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Re: show function
by Hamilton Richards 03 Dec '03

03 Dec '03

At 10:05 PM -0500 12/2/03, rui yang wrote: >I want to print a function which itself have some functions as it's parameters >and will return some functions as the results, and I want to print out the >result, does anyone knows how to define the instance declaration of show class >to this function type? [moving to Haskell Cafe] The mechanics of such an instance definition are simple enough. Letting F be your function type, this would do it: > instance Show F where > showsPrec f = ... some String ... The hard question is what the result should be. Unlike the types already in Show (such as Bool, Int, Char, etc.), there's no canonical representation for functions (other than a complete list of argument-result pairs, which would not be very useful). Regards, --Ham -- ------------------------------------------------------------------ Hamilton Richards, PhD Department of Computer Sciences Senior Lecturer The University of Texas at Austin 512-471-9525 1 University Station C0500 Taylor Hall 5.138 Austin, Texas 78712-0233 ham(a)cs.utexas.edu hrichrds(a)swbell.net ------------------------------------------------------------------

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Mistake in Haskell wiki MonadError?
by Graham Klyne 02 Dec '03

02 Dec '03

At: http://www.haskell.org/hawiki/MonadError I see: [[ instance Monad (Either e) where return = Right (Left e) >>= _ = e (Right a) >>= f = f a ]] Shouldn't that be: [[ (Left e) >>= _ = Left e ]] ? #g ------------ Graham Klyne For email: http://www.ninebynine.org/#Contact

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Re: Why are strings linked lists?
by Mark Carroll 01 Dec '03

01 Dec '03

(shifting to Haskell-Cafe) On Fri, 28 Nov 2003, Donald Bruce Stewart wrote: > ajb: (snip) > > As a matter of pure speculation, how big an impact would it have if, in > > the next "version" of Haskell, Strings were represented as opaque types > > with appropriate functions to convert to and from [Char]? Would there be > > rioting in the streets? I'd be sad to lose some convenient list-based string type because I make a lot of use of the fact that strings are lists in processing them. > You could look at GHC's FastString representation (used internally). > It is in $fptools/ghc/compiler/utils/FastString.lhs It does make sense to have a rather faster form of string conveniently available in /some/ form. -- Mark

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Multiple functions applied to a single value
by Graham Klyne 28 Nov '03

28 Nov '03

There's a possible programming idiom that I repeatedly find myself thinking about as I write my code. It feels like a kind of dual of fold, except that it's not necessarily confined to lists. I'm wondering if anything like this is discussed in the literature, or implemented in the libraries... Scene-setting: broadly speaking, fold takes a function and applies it to several values, and combines the results in some way. What I'm thinking of is a combinator or suchlike that takes two or more functions, applies them (separately) to a single value, then combines the results of those combinations. One function that I have defined and use in my code is called 'flist': flist :: [a->b] -> a -> [b] flist fs a = map ($ a) fs (which is similar to Monad.ap, except that the 'a' parameter is not a list/monad). I could imagine combining this with a fold in some cases. A more generalized form of this that works with arbitrary monads was suggested by Derek Elkin: fmonad :: Monad m => m (a->b) -> a -> m b fmonad fm a = do { f <- fm ; return $ f a } But not all cases I encounter involve lists or monads. A different case might look like this: > eval :: (b->c->d) -> (a->b) -> (a->c) -> (a->d) > eval f g1 g2 a = f (g1 a) (g2 a) So, for example, a function to test of the two elements of a pair are the same might be: > pairSame = eval (==) fst snd giving: > pairSame (1,2) -- false > pairSame (3,3) -- true Or a function to subtract the second and subsequent elements of a list from the first: > firstDiffRest = eval (-) head (sum . tail) > firstDiffRest [10,4,3,2,1] -- 0 That's about as far as I've taken this, but it feels as if it might be part of a more general pattern. Hence my question: is there anything like this in the libraries? #g ------------ Graham Klyne For email: http://www.ninebynine.org/#Contact

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Re: ANNOUNCE: Hugs98 November 2003 release
by Ross Paterson 26 Nov '03

26 Nov '03

[moving to haskell-cafe] On Tue, Nov 25, 2003 at 08:31:12PM +0100, Vincenzo aka Nick Name wrote: > Alle 17:51, martedì 25 novembre 2003, Ross Paterson ha scritto: > > - Integrated .NET support (on Windows). > > What does this exactly mean? See the dotnet/doc subdirectory of the bundle, or http://galois.com/~sof/hugs98.net/ > And, besides, is there any hope of supporting mono? I would guess it's possible, if someone volunteers to do the work. Can't see the point myself.

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RE: Hypothetical reasoning in type classes
by Simon Peyton-Jones 24 Nov '03

24 Nov '03

Yes, absolutely. See http://research.microsoft.com/~simonpj/Papers/derive.htm Section 7, and Trifanov's paper at the Haskell Workshop 2003 Simon | -----Original Message----- | From: haskell-cafe-bounces(a)haskell.org [mailto:haskell-cafe-bounces@haskell.org] On Behalf Of Ken | Shan | Sent: 13 November 2003 05:40 | To: haskell-cafe(a)haskell.org | Subject: Hypothetical reasoning in type classes | | Hello, | | Has anyone thought about adding hereditary Harrop formulas, in other | words hypothetical reasoning and universal quantification, to the | instance contexts in the Hsakell type class system? Just today (and not | only today) I wanted to write instance definitions like | | instance (forall a. C a => D a) => E [a] where ... | | This is analogous to wanting to write a rank-2 dictionary constructor | | (forall a. C a -> D a) -> E [a] | | at the term level, but with type classes, this dictionary constructor | should be applied automatically, in a type-directed fashion, at compile | time. The theory behind such instance contexts doesn't seem so | intractable; indeed it looks decidable to my cursory examination. The | opreational intuition is that we would like the type checker to generate | an eigenvariable "a" and perform hypothetical reasoning. | | I would also like to quantify universally over type classes; in other | words, if "?" is the kind of a type class constraint (aka a dictionary | type; perhaps "o" would be a better choice of notation), then I would | like to define not just types of kind "*->*->?" (aka type classes) or | kind "(*->*)->?" (aka constructor classes), but also types of kind | "(*->?)->(*->?)". But that's for another day... | | Ken | | -- | Edit this signature at http://www.digitas.harvard.edu/cgi-bin/ken/sig | "Hi, my name is Kent, and I let people change my .sig on the internet." | "Hi, Ken!" | "Put midgets back in midget porn!" -- Not authorized by Ken Shan. Supported by | the association of midget porn workers, Local 9823.

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Haskell Server Pages
by "Grigory Makeev 23 Nov '03

23 Nov '03

Hello, I'm interested in writing a personal home-page in a functional way, and i'm in need of some clues. Is there any resource, devoted exactly to the functional approaches to the web programming and web design? I've seen a WASH project, but i'm not really saticefied with the way pages look there. What i trully need is something like Haskell Server Pages system, from Erik Meijer's "Haskell Server Pages. Functional programming and the Battle for the Middle Tier" paper, but i haven't found any implementation of that. I'm not very good at either web programming, or Haskell, i'm just a fan of it, and there's no way i can implement that myself. Is anyone writing currently something like table = <table border="1"> <% mkRows cells%></table> Can anyone provide me with some links? Best regards, Grigory

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collection library in Haskell
by Alexandru D. Salcianu 21 Nov '03

21 Nov '03

Hello! Can somebody recommend me a good collection library in Haskell? I'm interested in more than one Set implementation, one FiniteMap implementation etc. (we already have this in the standard Haskell'98 library). What I'm searching for is a library where I have the Set, FiniteMap etc. specifications (i.e., typeclasses), and several implementations (i.e., instances) for them. E.g., we can have a list-based implementation of Set (for small sets), but also a tree-based implementation (for larger sets). So, I would like a library with something like this -- sa is a type corresponding to sets of a's -- requires multi-parameter typeclasses class (Eq a) => Set a sa where addSetElem :: sa -> a -> sa -- add an element to a set searchElem :: sa -> a -> Bool -- search an element in a set ... -- list based Set implementation instance (Eq a) => Set a [a] where ... -- ordered-tree Set implementation instance (Ord a) => Set a (OrdTree a) where ... and so on. Furthermore, I'm interested in a library that can be compiled / interpreted by a mainstream tool, e.g., ghc, hugs etc. I've already started writting such a library (mainly for didactic purposes), but I would like to know if an enstablished collection library already exists. Thanks, Alex

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Resolving Ambiguous Types
by Jared Warren 21 Nov '03

21 Nov '03

Thanks to Tom Pledger, Brian Boutel, and Keith Wansbrough for helping me with my last type problem, but now I have another one: :) ----- I understand why it is impossible to infer the intermediate type in `show . read`, but I was hoping someone could help me grasp why this (and only this) is problematic: > class Fooable a where foo :: a > instance Fooable Char where foo = '0' > class Barable a where bar :: a -> String > instance Barable Char where bar = const "Zero" `bar foo` generates an ambiguous type error; but anyone can deterministically see that there is only one intermediate type that satisfies the classes. Why can't the compiler figure it out? Are there any compiler directives, language extensions, or Template Haskell tricks I can use to make the compiler smarten up? (The requirements of my project rule out explicit type signatures.) If not, does anyone know if Chameleon's type system can avoid this problem? Thanks for your help, ~ Jared Warren <warren(a)cs.queensu.ca> Computing Science, Queen's University

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Re: Partially ordered collections (revisited)
by Tom Pledger 21 Nov '03

21 Nov '03

Graham Klyne writes: : | Ah, yes. I've grown a little wary of foldl, but I see it's | appropriate in this case. As it's part of the standard prelude, I | personally have no qualms about depending upon it ... or should I? (Hope you don't mind my returning this to haskell-cafe.) Most of foldl's bad reputation comes from stack overflows, because it builds deeply nested suspensions such as foldl f z [1..99] --> f (... (f (f z 1) 2) ...) 99 *regardless* of the strictness of f. Contrast that with foldr foldr f z [1..99] --> f 1 (foldr f z [2..99]) which enters f straight away. Your particular f, i.e. add, is strict in both its parameters, so I don't think the choice between foldl and foldr will affect whether you get stack overflows. However, if you do get stack overflows, try a strict folding function, such as a copy of Hugs's Prelude.foldl' (not exported). - Tom

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