January 2008 - Haskell-Cafe

Field updates in a state monad
by Michael Roth 11 Jan '08

11 Jan '08

Hello list, still playing with monads and states, I have the following question: Given: import Control.Monad.State.Lazy data MyData = MyData { content :: String } foobar :: State MyData String foobar = do gets content Ok, that looks nice and tidy. But: foobar2 :: State MyData () foobar2 = do modify $ \x -> x { content = "hello haskell"} ...looks not so nice. Exists there a way to write this cleaner without writing countless "set_xyz" helper functions? Michael

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Re: [Haskell-cafe] Re: Why purely in haskell?
by Miguel Mitrofanov 11 Jan '08

11 Jan '08

> However, the fact that (0 / 0) == (0 / 0) yields False is quite shocking. Just for the record: the following is from Firebug (JavaScript debugger for Firefox) session: >>> a = 0/0 NaN >>> a == a false >>> a === a false

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Haskell-mode 2.4
by Stefan Monnier 11 Jan '08

11 Jan '08

Here's for Xmas, version 2.4 of the Haskell-mode package. Haskell-mode is an Elisp package to support editing Haskell in Emacs. See the home page at http://www.iro.umontreal.ca/~monnier/elisp/ -- Stefan Changes since 2.3: * Update license to GPLv3. * New derived major mode for .hsc files. * Removed the C-c C-r binding to reload a file. You can still call inferior-haskell-reload-file (and/or bind it to your favorite key, including C-c C-r) or you can now use C-u C-c C-l. * C-c C-d looks up the symbol at point in the Haddock docs. * Haddock comments are highlighted with font-lock-doc-face if it exists. * Use `tex' rather than `latex' for haskell-literate. * inf-haskell.el tries to find the root of the module hierarchy to determine the root of a project (either by looking for a Cabal file or relying on the `module' declaration line). If all works well, this will make C-c C-l automatically switch to the root dir, so that dependencies in other directories are automatically found. If it doesn't, complain and/or set inferior-haskell-find-project-root to nil. * The new command haskell-hoogle helps you query Hoogle from Emacs.

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Displaying steps in my interpreter
by Victor Nazarov 11 Jan '08

11 Jan '08

Hello, I have little practice in Haskell. And I look forward for suggestions on how to improve the code. Code is not working: some definitions are missed. The goal of the code is to implement the evaluator for untyped lambda-calculus. The main problem is how to display each step of reduction? More detailed questions follows in the end of the code. Terms of lambda-calculus is the following data-type: > > data (Binder bndr) => > Term bndr = > Var bndr > | Lam bndr (Term bndr) > | App (Term bndr) (Term bndr) > Binder class is used to generate new unique binders and to compare binders > > class (Eq bndr) => Binder bndr > where {- ... -} > Next we'll define the evaluator. Evaluator returns the WHNF for each well-formed term, or _|_ if there is no WHNF Straight forward version: whnf :: Term bndr -> Term bndr whnf = reduce [] where reduce (a:as) (Lam b i) = reduce as $ subst b a i reduce as (App f a) = reduce (a:as) f reduce as term = foldl term App as But our goal is to perform only one step of reduction. And to display term after each reduction We refactor the original definition to perform this: > > whnfGen :: (Maybe String -> Bool) -> Term bndr -> Term bndr > whnfGen isFinal = runSteps isFinal $ reduce' [] > > where reduce' (a:as) (Lam b i) = markStep "beta" >> reduce as $ subst b a i > reduce' as (App f a) = reduce (a:as) f > reduce' as term = unwind as term > > reduce as term = > do final <- checkFinal > if final > then unwind as term > else reduce' as term > > unwind as term = return $ foldl term App as > Steps is the monad: > > newtype Steps mark a = > Steps { unSteps :: StateT (Maybe mark) (Reader (Maybe mark -> Bool)) a } > > instance Monad (Steps mark) > where (Steps a) >>= f = Steps $ a >>= \x -> unSteps (f x) > return a = Steps $ return a > > runSteps :: (Maybe mark -> Bool) -> Steps a -> (Maybe mark, a) > runSteps isFinal act = runReader (runStateT (unSteps act) Nothing) isFinal > > checkFinal :: Steps mark Bool > checkFinal = > do st <- Steps $ get > isFinal <- Steps $ lift $ ask > return $ isFinal st > > markStep :: mark -> Steps mark () > markStep = Steps . put . Just > Normal whnf can be written as follows: > > whnf = whnfGen (const False) > To print the reduction steps we can use the following code > > printReductions t = > do t' <- whnfGen isJust t > print t' > printReductions t' > Is there any better ways to implement printReductions? Is there a way to abstract out this steps pattern? For example if I want to show the process of converting lambda-terms to combinatory logic terms (I, K, S basis). I'll have to implement the same pattern. I can reuse Steps monad, but, I'll have to use markStep, checkFinal and special version of recursion. Is there a way to abstract this? Is there a more effective way to perform this? In printReductions I have to rescan the syntax tree over and over again to find the redex. Is there a way to save the position in the tree, to print the tree and then to resume from saved position? Thanx for reading this :) -- vir http://vir.comtv.ru/

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The Monad.Reader (10) - Second call for copy
by Wouter Swierstra 11 Jan '08

11 Jan '08

Call for Copy The Monad.Reader - Issue 10 It is not too late to consider writing something for the next issue of The Monad.Reader. The deadline for Issue 10 is ** January 25, 2007 ** It doesn't matter if you're an established academic or if you have only just started learning Haskell, if you have something to say, please write article for The Monad.Reader. The Monad.Reader is a electronic magazine about all things Haskell. It is less-formal than journal, but somehow more enduring than a wiki- page. There have been a wide variety of articles, including: exciting code fragments, intriguing puzzles, book reviews, tutorials, and even half-baked research ideas. * Submission Details * Get in touch with me if you intend to submit something -- the sooner you let me know what you're up to, the better. Please submit articles for the next issue to me by e-mail (wss at cs.nott.ac.uk). Articles should be written according to the guidelines available from http://www.haskell.org/haskellwiki/The_Monad.Reader Please submit your article in PDF, together with any source files you used. The sources will be released together with the magazine under a BSD license. If you would like to submit an article, but have trouble with LaTeX please let me know and we'll sort something out. All the best, Wouter This message has been checked for viruses but the contents of an attachment may still contain software viruses, which could damage your computer system: you are advised to perform your own checks. Email communications with the University of Nottingham may be monitored as permitted by UK legislation.

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Re: [Haskell-cafe] Re: Why purely in haskell?
by Miguel Mitrofanov 11 Jan '08

11 Jan '08

> As GNU is not Unix, NaN is not a number, Since NaN /= NaN, I think, we should decipher "NaN" as "Not a NaN" instead.

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Subtypes and Co/Contra-variance
by Ashley Yakeley 11 Jan '08

11 Jan '08

Consider this: type Super = forall a. [a] type Sub = forall b. b cast0 :: Sub -> Super cast0 s = s cast1a :: forall p. (Super -> p) -> (Sub -> p) cast1a sp x = sp x cast1b :: forall p. (Super -> p) -> (Sub -> p) cast1b sp = sp This compiles except for cast1b (ghc -c -fglasgow-exts): Occurs check: cannot construct the infinite type: a = [a] When generalising the type(s) for `cast1b' I get a similar result when I try a different Super and Sub: type Super = (?x :: Bool) => Int type Sub = Int Couldn't match expected type `Sub' against inferred type `Super' When matching `(?x::Bool) => Int' and `Int' Expected type: Sub -> p Inferred type: Super -> p In the expression: sp In either case, if cast1a compiles, shouldn't cast1b compile? -- Ashley Yakeley

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Tim Sweeney (the gamer)
by Galchin Vasili 11 Jan '08

11 Jan '08

Hello, I have been reading with great interested Tim Sweeney's slides on the Next Generation Programming Language. Does anybody know his email address? Kind regards, Vasili

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RE: [Haskell-cafe] Is there anyone out there who can translate C# generics into Haskell?
by Nicholls, Mark 10 Jan '08

10 Jan '08

[snip] >-- C#: interface IX1 { String foo1(int); } >class IX1 obj where > foo1 :: Int -> obj -> String Yep...I think that's what I'd do....though I would have done... "foo1 :: obj -> Int -> String" Does that matter? >-- C#: interface IX2<A> { String foo2(A); } >class IX2 obj a where > foo2 :: a -> obj -> String Ok same here >--C#: interface IX3<A> where A : IY { String foo3(A); } >class IY a where {- ... -} >class IY a => IX3 obj a where > foo3 :: a -> obj -> String Yep I think again I would have guessed at that >--C#: interface IX4<A> : IZ where A : IY >class IZ a where {- ... -} >class (IY a, IZ obj) => IX4 obj a where > foo4 :: a -> obj -> String Hmmmm...this would have been one of my guesses....but let me have a go... >This assumes your "objects" are immutable, otherwise you would have to >return (obj,String) instead of just String and then you most likely want to >use the state monad and "do notation" to make functional programming look >more like imperative programming. This is fine....my oop is largely immutable. >You really have to drop the OO way of thinking, >which I find the hardest :) >Haskell's type classes are more powerful in some sense than C# interfaces; >for example, in C# you can't attach an interface to any class (take for >example the Point struct), it has to be your own class, while in Haskell, >you can implement an instance of a type class for any datatype! OK but I was going to go onto Interface IX<A> where A : IX<A> { } And Interface IX<A,B> where A : B { } Where I cannot see a way to do the above in Haskell at all....as interfaces effectively operator on type classes not types....which seems inherently more powerful But if these could be done in Haskell the see what could be made of stuff like....which is obviously problematic in C# it obviously doesn't work....but potentially does make 'sense'. Interface IX<A> : A { } >Hope this helps a bit. As I'm new myself to Haskell, so take this with a >"grain of salt". It does...I will have a go with your sample answers. >Once you bite the bullet, I found Haskell a lot more fun than C#, although >C# of course comes with a gigantic .NET framework and tools... I'm looking at Haskell because of the formality of it's type system....but I'm actually not convinced it is as powerful as an OO one....i.e. OO ones operatate principally (in Haskell speak) on "type classes" not "types" >Peter -----Original Message----- From: haskell-cafe-bounces(a)haskell.org [mailto:haskell-cafe-bounces@haskell.org] On Behalf Of Nicholls, Mark Sent: Wednesday, January 02, 2008 5:41 PM To: haskell-cafe(a)haskell.org Subject: [Haskell-cafe] Is there anyone out there who can translate C# generics into Haskell? I'm trying to translate some standard C# constucts into Haskell... some of this seems easy.... Specifically 1) Interface IX { } 2) Interface IX<A> { } 3) Interface IX<A> Where A : IY { } 4) Interface IX<A> : IZ Where A : IY { } I can take a punt at the first 2....but then it all falls apart _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe(a)haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

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The Worker/Wrapper Transformation
by Achim Schneider 10 Jan '08

10 Jan '08

...is a paper about automatic specialisation of functions by unboxing arguments, one could say. I'm only on page 6, but already survived the first formalisms, which is bound to mean that the rest of the paper is likewise accessible, as hinted on at ltu. http://www.cs.nott.ac.uk/~gmh/wrapper.pdf The transformation itself is mindbogglingly easy, which makes this a good start: You only have to understand the formalisms, not so much what the formalisms are representing. To quote spj: It usually turns out to be more interesting and challenging than it seemed at first. I'm tempted to write that this is a paper for everyone trying to figure out what the heck Jonathan is talking about. -- (c) this sig last receiving data processing entity. Inspect headers for past copyright information. All rights reserved. Unauthorised copying, hiring, renting, public performance and/or broadcasting of this signature prohibited.

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