April 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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Export Haskell Libraries
by SevenThunders 12 Feb '08

12 Feb '08

I've finished a good sized Haskell project which I now must expose as a library (along with a lot of C code) to my fellow engineers. I had originally devoped my code on Windows XP and managed to learn how to wrap my Haskell code in a DLL and create an MSVC linkable library stub for the DLL. Although involved, the output in windows is relatively transparent in that as long as the paths are available to the Haskell runtime libraries and my DLL, linking is straightforward. Now however I find I must export the same libraries to linux. I decided to get 'smart' and repackage my build tools using CMAKE and cabal. Building the C object files is no problem and even configuring Cabal to generate my executables is not too painful. However the pain begins when one tries to link any Haskell object files or libraries to an external C program. Using ghc -v during the build reveals that gcc has about 50 -u flag arguments and links to 9 or 10 different libraries. Given that my little component is just a tiny part of a large software project involving many more engineers and many man years of work, it's not going to be fun to link my little library using all these crazy build flags to a large external project, especially when order is important. Wouldn't it be nice if there were a single run time library I could link to, (or even generate on the fly) that didn't have this intricate build step. How is ghc/Haskell supposed to integrate with the rest of the software world? A related issue in the building of my export libraries is cabal itself. I want to package my own library with some external object files etc and some Haskell code. It's easily done with ar and the correct object files. Unfortunately it is not clear from the documentation how I can force cabal to place the object files and generated stub files in a place where I can use them. (CMake is kind of forcing me to have them reside with the source for reasons I don't want to go into here.) I instead find myself copying the object files (*.o) back into the source directory from an obscure tmp directory several layers down from the source directory. This is not particularly portable and is probably quite fragile once someone decides to change the directory hiearcharchy in cabal. I guess I am surprised that on linux, with all of it's amazing software development tools, that Haskell export libraries would be this tricky to develop. -- View this message in context: http://www.nabble.com/Export-Haskell-Libraries-tf3569747.html#a9972866 Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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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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faster factorial function via FFI?
by Dan Drake 07 Jun '07

07 Jun '07

Hello everyone, I have some code in which the bottleneck is the factorial function. I began by using a naive fac n = product [1..n] but it looks like there are faster ways to do it. I could try to look up those faster algorithms and implement them, but I'm guessing that using libgmp's factorial function is the best way to do it. I'm a poor programmer, so I don't trust myself to implement those algorithms properly. Hence I need to use FFI. ...but ghc already uses libgmp for Integers, so shouldn't I be able to somehow call libgmp's factorial function? Using regular FFI stuff, it looks like this might get complicated, since the function doesn't return one of the usual foreign types, so I'd need to mess around to get the result into an Integer, which is what I need. Is there an easy way to do this? It might also be faster to use a lookup table, since most of the time I'll be taking factorials of relatively small numbers. Thanks, Dan -- Ceci n'est pas une .signature.

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How Albus Dumbledore would sell Haskell
by Dan Weston 21 May '07

21 May '07

It is unscientific to ask the (highly biased) people on this list how to sell Haskell. A focus group of the target audience is clearly called for. Having said that, I will now violate my own advice. Knowledge of the audience is critical to the success of a presentation. Simon (aka Dumbledore) is speaking to four different Houses: scientists (Ravenclaw), engineers (Hufflepuff), entrepreneurs (Slytherin), and managers (Griffindor). My advice to him is: You have already won over the scientists. Stop priming the pump. Algebraic runes, spells from the Book of Category Theory, bananas, lenses, arrows, cata, ana, hylo, endo, all is vanity. You cannot interest the engineer with new tricks. He has spent too long mastering difficult tricks to work around limitations to want to see them eliminated. His competitive advantage and prized reputation are at stake. Get him to believe that he is on the verge of being "left behind". You cannot win over the entrepreneur with promises of "easier and more robust". This translates to "anyone can do it" and the valuable "trade secret" of arcane wizardry is now devalued. Show something exciting that cannot be done (or would never have been attempted) without FP. Monads, CPS, existential data types, or other high-priestess wizardry are no impediment (less competition from lesser peers). But marginal improvement is unimpressive. A new language brings high risk, and must have high reward. Examples must be powerful (not another prime number generator). Effective complexity management, correctness reasoning in concurrent/distributed processing, and so on will get their interest. Risk is scary for managers. "Great promise" promises the manager only headaches. This translates to "no one can be hired to do it" and "those that can will shake me down for more money". You must promote instead the *inevitability* of Haskell's success: the fact that Haskell has recently dominated the ICFP Programming Contest, the strongly upward trend of Haskell taught at universities (hopefully there is a strongly upward trend?), some anecdotal measure of the predisposition to relatively better "quality" of those who seek to learn Haskell. Allay the unspoken fear that there are always plenty of Haskellers to cause trouble in a company (design and implement great tasks), but never enough more mediocre Haskellers to keep out of trouble (content to do "maintenance" tasks and minor upgrades to existing software) by marketing Haskell as a recruitment tool to attract the best, a motivational tool to reenergize engineers (see above) who've let their skills go stale in larger companies, and a staff-development tool to spur internal friendly competition to modernize their skills (and excuse wage stagnation in those that do not). Since "inevitability" is a hard sell for Haskell right now, avoid mentioning ML or Erlang. There needs to be only one FPL for a manager, or he will fret about VHS vs. Betamax syndrome (and he won't want to have been the one to invest in a Betamax). Don't show any Haskell function with the prefix "unsafe". Even now, I find the name unsettling. The Harry Potter series makes money because we can't do magic but we want to. It is human nature that what is given away will never be thought to have value. Don't give away the magic of Haskell. Let the audience in on the secret that the Haskell wizarding world is doing great things with it for glory and profit, and others will steal the secret for themselves readily enough. If Simon can do all this, then he really is worthy of the name Dumbledore. :) Dan

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Behavior of groupBy
by Hans van Thiel 21 May '07

21 May '07

Hello All, The standard function groupBy of List.hs doesn't work as I expect in this case: groupBy (\x y -> (last x) == (last y)) ["abc", "bd","cac"] results in: [["abc"],["bd"],["cac"]] where I want: [["abc","cac"], ["bd"]] Am I doing something wrong, or is this a bug? The function is defined (in List.hs in the hugs Data library) as : -- | The 'groupBy' function is the non-overloaded version of 'group'. groupBy :: (a -> a -> Bool) -> [a] -> [[a]] groupBy _ [] = [] groupBy eq (x:xs) = (x:ys) : groupBy eq zs where (ys,zs) = span (eq x) xs Thanks, Hans van Thiel

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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

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Follow up on 'GroupBy behavior'
by Hans van Thiel 21 May '07

21 May '07

Thanks again for the help, and, to follow up, this now does what I need.. -- partitions a list according to an equivalence relation partition1 :: (a -> a -> Bool) -> [a] -> ([a],[a]) partition1 eq ls = partition ((head ls) `eq`) ls -- partitionBy :: (a -> a -> Bool) -> [a] -> [[a]] partitionBy eq [] = [] partitionBy eq ls = (fst x):(partitionBy eq (snd x)) where x = partition1 eq ls Regards, Hans van Thiel

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