- Beginners - Haskell.org

A possibly foolish question
by Alan Cameron 12 Feb '09

12 Feb '09

Is it possible to have a Haskell function or functions included in a C++ program's structure and used as a procedure? Alan Cameron

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Help with RWH exercises wanted
by Alan Cameron 12 Feb '09

12 Feb '09

I am baffled by what the correct solution to the exercises in Recursive type section of Chapter 3 should be. <Quote> 1. Write the converse of fromList for the List type: a function that takes a List a and generates a [a]. 2. Define a tree type that has only one constructor, like our Java example. Instead of the Empty constructor, use the Maybe type to refer to a node's children. </Quote> fromList is defined in listADT.hs thus -- file: ch03/ListADT.hs fromList (x:xs) = Cons x (fromList xs) fromList [] = Nil Tree is defined in Tree.hs thus -- file: ch03/Tree.hs data Tree a = Node a (Tree a) (Tree a) | Empty deriving (Show) At this point in the book I fail to find any previous examples which might lead me to my own solution and there are no "Answers to Exercises" as far as I can see. I have a feeling that this might be a fundamental piece of knowledge to assist me in further reading of the book. Alan Cameron

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Deforesting binary tree operations?
by Neal Alexander 12 Feb '09

12 Feb '09

Is it possible to deforest something like this? data Matrix a = Scalar a | Matrix (Matrix a,Matrix a,Matrix a,Matrix a) (*) (Matrix (a11,a12,a21,a22)) (Matrix (b11,b12,b21,b22)) = Matrix ( (a11 * b11) + (a12 * b21), (a11 * b12) + (a12 * b22), (a21 * b11) + (a22 * b21), (a21 * b12) + (a22 * b22) ) (*) (Scalar a) (Scalar b) = Scalar (a * b)

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Recursive Let
by Tom Poliquin 11 Feb '09

11 Feb '09

I'm working on learning arrows. This has led me to ArrowLoop, which then led me to trying to understand the following, > tracePlus b = let (c,d) = (d,b:d) > in c > > main = do > w <- return $ take 10 $ tracePlus 7 > print w > which yields [7,7,7,7,7,7,7,7,7,7] My confusion two pronged, 1) How let actually 'recurses' 2) How things get started (since d is undefined). I have some vague understanding of both issues but I want to make sure what's *really* going on so I don't base my Haskell learning experience on a house of cards. Help greatly appreciated. My ramblings 1) How does recursion happen? Being an ex-Schemer I recalled that let x = 3 get translated into something like (lambda (x) ... ) 3 So now there's a function involved. It's clear that b:d is (cons b d) another function. So with appropriate plumbing I could see the thing recurses but the Haskell viewpoint eludes me. 2) How do things get started? It seems that Haskell could figure out that 'd' is a list. If an uninitialized (undefined?) list contains [] then things are reasonably straightforward .. tracePlus gets applied to 7 (b:d) then becomes [7] (c,d) then becomes ([],7) Given there's an infinite loop (stream) then 'take' grabs 'c' (an []) and asks for another. Haskell's laziness works for us here. (b:d) then becomes [7,7] (c,d) then becomes (7,[7,7]) ... ... This is all fine. The only thing that subconsciously nags me is that we appear to 'return' each time which would imply that we would leave the closure causing 'd' to be undefined again. I know this isn't true and if I close my eyes, think really hard, and recall how streams are implemented in Scheme (with a value-promise pair; the closure is actually passed around) then I can see how this would all work. Back to the undefined list ... if it's *not* an [] and has something to do with bottom ( _|_ ) then my eyes glaze over and I have to go watch a Woody Allen movie to reset. Sorry for the rambling. I really like Haskell and I have found it very powerful. Any help greatly appreciated. Thanks, Tom

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A Comparison of Haskell and Scheme
by Benjamin L.Russell 10 Feb '09

10 Feb '09

Although somewhat dated (posted on October 24, 2006 4:39 PM, to be precise), here is an interesting comparison of Haskell and Scheme, by Mark C. Chu-Carroll, a PhD computer scientist who works as a software engineer at Google, on his blog, regarding whether someone should translate Douglas Hofstadter's columns introducing Scheme to replace the Scheme code with Haskell code: Good Math, Bad Math : Haskell and Scheme: Which One and Why? http://scienceblogs.com/goodmath/2006/10/haskell_and_scheme_which_one_a.php The entry discusses differences in syntax, typing, and semantics, and makes for interesting reading. Surprisingly, he states that Haskell syntax is easier to learn for beginners: >I've actually taught an introduction to computer class using Scheme, >and the syntax was a big problem. > >I think that syntactically, Haskell is a better language for beginners. >Haskell syntax is very redundant, which is good for people. Compare >these two little snippets: > >(define (fact n) > (if (= n 0) > 1 > (* n (fact (- n 1))))) > >fact n = if n == 0 > then 1 > else n * fact(n-1) > >Which is easier to read? And the Haskell can get even easier to read >and write using pattern matching: > >fact 0 = 1 >fact n = n * fact (n-1) > >Try this comparison: > >(define (fold init reducer list) > (if (eq? list ()) > init > (reducer (car list) (fold init reducer (cdr list))))) > >versus: > >fold init reducer [] = init >fold init reducer l:ls = reducer l (fold init reducer ls) > >The difference gets much more obvious with bigger pieces of code. >Between the deliberate redundancies in Haskell's syntax, and the way >that pattern matching lets you decompose programs, the Haskell is >significantly clearer. This is the first time that I have seen a claim that Scheme syntax was a big problem compared to Haskell syntax, but his claims make sense. It makes me wonder whether his students were true beginners.... If his claims are true, though, then we probably need a counterpart to SICP (see http://mitpress.mit.edu/sicp/) for Haskell. The closest I can think of is SOE (see http://www.haskell.org/soe/) any alternatives? There's also _The Haskell Road to Logic, Maths and Programming_ (see http://homepages.cwi.nl/~jve/HR/) and _Real World Haskell_ (see http://book.realworldhaskell.org/) but the former is really about using Haskell to learn discrete mathematics and logic, and the latter is not directed toward computer science majors, so they don't correspond very closely. If Haskell is well-suited as a language for beginners, then there shouldn't be any reason for not creating a Haskell alternative. -- Benjamin L. Russell -- Benjamin L. Russell / DekuDekuplex at Yahoo dot com http://dekudekuplex.wordpress.com/ Translator/Interpreter / Mobile: +011 81 80-3603-6725 "Furuike ya, kawazu tobikomu mizu no oto." -- Matsuo Basho^

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Appending to a list
by Francesco Bochicchio 09 Feb '09

09 Feb '09

Hi all, in my haskell exercises I often build list by appending values at the end. But I read somewhere that in haskell this is inefficient since there is no 'pointer to the tail of the list'. I've also seen some example of recursive functions which build the list tail-first and then in the base case of the recursion returns the reverse of the accumulated results, counting on the fact that in haskell 'reverse list' just means 'consume the list from the tail'. Just out of curiosity (I have no need to speed up my little programs with which I try to teach myself some haskell): is this a consolidated pattern? And are append really expensive? Ciao ------- FB

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Installing packages
by Tymur Porkuian 08 Feb '09

08 Feb '09

I'm trying to install wxcore-0.11.0 using Cabal. I've downloaded it from http://hackage.haskell.org/cgi-bin/hackage-scripts/package/wxcore-0.11.0, unpacked and tried to follow instructions from http://haskell.org/haskellwiki/Cabal/How_to_install_a_Cabal_package, but this is what I'm getting: D:\libraries\wxcore-0.11.0>runhaskell Setup configure Setup: sh: runGenProcess: does not exist (No such file or directory) What does this mean?

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Re: [Haskell-cafe] Re: [Haskell-beginners] Just how unsafe is unsafe
by Roel van Dijk 06 Feb '09

06 Feb '09

On Fri, Feb 6, 2009 at 5:22 PM, Alberto G. Corona <agocorona(a)gmail.com> wrote: > then Data.List.head Data.Maybe.fromMaybe etc are also unsafe?. Yes, I consider them unsafe. Whenever I see those functions I know that I have to look elsewhere to see if their preconditions hold. I would have preferred that listToMaybe was called head and the existing head called unsafeHead, partialHead or something else of that nature. > unsafe does > not mean "with possible errors". unsafeXXX convert something in the IO monad > into something that is not. So it can potentially contaminate your pure > code. > But Data.List.head applied to a empty list will interrupt the computation > abruptly, so your code using head will either run side effect free or not > run at all. I guess what unsafe should mean is a matter of taste. Personally I find correctness more important that pureness. An unsafe function will crash your program if evaluated when its preconditions do not hold. Whether that is because of impurity (segmentation fault?), a partial pattern match or a direct error "bla" is not that important. It might be important when determining why your program crashed, but the result is still the same. The ByteString library has a module called Data.ByteString.Unsafe with plenty of unsafeXXX functions. The comment for unsafeHead for example states: "A variety of head for non-empty ByteStrings. unsafeHead omits the check for the empty case, so there is an obligation on the programmer to provide a proof that the ByteString is non-empty." That perfectly reflects my own philosophy on this issue.

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Parsing a file with data divided into sections
by Patrick LeBoutillier 06 Feb '09

06 Feb '09

Hi all, I wanted to parse a file that looks like this: MONDAY JOHN JIM LINDA TUESDAY BILL BOB WEDNESDAY THURSDAY SAM TODD LARRY LUKE FRIDAY TED in order to count the number of people for each day. After a (very) long time and a lot of trial and error, I managed to do it with this program: import Char (isSpace) main = interact (unlines . countSections . lines) where countSections = map (show) . snd . foldr compileSections (0, []) compileSections line (n, acc) = if isSection line then (0, (line, n) : acc) else (n + 1, acc) isSection line = not . isSpace . head $ line which outputs: ("MONDAY",3) ("TUESDAY",2) ("WEDNESDAY",0) ("THURSDAY",4) ("FRIDAY",1) I had quite a hard time figuring out how to keep count of the number of records in each sections. Is there a more obvious way to handle these types of problems? Are there some builtins that could of made it easier? In Perl I would probably have used a hash and a variable to keep count of the current day, incrementing the hash value for each person until I got to the next day, but it's not obvious to me how to transpose this technique to functional programming. Thanks a lot, Patrick -- ===================== Patrick LeBoutillier Rosemère, Québec, Canada

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Just how unsafe is unsafe
by Andrew Wagner 06 Feb '09

06 Feb '09

So we all know the age-old rule of thumb, that unsafeXXX is simply evil and anybody that uses it should be shot (except when it's ok). I understand that unsafeXXX allows impurity, which defiles our ability to reason logically about haskell programs like we would like to. My question is, to what extent is this true? Suppose we had a module, UnsafeRandoms, which had a function that would allow you to generate a different random number every time you call it. The semantics are relatively well-defined, impurity is safely sectioned off in its own impure module, which is clearly labeled as such. How much damage does this do? Can we push the lines elsewhere? Is sectioning unsafeXXX into Unsafe modules a useful idiom that we can use for other things as well?

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