking ... C:/Software/haskell/ghc622/HSreadline.o: unknown symbol `_rl_redisplay_function' ghc.exe: unable to load package `readline' I have seen the follow in the user guide: "For some reason, Mingw ships with the readline library, but not with the readline headers. As a result, GHC (like Hugs) does not use readline for interactive input on Windows. You can get a close simulation by using an emacs shell buffer!" but I assume I need readline as util needs it and util is needed by data which is needed by wxcore. Cheers Mark

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