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Re: [Haskell] [newbye] 'Just a'
by Graham Klyne 10 Feb '05

10 Feb '05

[I've switched my response to the Haskell-cafe list] What do you intend if both (get a) and (get b) return Nothing? You might also be interested in the library function 'maybe': http://www.haskell.org/onlinereport/standard-prelude.html#$vmaybe or maybe (sic) Maybe.fromMaybe in: http://www.haskell.org/onlinereport/maybe.html Using one of these, I think your function could reasonably be expressed as a simple one-liner. #g -- At 19:33 04/02/05 +0100, Yuri D'Elia wrote: >Hi all. I'm experimenting with haskell and its type system. >I've done a function which scans a list, and returns "Just a" >value if the element is found, or Nothing. > >get :: a -> Maybe a > >and implemented > >getAorB :: Maybe a -> Maybe a -> a >getAorB a b = ... > >a function which returns "Just (get a)" or "Just (get b)" if (get a) is >Nothing (excluding failures in both). By now, I've implemented it in >terms of pattern matching: > >getAorB a b = f (get a) (get b) > where f (Just a) Nothing = a > f Nothing (Just a) = a > >but I'd like to know if there are other possible ways to do it, possibly >without enforcing an evaluation order like pattern matching does. > >_______________________________________________ >Haskell mailing list >Haskell(a)haskell.org >http://www.haskell.org/mailman/listinfo/haskell ------------ Graham Klyne For email: http://www.ninebynine.org/#Contact

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Contravariance
by Johan Glimming 10 Feb '05

10 Feb '05

Hi I have a problem with contravariance in the bananas in space setting. If you take catamorphism from a function space such as A=A->A to a type such as BxC, then the negative occurrence of X in the functor GXY=X->Y swaps around BxC into B+C. This means that maybe in this particular case Meijer-Hutton's original Haskell code for cata/ana does not work, since the type for cata would force BxC and B+C to be the same type. I might be wrong about this, and I wonder anybody can comment on this. Med vänliga hälsningar / Yours Sincerely, Johan Glimming

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Name My Functor Classes
by Ashley Yakeley 09 Feb '05

09 Feb '05

These classes seem like they might correspond to concepts in category theory: class Functor f where fmap :: (a -> b) -> f a -> f b -- must satisfy: -- fmap id = id -- fmap (p . q) = (fmap p) . (fmap q) class (Functor f) => C1 f where f1 :: f (a -> b) -> f a -> f b class (C1 f) => C2 f where return :: a -> f a -- must satisfy: -- fmap = f1 . return class (C2 f) => Monad f where (>>=) :: f a -> (a -> f b) -> f b -- must satisfy: -- fmap ab fa = fa >>= (return . ab) -- f1 fab fa = fab >>= (\ab -> fmap ab fa) -- (return a) >>= p = p a -- (fa >>= p) >>= q = fa >>= (\a -> p a >>= q) What are good names for C1, C2 and f1? -- Ashley Yakeley, Seattle WA

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Re: Top 20 ``things'' to know in Haskell
by John Hughes 09 Feb '05

09 Feb '05

> >Message: 9 >Date: Mon, 7 Feb 2005 10:31:30 -0500 >From: "Jacques Carette" <carette(a)mcmaster.ca> >Subject: [Haskell-cafe] Top 20 ``things'' to know in Haskell >To: <haskell-cafe(a)haskell.org> >Message-ID: <000701c50d2a%24189f5590%241b447182(a)cas.mcmaster.ca> >Content-Type: text/plain; charset="us-ascii" > >The recent post of Graham Klyne (below) reminds me that I have meant to ask: >is there a ``top 20'' things a serious programmer should know when writing >code in Haskell? Of course there is a lot of programming language theory >that would be great to know, but I mean really down-to-earth things like the >2 items below (module Maybe, the 'maybe' function). > >The Haskell libraries are quite large, and it is unrealistic to try to get >familiar with all of them right away. But getting a ``small'' list would be >very useful - I think of this as step 2 after one learns to get comfortable >with a language. I had done this (for Maple) for training new hires at >Maplesoft, and I definitely noticed that they became more idiomatic >programmers faster this way. > >Jacques > >PS: of course, this could already exist on haskell.org and/or the Wiki, but >not in an 'obvious' enough place as I missed it... > > Beginners might find this useful: http://www.cs.chalmers.se/Cs/Grundutb/Kurser/d1pt/d1pta/ListDoc/ It's guide to finding the right list function which leads you to it by asking you what you are trying to achieve. john >-----Original Message----- >From: haskell-cafe-bounces(a)haskell.org >[mailto:haskell-cafe-bounces@haskell.org] On Behalf Of Graham Klyne >Sent: February 7, 2005 10:09 AM >To: Yuri D'Elia; haskell-cafe(a)haskell.org >Subject: [Haskell-cafe] Re: [Haskell] [newbye] 'Just a' > >You might also be interested in the library function 'maybe': > http://www.haskell.org/onlinereport/standard-prelude.html#$vmaybe > >or maybe (sic) Maybe.fromMaybe in: > http://www.haskell.org/onlinereport/maybe.html > >#g >-- > > > > > > >

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Re: [Haskell-cafe] How to write (FxF' . delta) AxB --> F A
by Johan Glimming 08 Feb '05

08 Feb '05

> proj :: (Functor f) => (f:*:f') (a,b) -> f a > proj (Pair x _) = fmap fst x Yes I think that was what I needed. ;-) It seems I also need fsplit :: (Functor f, Functor f') => (f a, f' b) -> (f :*: f') (a,b) for same data (f:*:f') o = Pair (f o) (f' o) Which takes (FA, F'B) into FxF' (A,B). I am confused by the fact of having two instances of F and somehow I need to map on both of them simultaneously. Med vänliga hälsningar / Yours Sincerely, Johan Glimming

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RE: [Haskell-cafe] how to name third party libraries?
by Simon Peyton-Jones 08 Feb '05

08 Feb '05

Send mail to libraries(a)haskell.org, saying what's in your libraries, and suggesting names. The process usually converges pretty rapidly. Simon | -----Original Message----- | From: haskell-cafe-bounces(a)haskell.org [mailto:haskell-cafe-bounces@haskell.org] On Behalf Of S. | Alexander Jacobson | Sent: 07 February 2005 22:55 | To: haskell-cafe(a)haskell.org | Subject: [Haskell-cafe] how to name third party libraries? | | Is there documentation somewhere about how to name | third party libraries? | | For example, if I have HTTP and SMTP libraries, | what should I call them? | | -alex- | | | | ______________________________________________________________ | S. Alexander Jacobson tel:917-770-6565 http://alexjacobson.com | _______________________________________________ | Haskell-Cafe mailing list | Haskell-Cafe(a)haskell.org | http://www.haskell.org/mailman/listinfo/haskell-cafe

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help with some code. it doesn't whant to compile.
by pablo daniel rey 08 Feb '05

08 Feb '05

hello i'm new to haskell so i'm sorry if this is a stupid question, but i'm having problems with some basic code. the code : data Maybe Dir = Just Dir | Nothing data Dir = Left | Right | Up | Down data Piece = Vertical | Horizontal | CodeA | CodeB flow = [(Horizontal, Left, Left), (Horizontal, Right, Right), (Vertical, Down, Down), (Vertical, Up, Up), ................ etc ] fst :: (a,b,c) -> a fst (x,y,z) = x scnd :: (a,b,c) -> b scnd (x,y,z) = y third :: (a,b,c) -> c third (x,y,z) = z element :: [(Piece, Dir, Dir)] -> Maybe Dir element [] = Nothing element xs = Just (third (head xs)) chgDir :: Piece -> Dir -> Maybe Dir chgDir p d = element (filter (\x -> p == (fst x)) (filter (\x -> d == (scnd x)) flow)) the error i get : Instances of (Eq Dir, Eq Piece) required for definition of chgDir i don't know what's happening. help!!! thanks in advance

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how to name third party libraries?
by S. Alexander Jacobson 08 Feb '05

08 Feb '05

Is there documentation somewhere about how to name third party libraries? For example, if I have HTTP and SMTP libraries, what should I call them? -alex- ______________________________________________________________ S. Alexander Jacobson tel:917-770-6565 http://alexjacobson.com

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Re: [Haskell] help with some basic code that doesn't work
by Cale Gibbard 08 Feb '05

08 Feb '05

Delete data Maybe Dir = Just Dir | Nothing as it is unnecessary "Maybe a" is defined already for any type "a" (and doesn't make sense, Dir occuring before the = sign would indicate that it is a type variable, but it is uppercase), and also change data Dir = Left | Right | Up | Down data Piece = Vertical | Horizontal | CodeA | CodeB to data Dir = Left | Right | Up | Down deriving Eq data Piece = Vertical | Horizontal | CodeA | CodeB deriving Eq Equality isn't defined by default on user-defined datatypes, so that you can add your own definition. In this case your type is pretty simple, so you can use the derived instance of Eq, so that (==) and (/=) get defined for Dir and Piece. You should also add the line import Prelude hiding (Left, Right) as these are the names of the data constructors for the Either type, and you will get ambiguity errors if you don't rename them or hide the ones in the Prelude. Lastly, you should probably rename your 'fst' to 'frst' or some such, so that it doesn't overlap with the Prelude function of the same name. hope this helps, - Cale On Mon, 7 Feb 2005 20:36:55 +0000, pablo daniel rey <listas(a)pablodanielrey.com.ar> wrote: > hello > i'm new to haskell so i'm sorry if this is a stupid question, but i'm having problems with some basic code. > the code : > > data Maybe Dir = Just Dir | Nothing > data Dir = Left | Right | Up | Down > data Piece = Vertical | Horizontal | CodeA | CodeB > > flow = [(Horizontal, Left, Left), > (Horizontal, Right, Right), > (Vertical, Down, Down), > (Vertical, Up, Up), ................ etc ] > > fst :: (a,b,c) -> a > fst (x,y,z) = x > > scnd :: (a,b,c) -> b > scnd (x,y,z) = y > > third :: (a,b,c) -> c > third (x,y,z) = z > > element :: [(Piece, Dir, Dir)] -> Maybe Dir > element [] = Nothing > element xs = Just (third (head xs)) > > chgDir :: Piece -> Dir -> Maybe Dir > chgDir p d = element (filter (\x -> p == (fst x)) (filter (\x -> d == (scnd x)) flow)) > > the error i get : > > Instances of (Eq Dir, Eq Piece) required for definition of chgDir > > i don't know what's happening. > help!!! > thanks in advance > _______________________________________________ > Haskell mailing list > Haskell(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell >

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"Parsing" a string
by Dmitri Pissarenko 08 Feb '05

08 Feb '05

Hello! I have to read a PGM image and transform it into a list of Int values. I read the image (this is an ASCII PGM format) using the readFile function and get a string with the contents of the file. This string contains height and widht of the image at the beginning, and then the pixel values follow. I need to read the height and width, then "cut" them from the string, create an array (or finite map) of Int's (for this I need to know the height and width), and then recursively process the pixel values (i. e. put them into the array). The string is structured as follows: <string> P2 # comment # comment 320<WHITESPACE>243<WHITESPACE>255 130 130 130 130 130 130 130 130 130 130 </string> P2 is the magic number. All lines starting with # are comments and must be ignored. 320 and 243 are width and height. <WHITESPACE> is either a space, or tab, or newline. At the moment, I don't have an idea about how to read width and height. One possible approach would be to convert the string into a list of strings A, using newline as separator. Then, I could create list A' with comments removed. Then, A' can be transformed into a string A'' again. From A'' I know that width is contained between the end of P2 and the first occurrence of whitespace (Char.isSpace). I also know that height is contained between first and second occurrence of white space. I could use some sort of regular expression analogons to access width and height. I have several questions concerning this approach: 1) How can I transform a string into a list of strings, separated by some character (in Java one uses StringTokenizer for this) ? 2) How do the aforementioned "regular expressions' analogons" work in Haskell? Thanks in advance Dmitri Pissarenko -- Dmitri Pissarenko Software Engineer http://dapissarenko.com

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