- Beginners - Haskell.org

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

3 4

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.

2 1

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.

2 3

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) > ...

3 4

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

1 0

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.

3 7

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

2 1

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 >> >

1 0

Strange behavior
by Luca Ciciriello 29 Aug '11

29 Aug '11

I found a strange behavior in my code Here a (very) simplified version of my code: {-# LANGUAGE PArr #-} {-# OPTIONS -fdph-seq #-} module Main where import Control.Parallel import GHC.PArr parArr :: [:String:] parArr = [: "1", "2", "3", "4" :] isElement :: String -> Bool isElement x = x `elemP` parArr main :: IO () main = do putStrLn $ (show . isElement) "5" The aspected result is "False" ("5" isn't a member of parArr), but the result I get is "True". I've compiled this code with the command: ghc --make -Odph -threaded Test.hs -rtsopts -o test and launched with: test +RTS -N2 My system is MacOS X 10.7.1 with Xcode 4.1 and GHC 7.0.4 (I've found the same behavior on Windows). Its obvious I'm missing something. Could someone help me? Thanks. Luca.

3 4

problem using hsc2hs
by Graham Gill 29 Aug '11

29 Aug '11

I've been trying to build package unix-compat-0.2.2.1 on a Windows XP machine which has the latest Haskell package installed (2011.2.0.1). I use "cabal install unix-compat". (I've updated my package list.) I run into a problem when the file Extensions.hs is generated from Extensions.hsc using hsc2hs. Extensions.hs is created, but empty. When I run the cabal install with verbose on, I'm able to see the hsc2hs command which generates Extensions.hs. Running that hsc2hs command with verbose on, I see the steps where it creates intermediate files .c, .o and .exe, then runs the .exe, which sends its output to stdout, and that stdout is redirected into Extentions.hs. The intermediate files are deleted. Extensions.hs is empty. However, if I manually run each command which hsc2hs runs, Extensions.hs has the intended contents. I've reproduced the details below. Could this be a case of a problem with how lazy IO is used? Here are the details. This command (I've cut out the paths from Extensions* files, so they're all created in/read from the current directory), "C:\Program Files\Haskell Platform\2011.2.0.1\bin\hsc2hs.exe" --cc="C:\Program Files\Haskell Platform\2011.2.0.1\mingw\bin\gcc.exe" --ld="C:\Program Files\Haskell Platform\2011.2.0.1\mingw\bin\gcc.exe" --cflag=-fno-stack-protector --lflag=-fno-stack-protector --cflag=-D__GLASGOW_HASKELL__=700 --cflag=-Dmingw32_BUILD_OS --cflag=-Dmingw32_HOST_OS --cflag=-Di386_BUILD_ARCH --cflag=-Di386_HOST_ARCH --cflag=-DMISSING_POSIX_TYPES --cflag="-IC:\Program Files\Haskell Platform\2011.2.0.1\lib\directory-1.1.0.0\include" --cflag="-IC:\Program Files\Haskell Platform\2011.2.0.1\lib\old-time-1.0.0.6\include" --cflag="-IC:\Program Files\Haskell Platform\2011.2.0.1\lib\Win32-2.2.0.1\include" --cflag="-IC:\Program Files\Haskell Platform\2011.2.0.1\lib\bytestring-0.9.1.10\include" --cflag="-IC:\Program Files\Haskell Platform\2011.2.0.1\lib\base-4.3.1.0\include" --cflag="-IC:\Program Files\Haskell Platform\2011.2.0.1\lib/include" --cflag="-IC:\Program Files\Haskell Platform\2011.2.0.1\lib/include" --lflag="-LC:\Program Files\Haskell Platform\2011.2.0.1\lib\directory-1.1.0.0" --lflag="-LC:\Program Files\Haskell Platform\2011.2.0.1\lib\old-time-1.0.0.6" --lflag="-LC:\Program Files\Haskell Platform\2011.2.0.1\lib\old-locale-1.0.0.2" --lflag="-LC:\Program Files\Haskell Platform\2011.2.0.1\lib\filepath-1.2.0.0" --lflag="-LC:\Program Files\Haskell Platform\2011.2.0.1\lib\Win32-2.2.0.1" --lflag=-luser32 --lflag=-lgdi32 --lflag=-lwinmm --lflag=-ladvapi32 --lflag=-lshell32 --lflag=-lshfolder --lflag="-LC:\Program Files\Haskell Platform\2011.2.0.1\lib\bytestring-0.9.1.10" --lflag="-LC:\Program Files\Haskell Platform\2011.2.0.1\lib\base-4.3.1.0" --lflag=-lwsock32 --lflag=-luser32 --lflag=-lshell32 --lflag="-LC:\Program Files\Haskell Platform\2011.2.0.1\lib\integer-gmp-0.2.0.3" --lflag="-LC:\Program Files\Haskell Platform\2011.2.0.1\lib\ghc-prim-0.2.0.0" --lflag="-LC:\Program Files\Haskell Platform\2011.2.0.1\lib" --lflag=-lm --lflag=-lwsock32 --lflag="-LC:\Program Files\Haskell Platform\2011.2.0.1\lib" --verbose -o Extensions.hs Extensions.hsc run in a Windows command shell, generates Extensions_hsc_make.c, Extensions_hsc_make.o, Extensions_hsc_make.exe, and then runs .\Extensions_hsc_make.exe >Extensions.hs After this the intermediate files Extensions_hsc_make.* are deleted. Extensions.hs has no contents. Since I ran with verbose on, I could see each step. Running each step individually: 1) create Extensions_hsc_make.c from Extensions.hsc, using --no-compile flag to hsc2hs: "C:\Program Files\Haskell Platform\2011.2.0.1\bin\hsc2hs.exe" --cc="C:\Program Files\Haskell Platform\2011.2.0.1\mingw\bin\gcc.exe" --ld="C:\Program Files\Haskell Platform\2011.2.0.1\mingw\bin\gcc.exe" --cflag=-fno-stack-protector --lflag=-fno-stack-protector --cflag=-D__GLASGOW_HASKELL__=700 --cflag=-Dmingw32_BUILD_OS --cflag=-Dmingw32_HOST_OS --cflag=-Di386_BUILD_ARCH --cflag=-Di386_HOST_ARCH --cflag=-DMISSING_POSIX_TYPES --cflag="-IC:\Program Files\Haskell Platform\2011.2.0.1\lib\directory-1.1.0.0\include" --cflag="-IC:\Program Files\Haskell Platform\2011.2.0.1\lib\old-time-1.0.0.6\include" --cflag="-IC:\Program Files\Haskell Platform\2011.2.0.1\lib\Win32-2.2.0.1\include" --cflag="-IC:\Program Files\Haskell Platform\2011.2.0.1\lib\bytestring-0.9.1.10\include" --cflag="-IC:\Program Files\Haskell Platform\2011.2.0.1\lib\base-4.3.1.0\include" --cflag="-IC:\Program Files\Haskell Platform\2011.2.0.1\lib/include" --cflag="-IC:\Program Files\Haskell Platform\2011.2.0.1\lib/include" --lflag="-LC:\Program Files\Haskell Platform\2011.2.0.1\lib\directory-1.1.0.0" --lflag="-LC:\Program Files\Haskell Platform\2011.2.0.1\lib\old-time-1.0.0.6" --lflag="-LC:\Program Files\Haskell Platform\2011.2.0.1\lib\old-locale-1.0.0.2" --lflag="-LC:\Program Files\Haskell Platform\2011.2.0.1\lib\filepath-1.2.0.0" --lflag="-LC:\Program Files\Haskell Platform\2011.2.0.1\lib\Win32-2.2.0.1" --lflag=-luser32 --lflag=-lgdi32 --lflag=-lwinmm --lflag=-ladvapi32 --lflag=-lshell32 --lflag=-lshfolder --lflag="-LC:\Program Files\Haskell Platform\2011.2.0.1\lib\bytestring-0.9.1.10" --lflag="-LC:\Program Files\Haskell Platform\2011.2.0.1\lib\base-4.3.1.0" --lflag=-lwsock32 --lflag=-luser32 --lflag=-lshell32 --lflag="-LC:\Program Files\Haskell Platform\2011.2.0.1\lib\integer-gmp-0.2.0.3" --lflag="-LC:\Program Files\Haskell Platform\2011.2.0.1\lib\ghc-prim-0.2.0.0" --lflag="-LC:\Program Files\Haskell Platform\2011.2.0.1\lib" --lflag=-lm --lflag=-lwsock32 --lflag="-LC:\Program Files\Haskell Platform\2011.2.0.1\lib" --no-compile -o Extensions.hs Extensions.hsc 2) create Extensions_hsc_make.o from Extensions_hsc_make.c: "C:\Program Files\Haskell Platform\2011.2.0.1\mingw\bin\gcc.exe" -c Extensions_hsc_make.c -o Extensions_hsc_make.o -fno-stack-protector -D__GLASGOW_HASKELL__=700 -Dmingw32_BUILD_OS -Dmingw32_HOST_OS -Di386_BUILD_ARCH -Di386_HOST_ARCH -DMISSING_POSIX_TYPES -I"C:\Program Files\Haskell Platform\2011.2.0.1\lib\directory-1.1.0.0\include" -I"C:\Program Files\Haskell Platform\2011.2.0.1\lib\old-time-1.0.0.6\include" -I"C:\Program Files\Haskell Platform\2011.2.0.1\lib\Win32-2.2.0.1\include" -I"C:\Program Files\Haskell Platform\2011.2.0.1\lib\bytestring-0.9.1.10\include" -I"C:\Program Files\Haskell Platform\2011.2.0.1\lib\base-4.3.1.0\include" -I"C:\Program Files\Haskell Platform\2011.2.0.1\lib/include" -I"C:\Program Files\Haskell Platform\2011.2.0.1\lib/include" -I"C:/Program Files/Haskell Platform/2011.2.0.1/lib/include/" 3) create Extensions_hsc_make.exe from Extensions_hsc_make.o: "C:\Program Files\Haskell Platform\2011.2.0.1\mingw\bin\gcc.exe" Extensions_hsc_make.o -o Extensions_hsc_make.exe -fno-stack-protector -L"C:\Program Files\Haskell Platform\2011.2.0.1\lib\directory-1.1.0.0" -L"C:\Program Files\Haskell Platform\2011.2.0.1\lib\old-time-1.0.0.6" -L"C:\Program Files\Haskell Platform\2011.2.0.1\lib\old-locale-1.0.0.2" -L"C:\Program Files\Haskell Platform\2011.2.0.1\lib\filepath-1.2.0.0" -L"C:\Program Files\Haskell Platform\2011.2.0.1\lib\Win32-2.2.0.1" -luser32 -lgdi32 -lwinmm -ladvapi32 -lshell32 -lshfolder -L"C:\Program Files\Haskell Platform\2011.2.0.1\lib\bytestring-0.9.1.10" -L"C:\Program Files\Haskell Platform\2011.2.0.1\lib\base-4.3.1.0" -lwsock32 -luser32 -lshell32 -L"C:\Program Files\Haskell Platform\2011.2.0.1\lib\integer-gmp-0.2.0.3" -L"C:\Program Files\Haskell Platform\2011.2.0.1\lib\ghc-prim-0.2.0.0" -L"C:\Program Files\Haskell Platform\2011.2.0.1\lib" -lm -lwsock32 -L"C:\Program Files\Haskell Platform\2011.2.0.1\lib" 4) and creating Extensions.hs using Extensions_hsc_make.exe: Extensions_hsc_make.exe >Extensions.hs This generates a file with the intended contents. There are two other files in the unix-compat package, Files.hsc and User.hsc, which run into the same problem. So what's the difference between running each step individually, which works, and running the combination of all steps, which gives me an empty file? cheers! Graham

1 0