March 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

15 18

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.

14 35

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)

14 16

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

12 22

Unresolved overloading error
by Scott Brown 21 May '07

21 May '07

I have written this code in Haskell which gives an unresolved overloading error. The function bernoulli should give the probability of j successes occuring in n trials, if each trial has a probability of p. > fac 0 = 1 > fac n = n * fac(n - 1) > binom n j = (fac n)/((fac j)*(fac (n - j))) > bernoulli n p j = (binom n j)*(p ^ j) * ((1 - p)^(n - j)) However, bernoulli 6 0.5 3 gives the error: ERROR - Unresolved overloading *** Type : (Fractional a, Integral a) => a *** Expression : bernoulli 6 0.5 3 Why doesn't Haskell infer the types? What kind of type casting or type definition can I use to fix the error? Send instant messages to your online friends http://au.messenger.yahoo.com

6 7

A wish for relaxed layout syntax
by Benjamin Franksen 21 May '07

21 May '07

Hi, I often run into the following issue: I want to write a list of lengthy items like this mylist = [ quite_lengthy_list_item_number_one, quite_lengthy_list_item_number_two, quite_lengthy_list_item_number_three ] With the current layout rules this is a parse error (at the closing bracket). Normally I avoid this by indenting everything one level more as in mylist = [ quite_lengthy_list_item_number_one, quite_lengthy_list_item_number_two, quite_lengthy_list_item_number_three ] but I think this is a little ugly. Same issue comes up with parenthesized do-blocks, I would like to write when (condition met) (do first thing second thing ) So my wish is for a revised layout rule that allows closing brackets (of all sorts: ']', ')', '}') to be on the same indent level as the start of the definition/expression that contains the corresponding opening bracket. Cheers Ben

14 17

Compiling GHC
by Chris Witte 11 Apr '07

11 Apr '07

I'm tying to compile GHC under mingw (winxp with mingw no cygwin), everything appears to go OK but when I run make install it fails to copy some files cp -rp ./../include/* /usr/local/include/mingw cp: cannot stat `./../include/*': No such file or directory make: [install] Error 1 (ignored) cp -rp ./../lib/gcc-lib/mingw32/3.4.2/* /usr/local/gcc-lib cp: cannot stat `./../lib/gcc-lib/mingw32/3.4.2/*': No such file or directory make: [install] Error 1 (ignored) cp -rp ./../lib/gcc/mingw32/3.4.2/* /usr/local/gcc-lib cp: cannot stat `./../lib/gcc/mingw32/3.4.2/*': No such file or directory make: [install] Error 1 (ignored) cp -rp ./../libexec/gcc/mingw32/3.4.2/* /usr/local/gcc-lib cp: cannot stat `./../libexec/gcc/mingw32/3.4.2/*': No such file or directory make: [install] Error 1 (ignored) cp ./../lib/*.* /usr/local/gcc-lib cp: cannot stat `./../lib/*.*': No such file or directory make: [install] Error 1 (ignored) cp ./gcc.exe /usr/local cp: cannot stat `./gcc.exe': No such file or directory make: [install] Error 1 (ignored) cp ./as.exe /usr/local/gcc-lib cp: cannot stat `./as.exe': No such file or directory make: [install] Error 1 (ignored) cp ./ld.exe /usr/local/gcc-lib cp: cannot stat `./ld.exe': No such file or directory make: [install] Error 1 (ignored) cp ./dllwrap.exe /usr/local/gcc-lib cp: cannot stat `./dllwrap.exe': No such file or directory make: [install] Error 1 (ignored) cp ./dlltool.exe /usr/local/gcc-lib cp: cannot stat `./dlltool.exe': No such file or directory make: [install] Error 1 (ignored) cp /c/ghc/ghc-6.6/bin/../perl.exe /usr/local cp /c/ghc/ghc-6.6/bin/../perl56.dll /usr/local for i in ANNOUNCE LICENSE README; do \ /bin/install -c -m 644 $i /usr/local; \ done and then when I try and run, say, GHCi I get $ ghcii.sh ___ ___ _ / _ \ /\ /\/ __(_) / /_\// /_/ / / | | GHC Interactive, version 6.7.20070323, for Haskell 98. / /_\\/ __ / /___| | http://www.haskell.org/ghc/ \____/\/ /_/\____/|_| Type :? for help. Loading package base ... linking ... ghc.exe: unable to load package `base' ghc.exe: C:/msys/1.0/local/HSbase.o: unknown symbol `_gettimeofday' any ideas on what could be causing this. Cheers Chris Witte

5 5

Re: Trouble trying to find packages for ubuntu linux
by Chad Scherrer 07 Apr '07

07 Apr '07

Brian, I had this exact problem, and I found this approach to work wonderfully: http://pupeno.com/2006/12/17/unstable-packages-on-ubuntu/ Chad

3 6

A question about functional dependencies and existential quantification
by oleg＠pobox.com 03 Apr '07

03 Apr '07

Jean-Marie Gaillourdet wrote > I am trying to do something like the following: > > {-# OPTIONS -fglasgow-exts -fallow-undecidable-instances #-} > > module TestCase where > > > > data Any root = forall pos sel . T root pos sel => ANY pos > > > > class T root pos sel | pos -> root, root -> sel where > > f :: pos -> sel -> Bool > > > > instance T root (Any root) sel where > > f (ANY p) s = f p s The code as posted seems to have a problem. The problem is noted by GHC 6.4 and hugs but is accepted by GHC 6.6. Such a change in GHC seems strange. The problem is not related to existentials, so we just drop them > {-# OPTIONS -fglasgow-exts -fallow-undecidable-instances #-} > module TestCase where > > data Any root = ANY > > class T root pos sel | pos -> root, root -> sel where > f :: pos -> sel -> Bool > instance T root (Any root) sel > > test x = f ANY x Hugs and GHC 6.4 both complain about the instance of T: Hush (September 2006 version) says ERROR "/tmp/j.hs":9 - Instance is more general than a dependency allows and GHC 6.4.2 says Illegal instance declaration for `T root (Any root) sel' (the instance types do not agree with the functional dependencies of the class) In the instance declaration for `T root (Any root) sel' I concur. The class declares T as being a ternary relation such that the following holds forall r p p' s s'. T(r,p,s) && T(r,p',s') -> s = s' Now, the instance `T root (Any root) sel' is satisfied when root=Int, sel = Bool and when root=Int, sel = Int. Does it not? That falsifies the above proposition. In other words, the instance T is not functional with respect to the first and the third arguments. That is not surprising. What is surprising is why GHC 6.6 accepts such an instance?

9 11

Re: [Haskell-cafe] Why the Prelude must die
by Vivian McPhail 03 Apr '07

03 Apr '07

I agree with Sven, but... What I want to push is a 'mathematically sound' numeric prelude. A proper numerical prelude should have bona fide mathematical obects like groups, rings, and fields underlying common numerical classes. It would be edifying to the student who discovered that the particular data type he is using is an inhabitant of a known class and can thus take advantage of known properties, presupplied as class methods. Reasoning and communication about programs, data types, and functions would be enhanced. [Conjecture 1 (2007). Haskell Mathematical Prelude and Mathematicians] If Haskell had a mathematically sound prelude then more mathematicians would use Haskell. Cheers, Vivian > > Message: 1 > Date: Sat, 24 Mar 2007 17:56:11 +0100 > From: Sven Panne <sven.panne(a)aedion.de> > Subject: Re: [Haskell-cafe] Why the Prelude must die > To: haskell-cafe(a)haskell.org > Cc: libraries(a)haskell.org > Message-ID: <200703241756.11121.sven.panne(a)aedion.de> > Content-Type: text/plain; charset="iso-8859-1" > > On Saturday 24 March 2007 03:48, Stefan O'Rear wrote: > > 1. Namespace pollution > > > > The Prelude uses many simple and obvious names. Most > programs don't > > use the whole Prelude, so names that aren't needed take up > namespace > > with no benefit. [...] > > Even though I think that the current Prelude is far from > perfect, one should not forget that is a very solid > foundation of a "common language": If one sees e.g. '(.)' or > 'map', it is immediately clear to everybody what this means, > without having to scan through (perhaps long) import lists. > Of course one could hide some parts of the Prelude etc., but > I think in the long run this only leads to confusion. > Redefining common things, heavy use of tons of self-defined > operators etc. all make maintenance much harder. > > Try reading Lisp code with heavy use of macros or C++ code > with tons of overloadings. This is more like Sudoku solving > than anything else, because there is no "common language" > between the author and the reader anymore. > > And taking away the prelude is a little bit like taking away > 'int', 'double', 'for', 'while' etc. from the C programmer... > > > 11. Committeeism > > > > Because the Prelude has such a wide audience, a strong committee > > effect exists on any change to it. This is the worst kind of > > committeeism, and impedes real progress while polluting the Prelude > > with little-used features such as fail in Monad (as opposed to > > MonadZero) and until. > > Depending on your viewpoint, you can see this as a plus. > Everybody agrees that finalizers are evil, but propose the > removal of that method from java.lang.Object to the Java people. :-) > > My proposal would be to incrementally improve the Prelude, > modularize it a bit more, fix the Num hierarchy, but > basically leave it as it is. > > Cheers, > S. > >

13 14