June 2003 - Haskell-Cafe

Why no findM ? simple Cat revisited
by Ahn Ki-yung 11 Nov '03

11 Nov '03

Simple Cat (revisitied) \begin{code} import IO findM f [] = return Nothing findM f (x:xs) = do { v <- x; if f v then return (Just v) else findM f xs } isLeft (Left _) = True isLeft _ = False main = findM (isLeft) (hCat stdin) where hCat h = try (hGetLine h) : hCat h \end{code} This is my answer for the question of my own, which is posted a couple There are mapM, filterM in the Haskell 98 Standard Library. But why no findM there ? As you can see from simple cat, it seems quite useful. I think fildM should be added to the module Monad. -- Ahn Ki-yung

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Simple monads
by Mark Carroll 15 Jul '03

15 Jul '03

Not really seeing why Unique is in the IO monad, not deeply understanding the use of Haskell extensions in the State source, and wanting to try to learn a bit more about monads, I thought I'd try to write my own monad for the first time: something for producing a series of unique labels. This is how it turned out: ========================================================================== module Label (Label, Labeller, newLabel) where import Monad newtype Label = Label Int deriving (Eq, Ord) newtype Labeller a = Labeller (Int -> (Int, a)) instance Monad Labeller where return r = Labeller (\n -> (n, r)) (Labeller g) >>= y = let f m = let (r, n) = g m Labeller h = y n in h r in Labeller f newLabel :: Labeller Label newLabel = Labeller (\n -> (n + 1, Label n)) runLabeller :: Labeller a -> a runLabeller (Labeller l) = snd (l minBound) labelTest :: Labeller [Int] labelTest = do Label a <- newLabel Label b <- newLabel Label c <- newLabel Label d <- newLabel return [a,b,c,d] main = print (runLabeller labelTest) ========================================================================== I was thinking that maybe, (a) People could point out to me where I'm still confused, as revealed by my code. Is it needlessly complicated? (b) My code may be instructive to someone else. -- Mark

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RE: Template Haskell question
by Simon Peyton-Jones 12 Jul '03

12 Jul '03

Yes, sorry, as Ian says, type splices just aren't implemented at the moment. The error message is uninformative. This is useful info though -- someone wants type splices! Simon | -----Original Message----- | From: haskell-cafe-admin(a)haskell.org [mailto:haskell-cafe-admin@haskell.org] On Behalf Of Ashley | Yakeley | Sent: 27 June 2003 23:57 | To: haskell-cafe(a)haskell.org | Subject: Template Haskell question | | Can anyone tell me what's wrong with this? | | -- ghc -fglasgow-exts -c TH.hs | module TH where | { | import Language.Haskell.THSyntax; | | class HasZero a where | { | zero :: a; | }; | | aninstance :: TypQ -> Q [Dec]; | aninstance t = [d| | | instance HasZero $t where -- error here | { | zero = 0; | }; | | |]; | | $(aninstance [t|Int|]) | $(aninstance [t|Integer|]) | } | | | $ ghc -fglasgow-exts -c TH.hs | TH.hs:14: Malformed context in instance header | | All I want to do is spin off a number instances for a number of types... | | -- | Ashley Yakeley, Seattle WA | | _______________________________________________ | Haskell-Cafe mailing list | Haskell-Cafe(a)haskell.org | http://www.haskell.org/mailman/listinfo/haskell-cafe

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UML diagrams of Haskell
by Bas van Dijk 30 Jun '03

30 Jun '03

Hi, It's maybe a weird question but do there exist UML diagrams of the Haskell language. They don't necessarily have to be UML diagrams, as long as they show the different concepts in the language (i.e.: functions, datatypes, patterns, etc.) and the relations between them(i.e.: a function consists of other functions, functions have a type). Bas.

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RE: Arrow Classes
by Simon Peyton-Jones 30 Jun '03

30 Jun '03

It may interest you guys to know that Ross Paterson and I have just put syntactic support for arrows into GHC. It's only in the CVS version at the moment, and is not documented at all, but those things will come. The type error messages should, I hope, be a lot better than those one can get by type checking pre-processed code. Simon | -----Original Message----- | From: haskell-cafe-admin(a)haskell.org [mailto:haskell-cafe-admin@haskell.org] On Behalf Of Joe | English | Sent: 28 June 2003 17:01 | To: haskell-cafe(a)haskell.org | Subject: Re: Arrow Classes | | | Ashley Yakeley wrote: | > Wolfgang Jeltsch wrote: | > | > > This brings me to another point. One year ago we had a discussion on The | > > Haskell Mailing List concerning arrows. (The subject of the mails was just | > > "arrows".) The point was that it seemed strange to me that first and second | > > are included in the basic arrow class Arrow while left and right have their | > > extra class ArrowChoice. Not only that it seemed strange to me but it made | > > impossible to make Parser baseMonad an instance of Arrow. Parser baseMonad | > > has nice implementations of pure and (>>>) but none of first or second. | > | > I agree. My own Arrow module hierarchy looks more or less like this: | > | > class Compositor comp where [...] | > class (Compositor arrow) => Arrow arrow where [...] | > class (Arrow arrow) => ProductArrow arrow where [...] | > class (Arrow arrow) => CoproductArrow arrow where [...] | > class (ProductArrow arrow,CoproductArrow arrow) => FullArrow arrow | > instance (ProductArrow arrow,CoproductArrow arrow) => FullArrow arrow | > class (Arrow arrow) => ArrowFix arrow where [...] | > class (FullArrow arrow) => ApplyArrow arrow where [...] | | | On that topic, see below for what mine looks like | (from HXML, <URL: http://www.flightlab.com/~joe/hxml/ >). | | I started off with Hughes' conventions, but for some | reason could never remember the difference between &&& and ***, | or between ||| and +++. I found &&&, >&<, |||, >|< to have | better mnemonic value. This also frees up +++ for ArrowPlus, | which -- in HXML applications -- is frequently used and should | thus be easy to type. | | When using the ArrowChoice operators, I kept tripping over all | the 'Either' coproduct types, so added some syntactic sugar | (borrowed from HaXML): | | data Choice a = a :> a | class (Arrow a) => ArrowChoice a where | [ ... ] | ( ?>) :: (b -> Bool) -> Choice (a b c) -> a b c | (>?>) :: a b Bool -> Choice (a b c) -> a b c | | I found "p ?> f :> g" much more pleasant to use. | | (I also like the idea of splitting the product operators out of | the base Arrow class -- will consider doing that in my library). | | -- | | infixr 5 +++ | infixr 3 >&<, &&& | infixr 2 >|<, |||, ?>, >?>, :> | infixl 1 >>> | | class Arrow a where | arr :: (b -> c) -> a b c | (>>>) :: a b c -> a c d -> a b d | apfst :: a b c -> a (b,x) (c,x) | apsnd :: a b c -> a (x,b) (x,c) | (>&<) :: a b c -> a d e -> a (b,d) (c,e) | (&&&) :: a b c -> a b d -> a b (c,d) | liftA2 :: (b -> c -> d) -> a e b -> a e c -> a e d | aConst :: c -> a b c | idArrow :: a b b | -- Minimal implementation: arr, >>>, apfst or >&< | | data Choice a = a :> a | class (Arrow a) => ArrowChoice a where | apl :: a b c -> a (Either b d) (Either c d) | apr :: a b c -> a (Either d b) (Either d c) | (>|<) :: a b c -> a d e -> a (Either b d) (Either c e) | (|||) :: a b c -> a d c -> a (Either b d) c | ( ?>) :: (b -> Bool) -> Choice (a b c) -> a b c | (>?>) :: a b Bool -> Choice (a b c) -> a b c | -- Minimal implementation: >|< or apl | | class (Arrow a) => ArrowApply a where | app :: a (a b c,b) c | | class (Arrow a) => ArrowZero a where | aZero :: a b c | aMaybe :: a (Maybe c) c | aGuard :: (b -> Bool) -> a b b | | class (Arrow a) => ArrowPlus a where | (+++) :: a b c -> a b c -> a b c | | | | --Joe English | | jenglish(a)flightlab.com | _______________________________________________ | Haskell-Cafe mailing list | Haskell-Cafe(a)haskell.org | http://www.haskell.org/mailman/listinfo/haskell-cafe

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RE: UML diagrams of Haskell
by Hal Daume 30 Jun '03

30 Jun '03

There's something like UML, called FAD (Functional Analysis and Design, I believe). It was the topic of Daniel Russell's PhD thesis. See http://www.cs.kent.ac.uk/pubs/2001/1152/index.html for more. -- Hal Daume III | hdaume(a)isi.edu "Arrest this man, he talks in maths." | www.isi.edu/~hdaume > -----Original Message----- > From: haskell-cafe-admin(a)haskell.org > [mailto:haskell-cafe-admin@haskell.org] On Behalf Of Bas van Dijk > Sent: Sunday, June 29, 2003 2:50 PM > To: HaskellCafe Mailinglist > Subject: UML diagrams of Haskell > > > Hi, > > It's maybe a weird question but do there exist UML diagrams > of the Haskell > language. > They don't necessarily have to be UML diagrams, as long as > they show the > different concepts in the language (i.e.: functions, > datatypes, patterns, > etc.) and the relations between them(i.e.: a function > consists of other > functions, functions have a type). > > Bas. > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe >

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RE: Simple monads
by Hal Daume 29 Jun '03

29 Jun '03

> (a) People could point out to me where I'm still confused, as > revealed by > my code. Is it needlessly complicated? looks pretty reasonable to me :) as to why Unique is in the IO monad is probabyl because if it were in any other monad, you could start the monad twice and thus get a repeat of each ID (if that makes sense). The only way around this is to put it in IO.

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Bom dia haskell-cafe
by Marta 28 Jun '03

28 Jun '03

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Template Haskell question
by Ashley Yakeley 27 Jun '03

27 Jun '03

Can anyone tell me what's wrong with this? -- ghc -fglasgow-exts -c TH.hs module TH where { import Language.Haskell.THSyntax; class HasZero a where { zero :: a; }; aninstance :: TypQ -> Q [Dec]; aninstance t = [d| instance HasZero $t where -- error here { zero = 0; }; |]; $(aninstance [t|Int|]) $(aninstance [t|Integer|]) } $ ghc -fglasgow-exts -c TH.hs TH.hs:14: Malformed context in instance header All I want to do is spin off a number instances for a number of types... -- Ashley Yakeley, Seattle WA

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RE: haskell array access
by Hal Daume 27 Jun '03

27 Jun '03

> At best case for Haskell, 15.5 times slower. The thing about > bounds checking, > in Haskell it's always there. In C, you might have it, you > might not there is > no certainty by the language, only by design and > implementation. So with C, > one is free to live dangerously. If you're using GHC, there's Data.Array.Base.unsafeAt which is the one which doesn't do bounds checking.

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