August 2011 - Beginners

Iterating a monadic action with memoization
by Tim Cowlishaw 29 Jan '12

29 Jan '12

Hi all, I was wondering if any of you knew whether the following was possible, (I discussed this a little on #haskell at the weekend but didn't quite get to the bottom of it): Say I have a monadic action: > f :: a -> m a and an initial value of type a. I'd like to iterate this action and collect the results of each iteration in a list, as with 'iterate' on normal functions: > iterate (>>= f) . return $ a However, I would also like to memoize the intermediate results of the iteration, such that the entire iteration runs in O(n) time rather than O(n^2). This also implies a slight semantic difference, as side-effecting or non-deterministic actions will not be repeated in each iteration, while maintaining the laziness of the iteration function. Is it possible to do this? Many thanks, Tim

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Help with enumerator pipeline
by Michael Xavier 01 Sep '11

01 Sep '11

I've just recently started learning how to use the enumerator library. I'm designing a utility that parses a file where each line is a JSON object. I've designed it to look like: source enumerator (enumHandle pretty much) -> chunk by lines (enumeratee) -> parse a line into an Object (enumeratee) -> filter objects based on a criteria (enumeratee) -> limit some keys from each object (enumeratee) -> encode the object into a lazy bytestring (enumeratee) -> output the file to stdout (iteratee) I'm having difficulties with the types, particularly composing the enumeratees in the middle. Someone in #haskell said that's a good case for >=> from Control.Monad but that seems to not like it if an enumeratee changes the type between the input and output. Here's the relevant types: type Object = Map Text Value pipeline :: MonadIO m => (Enumerator ByteString IO ()) -> [Text] -> [Filter] -> Iteratee a m () pipeline s rfs fs = s $$ splitLines >=> parseLine >=> (filterObjects fs) >=> (restrictFields rfs) >=> encoder $$ output splitLines :: Monad m => Enumeratee ByteString ByteString m b parseLine :: Monad m => Enumeratee ByteString Object m b filterObjects :: Monad m => [Filter] -> Enumeratee Object Object m b restrictObjects :: Monad m => [Text] -> Enumeratee Object Object m b encoder :: Monad m => Enumeratee Object LBS.ByteString m b output :: MonadIO m => Iteratee LBS.ByteString m () I'm using >=> because I'd prefer to compose them left-right for readability Here's the error I get Couldn't match expected type `ByteString' with actual type `M.Map Text Value' Expected type: Data.Enumerator.Step ByteString m b1 -> Iteratee ByteString m b2 Actual type: Enumeratee ByteString Object m0 b0 In the first argument of `(>=>)', namely `parseLine' In the second argument of `(>=>)', namely `parseLine >=> (filterObjects fs) >=> (restrictFields rfs) >=> encoder' Any ideas? -- Michael Xavier http://www.michaelxavier.net LinkedIn <http://www.linkedin.com/pub/michael-xavier/13/b02/a26>

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Help me improve my code
by Neuman Vong 31 Aug '11

31 Aug '11

Hi Haskell people, I'm pretty new to Haskell still. There were a bunch of things I didn't know how to do in the following script, I'm hoping some people on this list can help with. For example, I had trouble returning an ExitCode and using getProgName without getting a compile-time type error. I feel like I'm doing something wrong with the Text/[Char] conversions too. I'd also really appreciate any style tips. Thanks in advance! {-# LANGUAGE OverloadedStrings #-} module Main where import System (getArgs) import System.IO (hPutStrLn, stderr) import Data.Text (pack, splitOn, length, isPrefixOf, Text) import Prelude hiding (length) data Rate = Rate Text Text deriving (Show) findBestPrefix number rates = foldl1 longestPrefix $ matching rates where getLength rate = length $ getPrefix rate getPrefix (Rate prefix _) = prefix getPrice (Rate _ price) = price longestPrefix r1 r2 = if getLength r1 > getLength r2 then r1 else r2 matching rates = [ rate | rate <- rates, getPrefix rate `isPrefixOf` number ] makeRates = map $ \line -> let (prefix:rate:_) = splitOn ", " (pack line) in Rate prefix rate main = getArgs >>= \args -> let findBest number rates = findBestPrefix number $ makeRates rates in case args of (arg:_) -> interact $ (++ "\n") . show . findBest (pack arg) . lines _ -> hPutStrLn stderr "Pass in a number as the first argument" -- neuman

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haskell stack trace..
by Sunil S Nandihalli 31 Aug '11

31 Aug '11

Hello everybody, I was wondering if there is a way to get the haskell stack trace when I throw the error. I found this http://hackage.haskell.org/trac/ghc/wiki/ExplicitCallStack but it seems like it is only a proposal to do something.. has anything happened? Thanks, Sunil.

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to modify the eagerness level of haskell compiler..
by Sunil S Nandihalli 31 Aug '11

31 Aug '11

Hi everybody, May be I am doing it wrong being a beginner.. but here is what I want.. I am trying to debug a stack overflow problem by printing a lot using "trace" commands. But it does not seem to seem to be printing in the expected order. so it has been misleading quiet a bit. Can somebody guide me as to how to modify the laziness of the program or some pointers on how to debug the stack overflow errors. Thanks in advance. Sunil.

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Reducing local variable duplication
by Michael Orlitzky 30 Aug '11

30 Aug '11

I have a series of unit tests like this, > test_volume1 :: Assertion > test_volume1 = > assertEqual "volume is correct" True (vol ~= (-1/3)) > where > p0 = (0, -0.5, 0) > p1 = (0, 0.5, 0) > p2 = (2, 0, 0) > p3 = (1, 0, 1) > ... > > test_volume2 :: Assertion > test_volume2 = > assertEqual "volume is correct" True (vol ~= (1/3)) > where > p0 = (0, -0.5, 0) > p1 = (2, 0, 0) > p2 = (0, 0.5, 0) > p3 = (1, 0, 1) > ... I would like to de-duplicate some of the values in the 'where' clause. Furthermore, I don't want to declare e.g. p0 and p3 at the top level, since later tests may need different values of p0,p3. What I would really like is something like the following. Is it possible? > let p0 = (0, -0.5, 0) > p3 = (1, 0, 1) > in > test_volume1 :: Assertion > test_volume1 = > assertEqual "volume is correct" True (vol ~= (-1/3)) > where > p1 = (0, 0.5, 0) > p2 = (2, 0, 0) > ... > > test_volume2 :: Assertion > test_volume2 = > assertEqual "volume is correct" True (vol ~= (1/3)) > where > p1 = (2, 0, 0) > p2 = (0, 0.5, 0) > ...

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Re: [Haskell-beginners] some tools like .Net reflector?
by anyzhen 30 Aug '11

30 Aug '11

thank you ------------------ Original ------------------ From: "Claude Lee"; Date: 2011年8月30日(星期二) 中午12:30 To: "anyzhen"; Cc: "beginners"; Subject: Re: [Haskell-beginners] some tools like .Net reflector? Hi, Haskell code usually compiles to native code, which means it can decompile to ASM, but not the original Haskell code, especially when optimization applied. However, GHC can produce some sort of bytecode "using LLVM as a backend", but I don't think it's designed for production use. See http://www.haskell.org/ghc/ for more details. Thanks, Claude PS: License seems to be your first concern on the issue, not decompilers... ;) 2011/8/30 anyzhen <jiangzhen3s(a)qq.com> hi guys. is it exists some tools in haskell platfrom, such like "reflector" or "DeAsmIL" in .Net platform ? sometime get a compiled version without source code,and i don't know how explore it. thanks any help. jiangzhen _______________________________________________ Beginners mailing list Beginners(a)haskell.org http://www.haskell.org/mailman/listinfo/beginners

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haskell error message
by Sunil S Nandihalli 30 Aug '11

30 Aug '11

Hi everybody, can somebody explain as to what the following error message means.. [1 of 1] Compiling Main ( main.hs, main.o ) SpecConstr Function `$wf{v s3mp} [lid]' has four call patterns, but the limit is 3 Use -fspec-constr-count=n to set the bound Use -dppr-debug to see specialisations SpecConstr Function `$wf{v X3sC} [lid]' has three call patterns, but the limit is 1 Use -fspec-constr-count=n to set the bound Use -dppr-debug to see specialisations Linking main ... Thanks, Sunil.

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some tools like .Net reflector?
by anyzhen 30 Aug '11

30 Aug '11

hi guys. is it exists some tools in haskell platfrom, such like "reflector" or "DeAsmIL" in .Net platform ? sometime get a compiled version without source code,and i don't know how explore it. thanks any help. jiangzhen

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Fwd: Strange behavior
by Luca Ciciriello 29 Aug '11

29 Aug '11

Begin forwarded message: > From: Luca Ciciriello <luca_ciciriello(a)hotmail.com> > Subject: Re: [Haskell-beginners] Strange behavior > Date: August 29, 2011 3:43:39 PM GMT+02:00 > To: Daniel Fischer <daniel.is.fischer(a)googlemail.com> > > I'm very sorry. My mistake. > foldP works as expected as shown by the code below: > > {-# LANGUAGE PArr, ParallelListComp #-} > {-# OPTIONS -fdph-seq #-} > > module Test where > > import GHC.PArr > > > boolArr = [: False, False, False :] > > isTrue :: Bool > isTrue = foldP (||) False [: x | x <- boolArr :] > > Here the result of "isTrue" is False, as expected. > So, seems that the only problem is about "elemP" > > Sorry again. > > Luca. > > > On Aug 29, 2011, at 3:28 PM, Daniel Fischer wrote: > >> Hi Luca, >> >> On Monday 29 August 2011, 11:57:05, you wrote: >>> Ok, Thanks. >>> This works fine. I found out that also mapP and sumP are working as >>> expected, but elemP and foldP do not behave correctly. >> >> Have you a short example of foldP behaving wrongly? >> The dph people will have it easier to find the bug if they have one. >> >> Cheers, >> Daniel >> >

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