- Haskell-Cafe - Haskell.org

Command line prompt templates
by Dougal Stanton 24 Nov '06

24 Nov '06

I was trying to write my own equivalent to Don Stewart's mkcabal but ended up getting sidetracked. I made some generalised prompts for use at the command line and wanted to get some feedback on them. The full code can be found at [1] but the basic summary is like this: > prompt :: String -> IO String Simple prompt - supply a question and receive the user's answer. > promptYesNo :: String -> Maybe String -> IO Bool A yes/no question, with an optional value to ask about. I suppose this could be simplified to just one string. > promptList :: String -> [String] -> Maybe Integer -> IO Integer Ask the user to choose from a list, where the result is the number chosen. The optional value is a default if the user doesn't pick a number. > promptListEdit :: String -> [String] -> Maybe Integer -> IO String As above, but the last option in the list is an invitation to provide your own answer. Again the optional value is the default choice if nothing else is chosen. The result here is necessarily the string rather than the integer cos I didn't want to complicate matters further with stuff like Either types. If you can see any flaws or have any suggestions please let me know! Cheers, D. [1]: <http://brokenhut.no-ip.org/~dougal/darcs/joincabal/CommandPrompts.hs>

4 3

Equivalent of if/then/else for IO Bool?
by Dougal Stanton 24 Nov '06

24 Nov '06

Is there some sort of equivalent of the if/then/else construct for use in the IO monad? For instance the following can get quite tedious: > do bool <- doesFileExist filename > if bool > then sth > else sth' Is there a more compact way of writing that? Something akin to: > condM (doesFileExist filename) (sth) (sth') Or maybe there's a more sensible way of doing the above that I've missed. I seem to use endless alternate conditions sometimes and there's bound to be a better way.

8 9

Re: [Haskell-cafe] working with lists of couples
by Clare 23 Nov '06

23 Nov '06

Yes the code you are suggesting is certainly linear and it takes without doubt time n. But I was looking for a solution using foldl that of course takes time n. The problem of the following solution is that it gives a reversed result, and of course i can't just reverse the result. couples = snd . foldl f (0,[]) where f (s,[]) x = (s+x, [(x,0)]) f (s,xs) x = (s+x, (:) (x,s) xs) Clare 2006/11/17, Valentin Gjorgjioski <valentin.gjorgjioski(a)ijs.si>: > > On 17.11.2006 21:04 Clare wrote: > > I'm not sure it takes time n couse of the operator ++ and the lazy > > stuffs in haskell. > Ok, you can use > buildCouples (x:xs) s = (x,s) : (buildCouples xs (x+s)) > instead of ++ > > this algorithm is linear, I don't know why(?) you think it is not. > > Valentin > > -- > Valentin Gjorgjioski > Bachelor of Computer Science > Department of Knowledge Technologies, Jozef Stefan Institute > Jamova 39, SI-1000 Ljubljana, Slovenia > Phone: +386 1 477 3343 > Fax: +386 1 477 3315 > Web: http://kt.ijs.si/ValentinGjorgjioski/ > Email: Valentin.Gjorgjioski(a)ijs.si > >

1 0

type keeping rounding, typeable (and a difficulty)
by isto 23 Nov '06

23 Nov '06

Hi, I've been trying to compile the following function (rounding to a desired degree): roundDec :: (Num a, Typeable a) => Int -> a -> a roundDec d a = let t = show (typeOf a) in case t of "Double" -> roundDDec d a "Complex Double" -> roundCDec d a otherwise -> a -- or something The two other functions are roundCDec :: (RealFloat a) => Int -> Complex a -> Complex a roundCDec d (c :+ b) = (roundDDec d c :+ roundDDec d b) and roundDDec :: (RealFloat a) => Int -> a -> a roundDDec d a = a -- or somegthing Compiler gives the following error message: Couldn't match expected type `Complex a' against inferred type `a1' (a rigid variable) `a1' is bound by the type signature for `roundDec' at FFT.hs:57:17 In the second argument of `roundCDec', namely `a' In the expression: roundCDec d a In a case alternative: "Complex Double" -> roundCDec d a If in the roundDDec a's are replaced with Double, there will be similar error message from the "Double"-line. The functionality can be written differently, but I wanted to try write rounding having in a signature at least "(Num a) => Int -> a -> a". Again, any help would be appreciated a lot! Thanks in advance! br, Isto

3 5

Type Error. Why!?
by Tim Newsham 23 Nov '06

23 Nov '06

Can anyone tell me why I get a type error in the following code? I have to define liftA2 twice to avoid the type error. Both times its defined identically but with different type signature. If I use the original liftA2 in place of liftA2' I get: whyerror.lhs:36:25: Ambiguous type variable `a' in the constraint: `Arrow a' arising from use of `>>>' at whyerror.lhs:36:25-27 Possible cause: the monomorphism restriction applied to the following: liftA2' :: forall b a1 b1 c. (a1 -> b1 -> c) -> a b a1 -> a b b1 -> a b c (bound at whyerror.lhs:36:1) unsplit' :: forall a1 b c. (a1 -> b -> c) -> a (a1, b) c (bound at whyerror.lhs:34:1) split' :: forall b. a b (b, b) (bound at whyerror.lhs:33:1) Probable fix: give these definition(s) an explicit type signature or use -fno-monomorphism-restriction ----- whyerror.lhs ---- > {-# OPTIONS_GHC -farrows #-} > module WhyError where > import Control.Arrow > newtype SimpleFunc a b = SimpleFunc { runF :: (a -> b) } > instance Arrow SimpleFunc where > arr f = SimpleFunc f > first (SimpleFunc f) = SimpleFunc (mapFst f) > where mapFst g (a,b) = (g a, b) > (SimpleFunc f) >>> (SimpleFunc g) = SimpleFunc (g . f) > split :: (Arrow a) => a b (b, b) > split = arr (\x -> (x,x)) > unsplit :: (Arrow a) => (b -> c -> d) -> a (b, c) d > unsplit = arr . uncurry > -- arr (\op (x,y) -> x `op` y) > liftA2 :: (Arrow a) => (b -> c -> d) -> a e b -> a e c -> a e d > liftA2 op f g = split >>> first f >>> second g >>> unsplit op > -- f &&& g >>> unsplit op > f, g :: SimpleFunc Int Int > f = arr (`div` 2) > g = arr (\x -> x*3 + 1) > h = liftA2 (+) f g > hOutput = runF h 8 > -- XXX I am getting type problems with split, unsplit and liftA2! why? > split' = arr (\x -> (x,x)) > unsplit' = arr . uncurry > -- liftA2' :: (Arrow a) => (b -> c -> d) -> a e b -> a e c -> a e d > liftA2' op f g = split' >>> first f >>> second g >>> unsplit' op > plusminus, double, h2 :: Kleisli [] Int Int > plusminus = Kleisli (\x -> [x, -x]) > double = arr (* 2) > h2 = liftA2 (+) plusminus double > h2Output = runKleisli h2 8 > main = do > print hOutput > print h2Output Tim Newsham http://www.thenewsh.com/~newsham/

5 6

LLVM back end
by Michael T. Richter 23 Nov '06

23 Nov '06

I've been eyeing LLVM[1] as interesting technology -- brief executive summary: a virtual machine suited as the back end of compiler output with optimised native code then coming from it as either JIT-based execution of the LLVM bytecode or as a further compilation step -- and couldn't help but immediately think of the possibility of one of the Haskell compiler projects providing an LLVM code generator. I think this would help in several areas: * it could make porting the compiler to other architectures -- including oddball ones that would be too small to otherwise support -- easier; * it could help remove the nigh-ubiquitous reliance upon GCC as a back-end (while I think that GCC is a pretty good piece of software, I'm not sure it's really suited to its current role as the "do-everything" back end); * it could leverage some of the really interesting work that's going on in optimisation technology by letting one VM's optimiser do the work for any number of languages; * it could improve interaction between source code written in multiple languages. Is this me opening up a Pandora's Box of ignorance here? Or is LLVM potentially interesting? (And were someone motivated into perhaps trying to make an LLVM back-end, where would one start to poke around in, say, the GHC codebase to even begin to implement this? And how insane would they be driven by the process?) [1]http://llvm.org -- Michael T. Richter Email: ttmrichter(a)gmail.com, mtr1966(a)hotpop.com MSN: ttmrichter(a)hotmail.com, mtr1966(a)hotmail.com; YIM: michael_richter_1966; AIM: YanJiahua1966; ICQ: 241960658; Jabber: mtr1966(a)jabber.cn "I have never seen the poor people being so familiar with their heads of state as they were with [Michele Duvalier]. It was a beautiful lesson for me. I've learned something from it." --Mother Theresa

2 2

Re: help with happy/alex
by Simon Marlow 23 Nov '06

23 Nov '06

robert bauer wrote: > Hi, > > I have a .y and .x file. The alex -g _.x gives me a .hs file and happy -g -a -c _.y gives me a .hs file. > I then use ghc -c alex.hs to get a a .o and a .hi file. This works great. > Unfortunately > ghc -c happy.y doesn't work -- it says that it the module name for the lexer doesn't match. I've attached the > .y and .x files. > > Also, I cannot figure out what I need to do in the parser to call the lexer and I don't know how to kick start the > parser -- I have no clue what to put in third, main.hs, module. > > Any help would be appreciated. You have a typo in the module name in TVScriptLexer.x. As for tying it together, I suggest taking a look at some of the examples, eg. http://darcs.haskell.org/alex/examples/tiny.y from the Alex distribution. Cheers, Simon

1 0

YCR2JS Programmers Guide Draft available
by Dimitry Golubovsky 23 Nov '06

23 Nov '06

Hi, I have finished typing in the draft of low-level programming guide for Yhc Core to Javascript converter. Everyone interested in future use of this tool is encouraged to read and review the Guide. Its purpose is to give some ideas about interaction of Haskell programs converted into Javascript with a web browser on the lowest possible level, without application frameworks and support libraries (just because these haven't been developed). Any feedback and ideas are appreciated. The document is located at: http://haskell.org/haskellwiki/Yhc/Javascript/Programmers_guide Thanks. -- Dimitry Golubovsky Anywhere on the Web

1 0

trivial ghc problem, help needed
by brad clawsie 23 Nov '06

23 Nov '06

i have a program tb.hs: --- module Main where import Network.URI (URI(..), URIAuth(..), parseURI) myFunc :: String -> Maybe URI myFunc u = parseURI u main = do { return () } --- when i attempt to build it with ghc, i get the following output: tb.o: In function `Main_myFunc_info': (.text+0x11): undefined reference to `networkzm2zi0_NetworkziURI_parseURI_closure' tb.o: In function `Main_myFunc_srt': (.rodata+0x0): undefined reference to `networkzm2zi0_NetworkziURI_parseURI_closure' collect2: ld returned 1 exit status any clue why? i would send this to the ghc list but i presume my issue is due to a trivial misunderstanding of the language thanks in advance

2 1

Articles on programming with (IO) exceptions in Haskell?
by Magnus Therning 22 Nov '06

22 Nov '06

I'm having problems finding a good introduction to exceptions. Yet Another Haskell Tutorial leaves it for chapter 10, which is empty. I'd prefer something that suits a noob. :-) /M -- Magnus Therning (OpenPGP: 0xAB4DFBA4) magnus(a)therning.org Jabber: magnus.therning(a)gmail.com http://therning.org/magnus Software is not manufactured, it is something you write and publish. Keep Europe free from software patents, we do not want censorship by patent law on written works. `Time is an illusion. Lunchtime doubly so.' `Very deep,' said Arthur, `you should send that in to the "Reader's Digest". They've got a page for people like you.' -- Ford convincing Arthur to drink three pints in ten minutes at lunchtime.

4 3