[Haskell-cafe] Re: Haskell example archive?
Echo Nolan [EMAIL PROTECTED] writes: Hello all, I recently read the post about a problem using the list monad, and I was wondering if there was an archive of monad usecases. If there is one, I'd like to see it, and if not it'd be a helpful part of the haskell community. Something like the example of the Maybe monad from All About Monads. I haven't found any comprehensive archive either, but there are examples spread across the net and the Hakskell Wiki. A special usecase of monads is imperative programming. A few examples can be found at http://haskell.org/hawiki/ImperativeHaskell Hopefully when enough examples and snippets have been written it will be easy for someone to collect and edit a very much complete collection of examples. Regards Peter ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Why doesn't this work?
Am Montag, 25. April 2005 08:16 schrieb Michael Vanier:
I've been trying to generate an infinite list of random coin flips in GHC
6.4, and I've come across some strange behavior:
--
import System.Random
data Coin = H | T deriving (Eq, Show)
-- Generate a random coin flip.
coinFlip :: IO Coin
coinFlip = do b - getStdRandom random
return (bool2coin b)
where
bool2coin True = H
bool2coin False = T
-- Generate an infinite list of coin flips.
coinFlips :: IO [Coin]
coinFlips = sequence cfs
where cfs = (coinFlip : cfs)
-- Print n of them.
test :: Int - IO ()
test n = do f - coinFlips
print (take n f)
--
Now when I do test 1 (for instance), it hangs forever. It seems as if
there is some kind of strictness constraint going on that I don't
understand. My understanding is that cfs is an infinite list of (IO Coin),
sequence lifts this to be IO [Coin] where [Coin] is an infinite list, and
then test should extract the infinite list of coin flips into f, take some
number of them, and print them. But instead, the system appears to be
trying to compute all the coin flips before taking any of them. Why is
this, and how do I fix it?
Thanks,
Mike
How to fix it:
test n = sequence (replicate n coinFlip) = print
another way to fix it: use unsafeInterleaveIO (I would not recommend it,
though)
import System.IO.Unsafe
coinFlips = do c - coinFlip
cs - unsafeInterleaveIO coinFlips
return (c:cs)
Why: because coinFlips has to be evaluated before the result can be passed to
'print . take n' (that's part of the IO monad, executing actions in
sequence). And this can't be done lazily with sequence:
sequence :: Monad m = [m a] - m [a]
{-# INLINE sequence #-}
sequence ms = foldr k (return []) ms
where
k m m' = do { x - m; xs - m'; return (x:xs) }
so
sequence (ac:acs) = foldr k (return []) (ac:acs)
= k ac (foldr k (return []) acs)
= do x - ac
xs - sequence acs
return (x:xs)
and if sequence acs fails, the overall computation fails and nothing can be
returned. The point is, the function 'k' from sequence is strict, and folding
a strict function always uses the entire list (unless an error occurs before
the end is reached).
Conclusion: sequence only finite lists, otherwise you'll get a Stack overflow.
Cheers,
Daniel
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Re: [Haskell-cafe] Why doesn't this work?
On Sun, 2005-04-24 at 23:16 -0700, Michael Vanier wrote: I've been trying to generate an infinite list of random coin flips in GHC 6.4, and I've come across some strange behavior: -- import System.Random data Coin = H | T deriving (Eq, Show) -- Generate a random coin flip. coinFlip :: IO Coin coinFlip = do b - getStdRandom random return (bool2coin b) where bool2coin True = H bool2coin False = T -- Generate an infinite list of coin flips. coinFlips :: IO [Coin] coinFlips = sequence cfs where cfs = (coinFlip : cfs) -- Print n of them. test :: Int - IO () test n = do f - coinFlips print (take n f) -- Now when I do test 1 (for instance), it hangs forever. It seems as if there is some kind of strictness constraint going on that I don't understand. My understanding is that cfs is an infinite list of (IO Coin), sequence lifts this to be IO [Coin] where [Coin] is an infinite list, and then test should extract the infinite list of coin flips into f, take some number of them, and print them. But instead, the system appears to be trying to compute all the coin flips before taking any of them. Why is this, and how do I fix it? My first guess is that this is because sequence is strict in it's list. This is the normal behaviour that you would expect for this function since otherwise the side effects from all the IO actions are not going to happen before it returns (which is the ordinary behaviour for IO actions; one of the main purposes of the IO monad is for sequencing side effects). You can lazily defer IO actions using unsafeInterleaveIO. However in this case that's probably not the most elegant approach. It might be better to make the coinFlip function pure (ie not in the IO monad) and instead to pass it a random number generator which it returns as an extra component of the result (having extracted a random value using 'random'). Then you can use getStdGen once and pass the result to a function which generates an infinite lazy list of random numbers by threading the generator between calls to coinFlip. (Or if you want to cheat you can use randoms which will do all this for you) See: http://haskell.org/ghc/docs/latest/html/libraries/base/System.Random.html Duncan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] String search algorithms
I'm a bit puzzled to find no sub-string search in the Haskell libraries (unless there's some neat composition of the existing Data.List functions that I've missed). Google doesn't help much either. I've found a KMP implementation: http://haskell.org/hawiki/RunTimeCompilation I'm after something that'll report the position of the first occurrence, like Java's String.indexOf(). Alistair. - * Confidentiality Note: The information contained in this message, and any attachments, may contain confidential and/or privileged material. It is intended solely for the person(s) or entity to which it is addressed. Any review, retransmission, dissemination, or taking of any action in reliance upon this information by persons or entities other than the intended recipient(s) is prohibited. If you received this in error, please contact the sender and delete the material from any computer. * ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] String search algorithms
On Mon, 25 Apr 2005, Bayley, Alistair wrote: I'm a bit puzzled to find no sub-string search in the Haskell libraries (unless there's some neat composition of the existing Data.List functions that I've missed). Google doesn't help much either. I've found a KMP implementation: http://haskell.org/hawiki/RunTimeCompilation I'm after something that'll report the position of the first occurrence, like Java's String.indexOf(). List.findIndex (List.isPrefixOf bla) (List.tails dfvbdbblaesre) ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
RE: [Haskell-cafe] String search algorithms
From: Henning Thielemann [mailto:[EMAIL PROTECTED] (unless there's some neat composition of the existing Data.List functions List.findIndex (List.isPrefixOf bla) (List.tails dfvbdbblaesre) Oh, so there is. Thanks. - * Confidentiality Note: The information contained in this message, and any attachments, may contain confidential and/or privileged material. It is intended solely for the person(s) or entity to which it is addressed. Any review, retransmission, dissemination, or taking of any action in reliance upon this information by persons or entities other than the intended recipient(s) is prohibited. If you received this in error, please contact the sender and delete the material from any computer. * ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] String search algorithms
On Mon, 25 Apr 2005, Henning Thielemann wrote: On Mon, 25 Apr 2005, Bayley, Alistair wrote: I'm a bit puzzled to find no sub-string search in the Haskell libraries (unless there's some neat composition of the existing Data.List functions that I've missed). Google doesn't help much either. I've found a KMP implementation: http://haskell.org/hawiki/RunTimeCompilation I'm after something that'll report the position of the first occurrence, like Java's String.indexOf(). List.findIndex (List.isPrefixOf bla) (List.tails dfvbdbblaesre) But I'm curious if you really need the index. Working with indexes on lists is quite inefficient. E.g. if you want to replace substrings you may want to use this implementation: replace :: forall a. (Eq a) = [a] - [a] - [a] - [a] replace src dst = foldr (\x xs - let y=x:xs in if isPrefixOf src y then dst ++ drop (length src) y else y) [] Prelude Data.List replace ocu orcu hocuspocus horcusporcus ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
RE: [Haskell-cafe] String search algorithms
From: Henning Thielemann [mailto:[EMAIL PROTECTED] But I'm curious if you really need the index. Working with indexes on lists is quite inefficient. E.g. if you want to replace substrings you may want to use this implementation: I'm not mutating the list, but I am extracting sublists. I'm toying with porting FIT and I'm starting with the Parse class, as recommended ( http://fit.c2.com/wiki.cgi?TipsForCoreImplementors ). They warn against using a generic parser: Parse is the internal representation of the test data. Don't be seduced by generalized html/xml parsers. This one has unique features. Make it work. Consider replacing it only after you've used this one a while. Their parser is in Java, and makes heavy use of String.indexOf(), and I'm just trying to port it more-or-less as-is. I appreciate that'll be somewhat inefficient, but until I understand all of the wrinkles in their implementation, I'll take the simplest approach. Alistair. - * Confidentiality Note: The information contained in this message, and any attachments, may contain confidential and/or privileged material. It is intended solely for the person(s) or entity to which it is addressed. Any review, retransmission, dissemination, or taking of any action in reliance upon this information by persons or entities other than the intended recipient(s) is prohibited. If you received this in error, please contact the sender and delete the material from any computer. * ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
RE: [Haskell-cafe] FFI and pointers to pointers
On 22 April 2005 16:18, Dimitry Golubovsky wrote: I am trying to generalize my knowledge about FFI declarations when dealing with pointers to pointers (import from C to Haskell). Maybe these are silly questions, but It seems to me, I am missing some understanding. Per the FFI Addendum: For a variable, we use import: int bar; foreign import ccall bar :: Ptr CInt The syntax you want is foreign import ccall bar :: Ptr CInt But note that GHC 6.4 has a bug whereby this doesn't work as expected when compiling via C. Either use the native code generator (-fasm) or wait for 6.4.1. Cheers, Simon ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
RE: [Haskell-cafe] FFI and pointers to pointers
On 25 April 2005 12:51, Simon Marlow wrote: On 22 April 2005 16:18, Dimitry Golubovsky wrote: I am trying to generalize my knowledge about FFI declarations when dealing with pointers to pointers (import from C to Haskell). Maybe these are silly questions, but It seems to me, I am missing some understanding. Per the FFI Addendum: For a variable, we use import: int bar; foreign import ccall bar :: Ptr CInt The syntax you want is foreign import ccall bar :: Ptr CInt oops, never mind. Your version was fine. Cheers, Simon ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] How to debug GHC
Hi, all, I'm developing a back end for GHC and I have the following problem: my program is throwing an empty list exception due to head [] and I need to compile GHC with -prof -auto-all in order to see the stack trace when running it with +RTS -xc -RTS. I changed the makefile but the option +RTS -xc -RTS was not recognized as an available RTS option Does anyone have any idea about how I can do that ? Thanks in advance, __ Monique Louise Monteiro Mestranda em Ciencia da Computacao Centro de Informatica - CIn - UFPE ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
