February 2007 - Haskell-Cafe

Cannot understand liftM2
by Nicola Paolucci 13 Feb '12

13 Feb '12

Hi All, I'm loving learning Haskell quite a bit. It is stretching my brain but in a delightfull way. I've googled, I've hoogled but I haven't found a clear explanation for what exactly liftM2 does in the context below. Using the cool lambdabot "pointless" utility I found out that: > \x -> snd(x) - fst(x) is the same as: > liftM2 (-) snd fst I like the elegance of this but I cannot reconcile it with its type. I can't understand it. I check the signature of liftM2 and I get: Prelude> :t liftM2 Prelude> liftM2 :: (Monad m) => (a1 -> a2 -> r) -> m a1 -> m a2 -> m r Can someone help me understand what's happening here ? What does a Monad have to do with a simple subtraction ? What is actually the "m" of my example ? I am sure if I get this I'll be another step closer to illumination ... Thanks, Nick

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Separate a string into a list of strings
by Sara Kenedy 18 Apr '08

18 Apr '08

Hi all, I want to write a function to separate a string into a list of strings separated by commas. Example: separate :: String -> [String] separate "Haskell, Haskell, and Haskell" = ["Haskell", "Haskell", "and Haskell"] If anyone has some ideas, please share with me. Thanks. S.

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speeding up fibonacci with memoizing
by Thomas Hartman 06 Nov '07

06 Nov '07

I just thought this was interesting, so I would share it. Thanks to whoever it was on #haskell who helped me, sorry I can't remember who. -- horribly slow. try slow_fibs 30, not too much higher than that and it hangs slow_fibs = map slow_fib [1..] slow_fib 1 = 1 slow_fib 2 = 1 slow_fib n = ( slow_fib (n-2) ) + (slow_fib(n-1)) -- versus, try memoized_fibs !! 10000 memoized_fibs = map memoized_fib [1..] memoized_fib = ((map fib' [0 ..]) !!) where fib' 0 = 0 fib' 1 = 1 fib' n = memoized_fib (n - 1) + memoized_fib (n - 2)

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Re: Genuine Eratosthenes sieve [Was: Optimization fun]
by Jens Blanck 23 Jul '07

23 Jul '07

> > The point about "Eratosthenes's sieve" is that it does not specify > algorithmically how to find the next number to be crossed. It does not > even define how to store (crossed) numbers, it stores them "on paper". But , I believe that Eratosthenes's sieve does specify how to store numbers and how to find the next to cross out. This is how I remember it: 0 List the numbers from 2 onwards. 1 Take first uncrossed number, say p. 2 Cross every pth number from there on (crossed or not). 3 Repeat from 1. That "crossed or not" appears above makes repeated filters very tricky. It is trivial with a mutable array, but that is not the preferred way so one needs to somehow merge the crossing out lists. I haven't looked at it really, but being sorted lists I would have thought that this merger would be easy without resorting to things like priority queues, could someone point this out to me? The above mayeasily be optimised to only listing odd numbers (still cross out every pth number in the list), and much less importantly by starting from whereever p*p is. Jens

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Creating static libraries using GHC
by Tom Hawkins 16 Mar '07

16 Mar '07

I have a chunk of Haskell code I would like wrap up and distribute as a library. Is there a way to build a static library (*.a) that includes my code plus the Haskell runtime, which C programs can easily link against? Here is what I have tried so far... ghc --make -fffi MyLib # Builds MyLib.o and MyLib_stub.o. gcc -c -I/usr/local/lib/ghc-6.4.1/include MyLibWrapper.c ar -r libMyLib.a MyLib.o MyLib_stub.o MyLibWrapper.o This works fine when I use ghc to compile and link a C program... ghc main.c libMyLib.a But if I use gcc, it throws a lot of unresolved references. I have no problem compiling with ghc, but the folks using my library probably won't have it installed. I added -v to ghc to see how it calls gcc; it seems to link in different libraries based on what Haskell libraries are being used, and it undefines a bunch is symbol references. Are there any switches to have ghc return a single archive with everything included? Please excuse my ignorance of the linking process. Thanks for any help! -Tom

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EnumSet and EnumMap
by Chris Kuklewicz 08 Mar '07

08 Mar '07

I could not quickly find anyone else writing this boiler plate, so I have posted this useful wrapper on the Haskell wiki at http://haskell.org/haskellwiki/EnumSet_EnumMap This uses a cheap newtype to wrap IntSet and IntMap so that you can store any Enum in them. It saves either writing many toEnum/fromEnum functions or using the slower and more generic Data.Set and Data.Map. The original motivation was to go from Map Char to IntMap. And as a bonus, the type signature of the newtype is the same kind as Data.Set and Data.Map (which matters when declaring instances...) > newtype EnumSet e = EnumSet {unEnumSet :: S.IntSet} > deriving (Monoid,Eq,Read,Show,Ord) > newtype EnumMap k a = EnumMap {unEnumMap :: M.IntMap a} > deriving (Eq,Read,Show,Ord,Monoid,Foldable,Functor) This has been quickly tested with GHC 6.6 and may contain typographical errors I have not caught yet. -- Chris

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Re: problems installing ghc 6.6 with extralibs (bad interface file)
by Thomas Hartman 08 Mar '07

08 Mar '07

Thanks. I incorporated these changes, and it cranks longer now before failing. But still fails, now with a seg fault. Does this just mean I don't have enough ram, or cpu, or ... Any ideas? thomas. ************* thartman@linodewhyou:~/haskellInstalls$ tail -n13 out.txt ../compiler/ghc-inplace -H16m -O -istage2/utils -istage2/basicTypes -istage2/types -istage2/hsSyn -istage2/prelude -istage2/rename -istage2/typecheck -istage2/deSugar -istage2/coreSyn -istage2/specialise -istage2/simplCore -istage2/stranal -istage2/stgSyn -istage2/simplStg -istage2/codeGen -istage2/main -istage2/profiling -istage2/parser -istage2/cprAnalysis -istage2/ndpFlatten -istage2/iface -istage2/cmm -istage2/nativeGen -istage2/ghci -Istage2 -DGHCI -DBREAKPOINT -package template-haskell -threaded -package readline -DUSE_READLINE -cpp -fglasgow-exts -fno-generics -Rghc-timing -I. -Iparser -package unix -package Cabal -package regex-compat -ignore-package lang -recomp -Rghc-timing -H16M '-#include "cutils.h"' -package-name ghc-6.6 -fgenerics -fno-cse -c main/DriverPipeline.hs -o stage2/main/DriverPipeline.o -ohi stage2/main/DriverPipeline.hi <<ghc: 489718944 bytes, 1371 GCs, 11780600/23325480 avg/max bytes residency (7 samples), 59M in use, 0.00 INIT (0.00 elapsed), 3.40 MUT (18.69 elapsed), 3.69 GC (3.91 elapsed) :ghc>> ../compiler/ghc-inplace -H16m -O -istage2/utils -istage2/basicTypes -istage2/types -istage2/hsSyn -istage2/prelude -istage2/rename -istage2/typecheck -istage2/deSugar -istage2/coreSyn -istage2/specialise -istage2/simplCore -istage2/stranal -istage2/stgSyn -istage2/simplStg -istage2/codeGen -istage2/main -istage2/profiling -istage2/parser -istage2/cprAnalysis -istage2/ndpFlatten -istage2/iface -istage2/cmm -istage2/nativeGen -istage2/ghci -Istage2 -DGHCI -DBREAKPOINT -package template-haskell -threaded -package readline -DUSE_READLINE -cpp -fglasgow-exts -fno-generics -Rghc-timing -I. -Iparser -package unix -package Cabal -package regex-compat -ignore-package lang -recomp -Rghc-timing -H16M '-#include "cutils.h"' -package-name ghc-6.6 -fgenerics -c main/GHC.hs -o stage2/main/GHC.o -ohi stage2/main/GHC.hi gcc: Internal error: Segmentation fault (program cc1) Please submit a full bug report. See <URL:http://gcc.gnu.org/bugs.html> for instructions. For Debian GNU/Linux specific bug reporting instructions, see <URL:file:///usr/share/doc/gcc-4.0/README.Bugs>. <<ghc: 595874452 bytes, 1818 GCs, 14564135/30202688 avg/max bytes residency (8 samples), 74M in use, 0.00 INIT (0.00 elapsed), 5.10 MUT (17.37 elapsed), 5.80 GC (12.08 elapsed) :ghc>> make[1]: *** [stage2/main/GHC.o] Error 1 make: *** [install] Error 1 ~/haskellInstalls thartman@linodewhyou:~/haskellInstalls$ sudo ./install-ghc-6.6.sh 2>&1 | tee out.txt thartman@linodewhyou:~/haskellInstalls$ cat install-ghc-6.6.sh if [ ! -f ghc-6.6-src.tar.bz2 ]; then wget http://www.haskell.org/ghc/dist/6.6/ghc-6.6-src.tar.bz2 fi if [ ! -f ghc-6.6-src-extralibs.tar.bz2 ]; then wget http://www.haskell.org/ghc/dist/6.6/ghc-6.6-src-extralibs.tar.bz2 fi tar -tvjf ghc-6.6-src.tar.bz2 tar -tvjf ghc-6.6-src-extralibs.tar.bz2 pushd ghc-6.6 (cd compat && make clean && make UseStage1=YES) (cd utils && make clean && make UseStage1=YES) ./configure make make install popd thartman@linodewhyou:~/haskellInstalls$ 2007/2/27, Thomas Hartman <tphyahoo(a)gmail.com>: > I incorporated these changes to my install script, and now it cranks > longer before failing, but still fails. Now with a seg fault. > > For some odd reason the final crash snipped below didn't get written > to out.txt, otherwise I would have attached it. > > Any ideas to get it working? > > Thanks, thomas. > > ********************************** > > thartman@linodewhyou:~/haskellInstalls$ cat install-ghc-6.6.sh > if [ ! -f ghc-6.6-src.tar.bz2 ]; then > wget http://www.haskell.org/ghc/dist/6.6/ghc-6.6-src.tar.bz2 > fi > > if [ ! -f ghc-6.6-src-extralibs.tar.bz2 ]; then > wget http://www.haskell.org/ghc/dist/6.6/ghc-6.6-src-extralibs.tar.bz2 > fi > > > tar -tvjf ghc-6.6-src.tar.bz2 > tar -tvjf ghc-6.6-src-extralibs.tar.bz2 > pushd ghc-6.6 > (cd compat && make clean && make UseStage1=YES) > (cd utils && make clean && make UseStage1=YES) > ./configure > make > make install > popd > thartman@linodewhyou:~/haskellInstalls$ > > thartman@linodewhyou:~/haskellInstalls$ sudo ./install-ghc-6.6.sh 2>&1 > | tee out.txt > > .... > > ../compiler/ghc-inplace -H16m -O -istage2/utils -istage2/basicTypes > -istage2/types -istage2/hsSyn -istage2/prelude -istage2/rename - > istage2/typecheck -istage2/deSugar -istage2/coreSyn > -istage2/specialise -istage2/simplCore -istage2/stranal > -istage2/stgSyn -istage2 > /simplStg -istage2/codeGen -istage2/main -istage2/profiling > -istage2/parser -istage2/cprAnalysis -istage2/ndpFlatten > -istage2/iface > -istage2/cmm -istage2/nativeGen -istage2/ghci -Istage2 -DGHCI > -DBREAKPOINT -package template-haskell -threaded -package readline > -DUSE_R > EADLINE -cpp -fglasgow-exts -fno-generics -Rghc-timing -I. -Iparser > -package unix -package Cabal -package regex-compat -ignore-package > lang > -recomp -Rghc-timing -H16M '-#include "cutils.h"' -package-name > ghc-6.6 -fgenerics -c main/GHC.hs -o stage2/main/GHC.o -ohi > stage2 > /main/GHC.hi > gcc: Internal error: Segmentation fault (program cc1) > Please submit a full bug report. > See <URL:http://gcc.gnu.org/bugs.html> for instructions. > For Debian GNU/Linux specific bug reporting instructions, see > <URL:file:///usr/share/doc/gcc-4.0/README.Bugs>. > > <<ghc: 595874452 bytes, 1818 GCs, 14564135/30202688 avg/max bytes > residency (8 samples), 74M in use, 0.00 INIT (0.00 elapsed), 5.10 MUT > (17 > .37 elapsed), 5.80 GC (12.08 elapsed) :ghc>> > make[1]: *** [stage2/main/GHC.o] Error 1 > make: *** [install] Error 1 > ~/haskellInstalls > > > > 2007/2/27, Simon Marlow <simonmarhaskell(a)gmail.com>: > > Thomas Hartman wrote: > > > I installed ghc 6.6 from source ok. But then when I tried installing > > > it with the "extralibs" to get all the functionality that had been > > > unbundled in 6.6, I hit a glitch. > > > > > > Anyone ever seen anything like this? > > > > > > In case it matters, this is ssh-ed in to a virtualized user mode linux > > > session. > > > > > > ***************************************** > > > > > > thartman@linodewhyou:~/haskellInstalls$ cat install-ghc-6.6.sh > > > if [ ! -f ghc-6.6-src.tar.bz2 ]; then > > > wget http://www.haskell.org/ghc/dist/6.6/ghc-6.6-src.tar.bz2 > > > fi > > > > > > if [ ! -f ghc-6.6-src-extralibs.tar.bz2 ]; then > > > wget http://www.haskell.org/ghc/dist/6.6/ghc-6.6-src-extralibs.tar.bz2 > > > fi > > > > > > > > > tar -tvjf ghc-6.6-src.tar.bz2 > > > tar -tvjf ghc-6.6-src-extralibs.tar.bz2 > > > pushd ghc-6.6 > > > ./configure > > > make > > > make install > > > popd > > > thartman@linodewhyou:~/haskellInstalls$ sudo ./install-ghc-6.6.sh 2>&1 > > > | tee out.txt > > > thartman@linodewhyou:~/haskellInstalls$ tail -n20 out.txt > > > ------------------------------------------------------------------------ > > > make[2]: Nothing to be done for `install'. > > > ------------------------------------------------------------------------ > > > == make install - --no-print-directory -r; > > > in /home/thartman/haskellInstalls/ghc-6.6/utils/ghc-pkg > > > ------------------------------------------------------------------------ > > > Creating a symbolic link from ghc-pkg-6.6 to ghc-pkg in /usr/local/bin > > > for i in ghc-pkg-6.6; do \ > > > /usr/bin/install -c -m 755 $i /usr/local/bin; \ > > > done > > > /usr/local/bin/ghc -H16m -O -cpp -Wall -fno-warn-name-shadowing > > > -fno-warn-unused-matches -i../../compat -ignore-package Cabal > > > -Rghc-timing -c Main.hs -o Main.o -ohi Main.hi > > > > > > Main.hs:19:0: > > > Bad interface file: ../../compat/Distribution/InstalledPackageInfo.hi > > > mismatched interface file versions: expected 6060, found 6 > > > <<ghc: 24591324 bytes, 4 GCs, 106128/106128 avg/max bytes residency (1 > > > samples), 16M in use, 0.00 INIT (0.02 elapsed), 0.25 MUT (0.42 > > > elapsed), 0.10 GC (0.12 elapsed) :ghc>> > > > make[2]: *** [Main.o] Error 1 > > > make[1]: *** [install] Error 1 > > > make: *** [install] Error 1 > > > > I'm guessing what happened is that during the install the build system decided > > to rebuild ghc-pkg (I'm not sure why), but by that time you had already > > installed the new compiler, and the previous interface files weren't compatible > > with the new compiler. (I don't know why that would be the case either, since > > both compilers are 6.6...). > > > > You might be able to work around it by > > > > $ (cd compat && make clean && make UseStage1=YES) > > $ (cd utils && make clean && make UseStage1=YES) > > > > Cheers, > > Simon > > > > >

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Haskell Cookbook?
by Alexy Khrabrov 07 Mar '07

07 Mar '07

So I'm picking up Haskell bit by bit, and I found the code examples transpiring here most useful. Reflecting why it's harder to pick up Haskell than say Ruby or Python, here's what I found -- those languages deal with a typical domain available to any programmer -- his own computer/system/shell. The artifacts are files, directories, timestamps, etc. The stuff every programmer understands in their sleep. APIs. So I loved the shell-script beautification thread. That's how I learn about new modules and their functions -- as use cases. I also found some pieces of Haskell strewn around people's websites, blogs, forums; even at paste.lisp.org. I'm gathering them for my own purpose and trying to compile and run them. Wouldn't it be nice -- in case it doesn't exist already -- a[n O'Reilly[-like]] Haskell Cookbook? That would be the best way to learn Haskell. I've found a wikibook on Haskell, but I look for a big bag of small, self-contained programs. Perhaps you esteemed veterans can dig your small scripts and paste them into a wiki? Examples needed -- how to connect to a database; to a web service (e.g. Amazon); read a csv file; represent data equivalent to a directory listing; a text file/XML; etc... Cheers, Alexy

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Literate haskell format unclear (implementation and specification inconsistencies)
by Isaac Dupree 06 Mar '07

06 Mar '07

-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 Trying to implement literate haskell[*], I realized several ways in which the correct behavior for unliterating (especially with regard to errors) was unclear. I have several cases which ghc, hugs and Haskell 98 have differing opinions on! The Report as it stands is far from a clear and complete specification (and I didn't find anything in the Haskell' wiki/trac about literate haskell). [*](particularly, to make DrIFT able to deal with TeX-style lhs - there's unfinished work in darcs repo http://isaac.cedarswampstudios.org/2007/DrIFT/ ) testing with: ghc: 6.4.2, 6.6(some) hugs: Hugs Version 20050308 nhc98: recent darcs (1.19) report: Haskell 98 (The Revised Report: December 2002), section 9.4, http://www.haskell.org/onlinereport/syntax-iso.html#sect9.4 A full set of .lhs test files for all the issues: darcs get http://isaac.cedarswampstudios.org/2007/LiterateHaskellTests or download http://isaac.cedarswampstudios.org/2007/LiterateHaskellTests-1.tar.gz or you can try just prefixing all examples with \begin{code} module Main where main = print str \end{code} or >module Main where > main = print str as appropriate... (please don't get mangled by mail programs, initial '>'s... ): 1.[UnmatchedBegin] If a \begin{code} starts a section of code, is \end{code} _required_ before the end of the file? report: unclear ghc: required hugs, nhc98: not required The report says "entirely enclosed between", but goes on to say "More precisely:" and give a description that is not at all precise in the matter of this question. 2.[AfterBeginOrEnd/{BeginWhite,EndWhite,BeginPrint,EndPrint}] Can a line beginning \begin{code} or \end{code} have additional stuff on the end, where the directive is understood and the additional stuff is ignored? report:[yes] hugs:[yesIffAdditionalStuffIsInvisible] ghc:[case beginningOfLine of "\end{code}" -> yes "\begin{code}" -> yesIffAdditionalStuffIsInvisible] nhc98:[UNLIT_IGNORED] where yesIffAdditionalStuffIsInvisible = if (all isSpace additionalStuff) then yes else UNLIT_IGNORED UNLIT_IGNORED means that if it was inside a code block then the line is treated as program text (so it's probably a syntax error) and if it was in a literate comment section it is treated as a non-empty literate comment line. Note that it takes a careful reading of the report: for begin, program code only begins on the _following_ line. Most seem to agree that it shouldn't mess up your program to have trailing whitespace on such a line (but at least nhc98 doesn't currently implement this). Is there any reason to allow NON-whitespace in that location? 3.[IgnoringStringLiterals/{A,B}] what does "(ignoring string literals, of course)" mean? that the following(A) makes str = "string gap:end{code}" and an unended code block(A), or that it makes an ended code block(B)? (A)--------- \begin{code} str = "string gap:\ \end{code}" - --------- report:unclear, hugs:A, ghc:B, nhc98:A This works for ghc, the result being "string gap:string gap ends": (B)--------- \begin{code} str = "string gap:\ \end{code}" \begin{code} \string gap ends" \end{code} - ----------- Note that behavior 1 requires a detailed knowledge of Haskell's syntax in order to unliterate a file, for a dubious benefit (if a string literal with string gaps is used like that, the programmer could just indent the second line!) 4.[ExtraBeginEnd/{ExtraBegin,ExtraEnd}] What happens if \begin{code} appears after another \begin{code} before an \end{code}; and what happens if an \end{code} appears without a code block previously having been started by a \begin{code}? stray end: ghc, nhc98:[UNLIT_IGNORED (-> probable successful compile)] hugs:[error "\end{code} encountered outside code block"] stray begin: ghc, nhc98:[UNLIT_IGNORED (-> probable syntax error)] hugs:[error "\begin{code} encountered inside code block"] 5.[LexicalUnitAcrossLiterateComment/{StringGap,BlockComment}] Can lexical units jump across literate comment gaps? report, ghc, hugs, nhc98: yes... Note that the Report specifies it by removing all non-program lines, rather than converting them to blank lines, but an additional blank line in the middle of a Haskell program NEVER makes a difference (except for line numbering, of course). - ---------- > str = "string gap:\ This might be a literate comment. > \ends here" - --------- ghc, hugs, nhc98: "string gap:ends here" or - -------- > str = "string" > {- a comment This might be a literate comment -} with weird character sequences. > ends here -} - ---------- ghc, hugs, nhc98: think it's a fine comment I mention this because allowing these makes it complicated to preserve literate comments in a translation to .hs, because, other than cases like these, prefixing literate comment lines with "-- " works fine.[*] However, banning these could make processing that wants to report errors end up more complicated. Maybe the report could/should say that it is "not advisable", as it does for mixing '>' and {code} styles? (Also it's confusing to the programmer - I wondered "can I (and should I) really do that?!" sometimes..) [*]Haddock style is a nuisance too, which is why there are two spaces added -- Haddock seems not to recognize such comments then, as desired. Or would it be better to take the other approach and say those should count as haddock comments? 6.[TeXBirdtrack/] I understand that "It is not advisable to mix these two styles in the same file." and the report doesn't even talk about how they mix, but now that I've gotten started on the implementation inconsistencies... Actually, despite the Report's advice against it, there seems to be a consensus on what the meaning of mixing the two styles is, which I'll describe below: Sensibly, ghc, hugs and nhc98 treat begin/end{code} lines as blank for the purposes of '>'-style comment checking (which is that a code and a non-blank literate comment line can't be adjacent); this works: [TeXBirdtrack/NoLayout]------------ >module Main where >{main = print str \begin{code} ;str = "string"} \end{code} ok - ------------ Note I didn't rely on the layout rule. This should work: [TeXBirdtrack/AlignedLayout]------------ >module Main where > main = print str \begin{code} str = "string" \end{code} ok - ------------ It does in hugs and nhc98, and according to http://hackage.haskell.org/trac/ghc/ticket/210 it does in GHC HEAD now (6.7) as well. As another example, this doesn't work, for the same reason that you can't start a line with '>' in a .hs file: [TeXBirdtrack/Wrong]------------ >module Main where > main = print str \begin{code} > str = "string" \end{code} ok - ------------ Hoping to start some discussion, Isaac -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.3 (GNU/Linux) Comment: Using GnuPG with Mozilla - http://enigmail.mozdev.org iD8DBQFF5gaoHgcxvIWYTTURAoF4AJwIjQ3hJ9jpwUgHiYgTB7IhN2so4QCdGCKU 96q4YIeakWtlBKOdAiFM+vU= =qzCQ -----END PGP SIGNATURE-----

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Tricks for making low level haskell hacking easier
by dons＠cse.unsw.edu.au 05 Mar '07

05 Mar '07

For those of you who like your Haskell code fast, then, if you're like me, reading Core output all day can give you headaches. Here's a couple of tricks I use to make optimising low level stuff easier. 1) Use Hscolour to pretty-ifiy the Core so its more parsable: ghc -O Foo.hs -ddump-simpl | HsColour -tty | less -r will colourise the Core, and pipe it into less (which will display the metachars). A screenshot: http://www.cse.unsw.edu.au/~dons/tmp/screen-core.png 2) Use vimdiff on asm output, checking the results for different ghc / gcc flags. Yes, sometimes you get down to the asm level, and it can make all the difference. It is useful when writing (*cough*) entries for the great language shootout, to compare the asm output. Currently I do this by compiling a program with different flags, or slightly different code, with -keep-tmp-files -no-recomp set, and then vimdiff the resulting asm files: http://www.cse.unsw.edu.au/~dons/tmp/asm-diff.png (before on the left, after generating pentium4 instructions on the right). Do people have any other tricks for making the tweaking of low level ghc output easier (that we should incorporate into vim/emacs support)? -- Don

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