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  • Good Haskell coding standards

    - by Alexey Romanov
    Could someone provide a link to a good coding standard for Haskell? I've found this and this, but they are far from comprehensive. Not to mention that the HaskellWiki one includes such "gems" as "use classes with care" and "defining symbolic infix identifiers should be left to library writers only."

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  • Why won't the following haskell code compile?

    - by voxcogitatio
    I'm in the process of writing a small lisp interpreter in haskell. In the process i defined this datatype, to get a less typed number; data Number = _Int Integer | _Rational Rational | _Float Double deriving(Eq,Show) Compiling this fails with the following error: ERROR "types.hs":16 - Syntax error in data type declaration (unexpected `|') Line 16 is the line w. the first '|' in the code above.

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  • Infinite loop in haskell? (newbie)

    - by Mike
    I'm just learning Haskell. I thought this would produce a factorial function... (within ghci) Prelude let ft 0 = 1 Prelude let ft n = n * ft (n - 1) Prelude ft 5 (hangs indefinitely, until ^C). Can someone point me in the right direction? Thanks!

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  • Haskell data serialization of some data implementing a common type class

    - by Evan
    Let's start with the following data A = A String deriving Show data B = B String deriving Show class X a where spooge :: a -> Q [ Some implementations of X for A and B ] Now let's say we have custom implementations of show and read, named show' and read' respectively which utilize Show as a serialization mechanism. I want show' and read' to have types show' :: X a => a -> String read' :: X a => String -> a So I can do things like f :: String -> [Q] f d = map (\x -> spooge $ read' x) d Where data could have been [show' (A "foo"), show' (B "bar")] In summary, I wanna serialize stuff of various types which share a common typeclass so I can call their separate implementations on the deserialized stuff automatically. Now, I realize you could write some template haskell which would generate a wrapper type, like data XWrap = AWrap A | BWrap B deriving (Show) and serialize the wrapped type which would guarantee that the type info would be stored with it, and that we'd be able to get ourselves back at least an XWrap... but is there a better way using haskell ninja-ery? EDIT Okay I need to be more application specific. This is an API. Users will define their As, and Bs and fs as they see fit. I don't ever want them hacking through the rest of the code updating their XWraps, or switches or anything. The most i'm willing to compromise is one list somewhere of all the A, B, etc. in some format. Why? Here's the application. A is "Download a file from an FTP server." B is "convert from flac to mp3". A contains username, password, port, etc. information. B contains file path information. A and B are Xs, and Xs shall be called "Tickets." Q is IO (). Spooge is runTicket. I want to read the tickets off into their relevant data types and then write generic code that will runTicket on the stuff read' from the stuff on disk. At some point I have to jam type information into the serialized data.

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  • Implementing a very simple 'Wine Rating System' in Haskell

    - by Alex N
    Hello, I have just started learning Haskell and have got stumped on how to add a rating to a custom data type. The data type I'm using has a name, a year and a tuple (userName and their rating), it looks like: data Wine = Wine String Int [Rating] deriving (Eq,Ord,Show,Read) type Rating = (String, Int) I wanted to allow a user to rate a given wine from a database, stored as [Wine] but cant figure out how to to it. Any pointers or suggestions would be greatly appreciated! Thanks.

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  • Haskell - interpreting a number

    - by Abstract
    I have a number 9877342931235. Using Haskell, I need to show it as: 987-734293-123-5 i've tried interspersing the list but of course that puts '-' between every digit. How would I do it to yield the actual result?

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  • Haskell and random numbers

    - by John D.
    Hi, I've been messing with Haskell few days and stumbled into a problem. I need a method that returns a random list of integers ( Rand [[Int]] ). So, I defined a type: type Rand a = StdGen -> (a, StdGen). I was able to produce Rand IO Integer and Rand [IO Integer] ( (returnR lst) :: StdGen -> ([IO Integer], StdGen) ) somehow. Any tips how to produce Rand [[Int]]?

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  • Haskell Tuple Size Limit

    - by SHiNKiROU
    Why I can't construct large tuples in Haskell? Why there's a tuple size limit? Prelude> (1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1) <interactive>:1:0: No instance for (Show (t, t1, t2, ... t23)) arising from a use of `print' at <interactive>:1:0-48 Possible fix: add an instance declaration for (Show (t, t1, t2, ... t23)) In a stmt of a 'do' expression: print it

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  • Haskell Binary Tree Function (map)

    - by Bizarro
    How can i define a Haskell function which will apply a function to every value in a binary tree? So i know that it is similar to the map function - and that its type would be: mapT :: (a - b) - Tree a - Tree b but thats about it...

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  • Haskell -> After parsing how to work with strings

    - by bito08
    Hello after doing the parsing with a script in Haskell I got a file with the 'appearance' of lists of strings. However when I call the file content with the function getContents or hGetContents, ie, reading the contents I get something like: String with lines (schematically what I want is: "[" aaa "," bbb "" ccc "]" - ["aaa", "bbb" "ccc"]). I have tried with the read function but without results. I need to work with these lists of strings to concatenating them all in a single list. Thanks.

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  • Haskell "Source reduction"

    - by Martin
    I'm revising for an upcoming Haskell exam and I don't understand one of the questions on a past paper. Google turns up nothing useful fst(x, y) = x square i = i * i i) Source reduce, using Haskells lazy evaluation, the expression: fst(square(3+4), square 8) ii) Source reduce, using strict evaluation, the same expression iii) State one advantage of lazy evaluation and one advantage of strict evaluation

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  • Does F# have an equivalent to Haskell's take?

    - by McMuttons
    In Haskell, there is a function "take n list" which returns the first n elements from a list. For example "sum (take 3 xs)" sums up the first three elements in the list xs. Does F# have an equivalent? I expected it to be one of the List-functions, but I can't spot anything that seems to match.

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  • pointers in haskell???

    - by curioComp
    hi, do you know if are there pointers in haskell? -If yes, how do you use them? Are there any problems with them? And why aren't they popular? -If no, is there any reason for it? Please help us!! :) Thank you so much!!

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  • Look up or insert new element to string list in Haskell

    - by nightscream
    So I want to have a function that takes a String and a list as an argument, and checks if that element is already on the list, if it is, returns the same list, if it isnt, adds it to the list and returns it, 'im a begginer with haskell so heres what I have tried with no sucess: check:: String ->[String] ->[String] check x [] = []++[x] check x (y:xs) | x==y = (y:xs) | otherwise = check x xs Can someone point me the way ? thks

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  • Haskell: monadic takeWhile?

    - by Mark Rushakoff
    I have some functions written in C that I call from Haskell. These functions return IO (CInt). Sometimes I want to run all of the functions regardless of what any of them return, and this is easy. For sake of example code, this is the general idea of what's happening currently: Prelude> let f x = print x >> return x Prelude> mapM_ f [0..5] 0 1 2 3 4 5 Prelude> I get my desired side effects, and I don't care about the results. But now I need to stop execution immediately after the first item that doesn't return my desired result. Let's say a return value of 4 or higher requires execution to stop - then what I want to do is this: Prelude> takeWhile (<4) $ mapM f [0..5] Which gives me this error: <interactive:1:22: Couldn't match expected type `[b]' against inferred type `IO a' In the first argument of `mapM', namely `f' In the second argument of `($)', namely `mapM f ([0 .. 5])' In the expression: takeWhile (< 4) $ mapM f ([0 .. 5]) And that makes sense to me - the result is still contained in the IO monad, and I can't just compare two values contained in the IO monad. I know this is precisely the purpose of monads -- chaining results together and discarding operations when a certain condition is met -- but is there an easy way to "wrap up" the IO monad in this case to stop executing the chain upon a condition of my choosing, without writing an instance of MonadPlus? Can I just "unlift" the values from f, for the purposes of the takeWhile? Is this a solution where functors fit? Functors haven't "clicked" with me yet, but I sort of have the impression that this might be a good situation to use them. Update: @sth has the closest answer to what I want - in fact, that's almost exactly what I was going for, but I'd still like to see whether there is a standard solution that isn't explicitly recursive -- this is Haskell, after all! Looking back on how I worded my question, now I can see that I wasn't clear enough about my desired behavior. The f function I used above for an example was merely an example. The real functions are written in C and used exclusively for their side effects. I can't use @Tom's suggestion of mapM_ f (takeWhile (&lt;4) [0..5]) because I have no idea whether any input will really result in success or failure until executed. I don't actually care about the returned list, either -- I just want to call the C functions until either the list is exhausted or the first C function returns a failure code. In C-style pseudocode, my behavior would be: do { result = function_with_side_effects(input_list[index++]); } while (result == success && index < max_index); So again, @sth's answer performs the exact behavior that I want, except that the results may (should?) be discarded. A dropWhileM_ function would be equivalent for my purposes. Why isn't there a function like that or takeWhileM_ in Control.Monad? I see that there was a similar discussion on a mailing list, but it appears that nothing has come of that.

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  • Haskell Monad bind currying

    - by Chime
    I am currently in need of a bit of brain training and I found this article on Haskell and Monads I'm having trouble with exercise 7 re. Randomised function bind. To make the problem even simpler to experiment, I replaced the StdGen type with an unspecified type. So instead of... bind :: (a -> StdGen -> (b,StdGen)) -> (StdGen -> (a,StdGen)) -> (StdGen -> (b,StdGen)) I used... bind :: (a -> c -> (b,c)) -> (c -> (a,c)) -> (c -> (b,c)) and for the actual function impelemtation (just straight from the exercise) bind f x seed = let (x',seed') = x seed in f x' seed' and also 2 randomised functions to trial with: rndf1 :: (Num a, Num b) => a -> b -> (a,b) rndf1 a s = (a+1,s+1) rndf2 :: (Num a, Num b) => a -> b -> (a,b) rndf2 a s = (a+8,s+2) So with this in a Haskell compiler (ghci), I get... :t bind rndf2 bind rndf2 :: (Num a, Num c) => (c -> (a, c)) -> c -> (a, c) This matches the bind curried with rndf2 as the first parameter. But the thing I don't understand is how... :t bind rndf2 . rndf1 Suddenly gives bind rndf2 . rndf1 :: (Num a, Num c) => a -> c -> (a, c) This is the correct type of the composition that we are trying to produce because bind rndf2 . rndf1 Is a function that: takes the same parameter type(s) as rndf1 AND takes the return from rndf1 and pipes it as an input of rndf2 to return the same type as rndf2 rndf1 can take 2 parameters a -> c and rndf2 returns (a, c) so it matches that a composition of these function should have type: bind rndf2 . rndf1 :: (Num a, Num c) => a -> c -> (a, c) This does not match the naive type that I initially came up with for bind bind f :: (a -> b -> (c, d)) -> (c, d) -> (e, f) Here bind mythically takes a function that takes two parameters and produces a function that takes a tuple in order that the output from rndf1 can be fed into rndf2 why the bind function needs to be coded as it is Why the bind function does not have the naive type

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  • How are lists implemented in Haskell (GHC)?

    - by eman
    I was just curious about some exact implementation details of lists in Haskell (GHC-specific answers are fine)--are they naive linked lists, or do they have any special optimizations? More specifically: Do length and (!!) (for instance) have to iterate through the list? If so, are their values cached in any way (i.e., if I call length twice, will it have to iterate both times)? Does access to the back of the list involve iterating through the whole list? Are infinite lists and list comprehensions memoized? (i.e., for fib = 1:1:zipWith (+) fib (tail fib), will each value be computed recursively, or will it rely on the previous computed value?) Any other interesting implementation details would be much appreciated. Thanks in advance!

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