February 2002 - Haskell-Cafe

Re: a universal printer for Haskell?
by Ashley Yakeley 19 Feb '02

19 Feb '02

At 2002-02-19 09:56, Richard Uhtenwoldt wrote: >>This is reflection! I'd rather not have Haskell contaminated with such >>things; > >Can you say a little more about why you think reflection is bad? >Does it make the language implementation run slower? It's ugly, and isn't part of the spirit of the language. You shouldn't be able to peer into types that way. If you decide to change the private structure of your types, you shouldn't have to worry about what effects it will have on the rest of the program. Consider: module A (Thing()) where { data Thing = MkThing Char; } module B where { import A; peek :: Thing -> Char; peek thing = (\(MApp _ (MChar c)) -> c) (reify thing); } B can look inside a private structure of A. The problem is not the Reify class, but the compiler's autogeneration of instances. >(I'd ask via private email, but when I emailed you in the past >I didn't get a reply.) That sounds like me... -- Ashley Yakeley, Seattle WA

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Re: a universal printer for Haskell?
by Richard Uhtenwoldt 19 Feb '02

19 Feb '02

Ashley Yakeley writes: >This is reflection! I'd rather not have Haskell contaminated with such >things; Can you say a little more about why you think reflection is bad? Does it make the language implementation run slower? (I'd ask via private email, but when I emailed you in the past I didn't get a reply.)

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performance of monads
by Eric Allen Wohlstadter 19 Feb '02

19 Feb '02

I see a lot of literature that says that monads "simulate" the effects of imperative programming concepts. It seems to me that the relative performance of monadic implementations must be equivalant to imperative ones to provide a strong case for functional programming. For example, in the Exception handling monad it seems that the "bind" function must be called continously to pass the Error value back to an error handling function. However, in imperative languages we can make an immediate non-local transfer of control. Similiar is the way State is handled. Do compilers for haskell do any sort of optimization on these monadic operations or is it all as ineffecient as it looks. Eric Wohlstadter UCDavis Software Tools Lab

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Re: a universal printer for Haskell?
by Ch. A. Herrmann 19 Feb '02

19 Feb '02

Hi Haskellers, >>>>> "Bernard" == Bernard James POPE <bjpop(a)cs.mu.OZ.AU> writes: Bernard> Handling functions is always going to be hard. Actually, a Bernard> related issue in Haskell is what do you do with partially Bernard> evaluated structures? Certainly in some circumstances you Bernard> don't want to force the value just so that you can write it Bernard> out. sorry if I wasn't precise enough. I don't want to force ANY evaluation on the value to be marshalled; in some cases the computation may even not terminate! Instead, the string is an encoding of the smallest part of the heap that is necessary to restore the information. My aim is to save a state which contains unevaluated closures to disk and resume the computation at a later time or to send a state to a remote location. The nice property of a functional language, in contrast to imperative languages, is that you can make a snapshot of your computation without any support by the runtime environment if, of course, those (un)marshalling functions are available. Bernard> If you compiled to byte code, life would be a lot Bernard> easier, however everything will get a lot messier if you Bernard> want to mix machine code and byte code. urgh... The problem is to encode program functions, not just functions that arise during the computation. Compiling to byte code is a different issue. Bernard> It would be nice if you could write out some data to file from Bernard> GHC and then read it in using NHC :) That'll be very complicated and require a deep discussion about an interface language. Bernard> I think Clean has a prototype persistence mechanism, I saw Bernard> a short demo late last year. I'm not sure how they Bernard> represent functional values. I think you can only read the Bernard> data from within the same program that wrote the data. That would be OK for me. It would even be OK if it is the same binary, i.e., not to be used in heterogeneous networks. Anyway, I don't like to change my favourite programming language unless it's better than Haskell and that'll be very likely Haskell-II :-) Cheers -- Christoph

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laziness again...
by ketil＠ii.uib.no 19 Feb '02

19 Feb '02

Hi, I'm a bit puzzled by this observatio that I made. I have a function that, pseudocoded, lookes somewhat like f i as bs cs = ins i (f (i+1) as) ++ ins i (f (i+1) bs) ++ ins i (f (i+1) cs) where ins i = manipulates the first element of the list Now, without the ins'es, the function appears to be lazy, i.e I can "take" a part of it without evalutating the rest. With the ins'es, the whole thing seems to be calculated as soon as I touch it. Is this obvious? Or is my observation wrong? -kzm -- If I haven't seen further, it is by standing in the footprints of giants

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Trouble with DtdToHaskell
by Shawn P. Garbett 18 Feb '02

18 Feb '02

-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 I've got a debian install (2.2.18kernel, 2.95.4 gnu c compiler) with ghc installed using hmake. I also found DtdToHaskell was installed on my computer. I run it on any Dtd and get the following: spg@Further:~/clean$ DtdToHaskell clean.dtd module DTD_clean where import Xml2Haskell {-Type decls-} {-Instance decls-} {-Done-} This also happens if I compile DtdToHaskell myself from the source. Any ideas what's misconfigured? - -- You're in a maze of twisty little statements, all alike. Public Key available from http://www.garbett.org/public-key -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.0.6 (GNU/Linux) Comment: For info see http://www.gnupg.org iD8DBQE8cQ/ZDtpPjAQxZ6ARAopFAJ4pePWkXo3TCtadkBR67zHHWoGPRwCfT8qL 86iZ8hdUPZHhbSA3DUVhB84= =k7Ax -----END PGP SIGNATURE-----

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re: order of evaluation ?
by Bernard James POPE 18 Feb '02

18 Feb '02

konst writes: > my question is, if i have an expression such as ((Const False) :&: > <subexp>), will <subexp> be reduced first (according to the definition > 'simplify (x :&: y) = simplify' ((simplify x) :&: (simplify y))') or > will laziness do the right thing, and emit (Const False) without looking > into <exp>? > i think the latter, but would appreciate a word from an expert. Hi Konst, There is an easy way to check, try making <subexp> an erroneous computation. There is such a value in the Prelude, it is called undefined: simplify ((Const False) :&: undefined) If this bombs then you know that simplify wasn't as lazy as you thought, since it must have tried to evaluated 'undefined'. On my version of hugs I get: Program error: {undefined} The important bits of code are: simplify (x :&: y) = simplify' ((simplify x) :&: (simplify y)) simplify' (x :&: (Not y)) | x==y = Const False simplify' ((Const False) :&: _) = Const False The order of the equations for simplify' is important. Effectively pattern matching causes evaluation in Haskell. To determine whether the first equation for simplify' should be used, the second argument of :&: must be evaluated to what is called "weak head normal form" (whnf). This means that the outermost constructor of that argument must be computed. Hence the computation with undefined fails in this case. However, what happens if you swap the order of the equations for simplify'? Doing so will give you the lazyness that you originally expected (for this particular example). Swapping the order of equations is not a silver bullet however, and you must be very careful with how you order them. One of the best places to learn about the operational semantics of languages like Haskell is "The Implementation of Functional Programming Languages" by Simon Peyton Jones. I think it is out of print, but you may find copies in your local uni library if you are lucky. For this particular example, pay close attention to the Pattern Matching Compiler section, which I think was done by Wadler. Cheers, Bernie.

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order of evaluation ?
by Konst Sushenko 17 Feb '02

17 Feb '02

hello, below is the code that i wrote as an excercise for myself (I am still learning haskell). it implements a straighforward way to simplify boolean expressions, and should be self-explanatory. my question is, if i have an expression such as ((Const False) :&: <subexp>), will <subexp> be reduced first (according to the definition 'simplify (x :&: y) = simplify' ((simplify x) :&: (simplify y))') or will laziness do the right thing, and emit (Const False) without looking into <exp>? i think the latter, but would appreciate a word from an expert. thanks konst PS: any coding suggestions, etc. are also welcome infixr 3 :&: infixr 2 :|: data Exp = Const Bool | Sym String | Not Exp | Exp :&: Exp | Exp :|: Exp instance Eq Exp where (Const x) == (Const y) = x==y (Sym x) == (Sym y) = x==y (Not x) == (Not y) = x==y (x :&: y) == (u :&: v) = x==u && y==v || x==v && y==u (x :|: y) == (u :|: v) = x==u && y==v || x==v && y==u _ == _ = False simplify (x :&: y) = simplify' ((simplify x) :&: (simplify y)) simplify (x :|: y) = simplify' ((simplify x) :|: (simplify y)) simplify (Not x) = simplify' (Not (simplify x)) simplify x = x simplify' (Not (Const True)) = Const False simplify' (Not (Const False)) = Const True simplify' (Not (Not x)) = x simplify' ((Not x) :&: y) | x==y = Const False simplify' (x :&: (Not y)) | x==y = Const False simplify' ((Not x) :|: y) | x==y = Const True simplify' (x :|: (Not y)) | x==y = Const True simplify' ((Const False) :&: _) = Const False simplify' (_ :&: (Const False)) = Const False simplify' ((Const True) :&: x) = x simplify' (x :&: (Const True)) = x simplify' ((Const True) :|: _) = Const True simplify' (_ :|: (Const True)) = Const True simplify' ((Const False) :|: x) = x simplify' (x :|: (Const False)) = x simplify' (x :&: y) | x==y = x simplify' (x :|: y) | x==y = x simplify' x = x

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Re: syntax...(strings/interpolation/here docs)
by cwitty＠newtonlabs.com 17 Feb '02

17 Feb '02

"Claus Reinke" <claus.reinke(a)talk21.com> writes: > Haskell definitely supports abstraction and composition, so we can > factor out application aspects (not just text) that need localisation, > and link them (dynamically?) with the main parts of our applications. > Some systematic approach would be useful, but apart from keeping > track of the issues raised in the standards committees, I don't see > why Haskellers should limit themselves to "the" standard way of > patching C#/Java apps with translated text fragments. I think there is a good reason to use standard localisation methods; it makes it cheaper/more likely to happen. It sounds like you're advocating localisation methods which would require the translators to know Haskell; this would make hiring translators more expensive (for a commercial proposition) or significantly reduce your pool of volunteers (if you rely on volunteer translators). Carl Witty

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add something to a list
by christophe certain 17 Feb '02

17 Feb '02

Hi, I'm a poor lonesome newbie in Haskell world, and I would like to add a string typed on the prompt to a list of strings which is already defined. It would look like something like : type Path = [String] currentPath::Path currentPath = [] getpiece ::IO String getpiece = do c <-getLine return c putpiece:: String->Path putpiece a = a:currentPath and then I could combine the two functions, but obviously it doesn't work. I dare understand that it's impossible isn't it ? Maybe the only way is to create a new [String] each time I want to add a new string ? No ? Christophe Certain

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