November 2003 - Haskell-Cafe

Is there an easier way to use guards in an expression...
by Graham Klyne 10 Nov '03

10 Nov '03

<aside> This is almost a reprise of a question that came up back in July [1]. At the time, Mark Jones posted a reference [2], but I'm not seeing how it is relevant to this issue of case expressions. [1] http://haskell.org/pipermail/haskell-cafe/2003-July/004708.html [2] ftp://ftp.research.bell-labs.com/dist/smlnj/papers/92-tr-aitken.ps </aside> The following code works, but it seems a bit clumsy: [[ mapXsdBoolean = DatatypeMap { -- mapL2V :: String -> Maybe Bool mapL2V = \s -> case s of s | matchT s -> Just True | matchF s -> Just False | otherwise -> Nothing -- mapV2L :: Bool -> Maybe String , mapV2L = Just . (\b -> if b then "true" else "false") } ]] The case expression seems rather an unwieldy way of getting guards into an expression. If this was an ordinary variable declaration rather than a field definition, I'd use a function definition with guards, but I can't see how to make that work with a lambda expression. Is there a neater way? And a supplementary question... do the prelude or standard libraries define any function like this: cond :: Bool -> a -> a -> a cond True a _ = a cond False _ b = b #g ------------ Graham Klyne For email: http://www.ninebynine.org/#Contact

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determining if a int is a power
by ddekker6＠chello.nl 10 Nov '03

10 Nov '03

hello all, im new to haskell and have to do some excersises. Well i ran into this problem. I have to determin if a list of int's contains a int that is a power. So i have been trying using "any" with a helper function which return a Bool if the int is a power and applying that to the list. I think that this aproach should work. Anyway the problem im having is in the helper function. I have been trying this: isPower n = let powerF = floor(sqrt(n)) in if (powerF*PowerF == n) then True else False but this doesnt work. Prelude says that the definition of isPower needs a fractional int (if i remember correctly) anyway it looks like something is wrong with my type's? Could you guys help me out? Is there a simple standard function i overlooked. Or what should i change? Thanks in advance guys/gals

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Converting IO Int --> Int
by Tim Stitt 09 Nov '03

09 Nov '03

Can anyone tell me how to convert a value of type IO Int to a value of type Int. I wrote a simple script to read an integer from the user using IO and then the value is returned from the IO function using return value with a return type of IO Int. In the calling function I wish to do a comparison with the user value and a hard-coded integer. Unfortunately I keep getting type errors between the Int and the IO Int. I would be grateful for any assistance in this. Thanks in advance, Tim Stitt

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Beta Reduction, undefined vs Type Classes
by Jared Warren 09 Nov '03

09 Nov '03

Consider: > class Thing t where > thing :: t > > instance Thing Int where > thing = 0 > > instance Thing Char where > thing = 'a' Can someone please explain why > fst (1,thing) ...gets an 'ambiguous type variable' error, but > fst (1,undefined) ...doesn't? And is there anything I can change to make the former work as well? Even modifying fst doesn't work: > fst' :: Thing b => (a,b) -> a > fst' (~a,~b) = a -- ~ Jared Warren <warren(a)cs.queensu.ca> Computing Science, Queen's University

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RE: automaticly create the ana, cata, hylomorphisms
by Marta Isabel Oliveira 08 Nov '03

08 Nov '03

Ok, i read the page but i'm still stuck. I'm reading some papers about cata, ana and hylomorphisms but i need to know where to start putting it to code. So, 1. in order to have a pre-processor, i need to have a module with ALL data types? 2. from this module, how could i create the cata, ana,... i mean, a)how can i represent the result in haskell b) how can i create the cata(whatever) without knowing all of the possible functions that may be inserted in order to it's cata be created? thansk a lot, lesina.

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RE: automaticly create the ana, cata, hylomorphisms
by Marta Isabel Oliveira 07 Nov '03

07 Nov '03

Thanks and sorry. Beside new in Haskell i'm also new in this archive lists. So, i was not aware i had sent with sucess the mail to the first list. lesina > -----Original Message----- > From: Keith Wansbrough [SMTP:Keith.Wansbrough@cl.cam.ac.uk] > Sent: Friday, November 07, 2003 3:00 PM > To: Marta Isabel Oliveira > Cc: haskell(a)haskell.org; haskell-cafe(a)haskell.org > Subject: Re: automaticly create the ana, cata, hylomorphisms > > > Hello programmers, > > > > I have a problem to solve and don't have a clue where to start. > > This is a homework question. Please read > > http://www.haskell.org/hawiki/HomeworkHelp > > for some helpful hints. > > By the way, you shouldn't post the same message to multiple lists - you should choose the right one (haskell-cafe in this case), or if you must, cross-post one message to both. This means people who subscribe to both won't see your message twice. > > --KW 8-) > -- > Keith Wansbrough <kw217(a)cl.cam.ac.uk> > http://www.cl.cam.ac.uk/users/kw217/ > University of Cambridge Computer Laboratory. >

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Re: automaticly create the ana, cata, hylomorphisms
by Keith Wansbrough 07 Nov '03

07 Nov '03

> Hello programmers, > > I have a problem to solve and don't have a clue where to start. This is a homework question. Please read http://www.haskell.org/hawiki/HomeworkHelp for some helpful hints. By the way, you shouldn't post the same message to multiple lists - you should choose the right one (haskell-cafe in this case), or if you must, cross-post one message to both. This means people who subscribe to both won't see your message twice. --KW 8-) -- Keith Wansbrough <kw217(a)cl.cam.ac.uk> http://www.cl.cam.ac.uk/users/kw217/ University of Cambridge Computer Laboratory.

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Haskell pre-processor
by Marta Isabel Oliveira 07 Nov '03

07 Nov '03

Hello programmers, I have a problem to solve and don't have a clue where to start. I need to do a program in haskell that creates automaticly the ana, cata and hylomorphisms to arbitraty types. It's sort of a pre-processer. How can i start? I have the HaPar.hs that has uses Haskell parser to extract the type declarations from a file containing a Haskell module. I also have the TDec1.hs that has some dataTypes defined(TDec1, CDec1, Type) and some Show instances. Can anyone help me? I'm totally new to Haskell :(

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Another fold question
by Patty Fong 06 Nov '03

06 Nov '03

Having struglled over this for the better part of a day and only becoming more frustrated the more i try to understand it, i once again seek help :) I understand how basic folds work, i.e foldr replaces (:) with some parameter and [] by another i.e foldr (+) 0 [1,2,3] becomes 1+(2+(3+0)) I also understand how to write my own fold function. What i don't understand quite is how to use them. given this data type and this fold function i wrote: data Music = Note Pitch Octave Duration | Silence Duration | PlayerPar Music Music | PlayerSeq Music Music | Tempo (Ratio Int) Music data Pitch = Cf | C | Cs type Octave = Int type Duration = Ratio Int foldMusic :: (Pitch -> Octave -> Duration -> a) -> (Duration -> a) -> (a -> a -> a) -> (a -> a -> a) -> (Ratio Int -> a -> a) -> Music -> a foldMusic n _ _ _ _ (Note pitch octive duration) = n pitch octive duration foldMusic _ s _ _ _ (Silence duration) = s duration foldMusic n s p1 p2 t (PlayerPar partOne partTwo) = p1 (foldMusic n s p1 p2 t partOne)(foldMusic n s p1 p2 t partTwo) foldMusic n s p1 p2 t (PlayerPar partA partB) = p2 (foldMusic n s p1 p2 t partA)(foldMusic n s p1 p2 t partB) foldMusic n s p1 p2 t (Tempo rate part) = t rate (foldMusic n s p1 p2 t part) I understand that when i use the foldMusic function i need to pass it 5 parameters. given the type signiature, why can i pass (+) as a parameter for p1 but not for n, what determines what can be passed as a parameter, because they all have the return type a?? I attempted to create a function that utilises the foldMusic function that counts the number of notes: count_notes :: Music -> Integer count_notes = foldMusic (\_-> \_ -> \_ -> 1) (\_ -> 0) (+) (+) (\_ -> \_ -> 0) it appears to work, i think. Yet i'm still not certain of how it does so. This confuses me, Is there anyway to represent other fold functions in a tree like representation as foldr (+) 0 would appear as such? + 1 \ + 2 \ + 3 \ 0 Regards, confused Patrick _________________________________________________________________ Hot chart ringtones and polyphonics. Go to http://ninemsn.com.au/mobilemania/default.asp

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Another Newbie question :)
by Patty Fong 05 Nov '03

05 Nov '03

I seem to understand the basic idea behind the fold function, i think but do not seem to be able to write them myself. My understanding is that it works by replacing every constructor with a fold ? Could anyone please point me in the direction of a suitable resource of attempt to explain this to me :) Cheers, Paty _________________________________________________________________ Hot chart ringtones and polyphonics. Go to http://ninemsn.com.au/mobilemania/default.asp

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