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Referring to several constructors at once
by Tomer Libal 12 Jan '09

12 Jan '09

Hello, I am trying to define a logical calculus using one data type for the rules. There are three types of rules and each has 0,1 or 2 assumptions (rule, rule1 and rule2 below). I have defined all the rules as different constructors so the difference between the types according to 0,1 or 2 assumptions is very weak. I would still like to be able to group the rules in types according to the number of assumptions in order to use pattern matching. Is there a simple way to do that or another way I should implement the data type such that I can refer to the rules both according to their number of assumptions and according to their type? 27 data Rule = Axiom {lowseq :: Sequent} 28 | WeakeningL {rule :: Rule, lowseq :: Sequent, foccur :: FormulaOccur} 29 | WeakeningR {rule :: Rule, lowseq :: Sequent, foccur :: FormulaOccur} 30 | ContractionL {rule :: Rule, lowseq :: Sequent, foccur :: FormulaOccur} 31 | ContractionR {rule :: Rule, lowseq :: Sequent, foccur :: FormulaOccur} 32 | PermutationL {rule :: Rule, lowseq :: Sequent} 33 | PermutationR {rule :: Rule, lowseq :: Sequent} 34 | Mix {rule1 :: Rule, rule2 :: Rule, lowseq :: Sequent, foccur :: FormulaOccur} 35 | NotL {rule :: Rule, lowseq :: Sequent, foccur :: FormulaOccur} 36 | NotR {rule :: Rule, lowseq :: Sequent, foccur :: FormulaOccur} 37 | AndL {rule1 :: Rule, rule2 :: Rule, lowseq :: Sequent, foccur :: FormulaOccur} 38 | AndR {rule1 :: Rule, rule2 :: Rule, lowseq :: Sequent, foccur :: FormulaOccur} 39 deriving (Eq, Show) Thanks, Tomer

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untilM and scanM
by Jan Snajder 11 Jan '09

11 Jan '09

Hi, is there a reason why there is no monadic version of "until" in the Haskell libraries? It would be defined as follows: untilM :: (Monad m) => (a -> Bool) -> (a -> m a) -> a -> m a untilM p f x | p x = return x | otherwise = f x >>= untilM p f The same applies to scanM, also not part of the libraries: scanM :: (Monad m) => (a -> b -> m a) -> a -> [b] -> m [a] scanM f q [] = return [q] scanM f q (x:xs) = do q2 <- f q x qs <- scanM f q2 xs return (q:qs) I often find myself in need for these. To me these seem idiomatic enough to be included in the library. But since they is not, I guess there must be another, more idiomatic way to do this. Thank you, Jan

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typeOf raises Error
by Frank Schwidom 11 Jan '09

11 Jan '09

Hi, if im calling (on the console) "typeOf 1" after importing Data.Typeable, i will gain "Integer", as i expected, but if i do so in code which is to compile, then there raises an error ------------ import Data.Typeable main= print (typeOf 1) ------------ $ ghc -c DT.hs $ ghc -c DT.hs DT.hs:4:15: Ambiguous type variable `t' in the constraints: `Num t' arising from the literal `1' at DT.hs:4:15 `Typeable t' arising from a use of `typeOf' at DT.hs:4:8-15 Probable fix: add a type signature that fixes these type variable(s) what can i do, to solve this problem? Regards

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First request for references
by Alan Cameron 10 Jan '09

10 Jan '09

Hi, I stumbled across Haskell while researching 'functional programming'. I have done some initial reading and have the following questions: 1. Are there any references to real-world applications using this language? I ask because most of the articles I have found seem to be of an extreme academic nature. I want to be sure that my time will not be wasted delving deeper. 2. Are there any tutorials with full code included which I can use on the HUGS system I have installed? I have found a few "A Gentle Introduction to Haskell 98" which assumes far too much previous knowledge to be immediately useful to me. Thanks for any replies. Alan Cameron

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how to print a floating-point number?
by Sergei Winitzki 08 Jan '09

08 Jan '09

Subject: how to print a floating-point number? hi, I am very new to Haskell. I am trying to benchmark the CPU time needed for a computation, and I can't figure out how to print a floating-point number. My code is as follows, and I expected it to work: import System.CPUTime main = do let result = some_computation print result time <- getCPUTime -- this is an Integer that needs to be divided by 1e12 to get time in seconds print (time / 1.0e12) -- I want this to print a floating-point number But this does not compile. Error message: No instance for (Fractional Integer) arising from use of `/' at fact1.hs:18:15-28 Possible fix: add an instance declaration for (Fractional Integer) In the first argument of `print', namely `(time1 / 1.0e12)' I thought this should work because e.g. 12 / 7 evaluates to 1.7142857142857142 in ghci. I understand this is some problem with types, but surely it is fixed not by adding any instance declarations but perhaps by using some Prelude or Numeric function. But which one? I tried everything I could find in the documentation: showFloat, adding ::Float everywhere, adding fromIntegral, etc.etc. - nothing works. All the books and the tutorials I looked at seem to discuss at length such nice things as Fibonacci numbers and recursive factorial functions rather than a practical problem like this. help will be much appreciated! Sergei

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Some confusion still with a YAHT example
by David Schonberger 08 Jan '09

08 Jan '09

Hello all. This is my first post and I'm essentially a rank beginner with Haskell, having used it briefly some 7 years ago. Glad to be back but struggling with syntax and understanding error messages. This post actually ties to a prior post, http://www.haskell.org/pipermail/beginners/2008-December/000583.html My troubles are with exercise 3.10, p45 of from HDIII's "Yet Another Haskell Tutorial" (YAHT). The exercise states: Write a program that will repeatedly ask the user for numbers until she types in zero, at which point it will tell her the sum of all the numbers, the product of all the numbers, and, for each number, its factorial. For instance, a session might look like: Note that the sample session accompanying the exercise suggests a session such as: Give me a number (or 0 to stop): 5 Give me a number (or 0 to stop): 8 Give me a number (or 0 to stop): 2 Give me a number (or 0 to stop): 0 The sum is 15 The product is 80 5 factorial is 120 8 factorial is 40320 2 factorial is 2 The following code handles the sum and products pieces--I built the module incrementally--but fails on the factorial part. In fact my current code, if it worked, would only output something like: The sum is 15 The product is 80 120 40320 2 But I'm not even getting that much. Here's the code: --begin code module AskForNumbers whereimport IO askForNums = do putStrLn "Enter a pos int or 0 to end: " numStr <- getLine let num = read numStr if num == 0 then return [] else do rest <- askForNums return (num:rest) listFactorial l = if length l == 0 then return 1 else do fact (head l) listFactorial (tail l) fact n = if n == 0 then return 1 else return foldr (*) 1 [1..n] f = do nums <- askForNums putStr ("Sum is " ++ (show (foldr (+) 0 nums)) ++ "\n") putStr ("Product is " ++ (show (foldr (*) 1 nums)) ++ "\n") listFactorial nums --end code Here is the error msg I get when I load into WinHugs (Sept 2006 version): ERROR file:.\AskForNumbers.hs:22 - Ambiguous type signature in inferred type *** ambiguous type : (Num a, Num [a], Monad ((->) [b]), Num c, Num (b -> [a] -> [a]), Enum a, Monad ((->) (c -> c -> c))) => a -> [b] -> [a] *** assigned to : fact Ambiguous type assigned to fact. Ok. Not sure what to make of that or how to correct it. I though I was passing fact an Integer, since I think nums is a list of Integers, since the sum and product lines in f work ok. Help? Thanks. David

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Excess mem consumption in file IO task
by Moritz Tacke 07 Jan '09

07 Jan '09

Hi! I have some resource problems when extracting data from a file. The task is as follows: I have a huge (500MB) binary file, containing some interesting parts and lots of rubbish. Furthermore, there is a directory that tells me the parts of the file (first- and last byte index) that contain the substrings I need. My approach to do this is to open the file and to pass the list of addresses along with the handle to a function that processes the list step-by-step and calls a subfunction which uses the handle to seek the start position of the interesting block, reads the block into a bytestring (lazy or not, didn't make any difference here) and calls the function that scans this byte string for the interesting part. Using this approach - which results in a data structure with an approximate size of 10 MB - the program uses hundreds of megabytes of RAM, which forces my computer to swap (with the obvious results...). I have right now two main suspects: The recursive function is tail-recursive, but I don't know whether the usual way to write these functions (with an accumulator etc) works in monadic code (the stage is, of course, the IO monad, and I am using the do-notation as I don't like the only other way I know, writing lambdas and lambdas and lambdas into the function body). The other problem I can imagine is the passing-around of the file handle, and the subsequent reading of byte strings: Are those strings somehow attached to the handle, and does the handle work in a different way than I expected, i.e. is the handle copied while using it as an argument for another function, and exists something like a register of handles that keeps the connection upright and, therefore, excludes the (handle, string)-chunk from garbage collection? I have, of course, been experimenting with the "seq" - function, but, honestly, I am not sure whether I got it right. Does a call to "identity $! (function arguments ...)" force the full evaluation of the function? Greetings! Moritz

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Yet another monad tutorial.
by Rafael Gustavo da Cunha Pereira Pinto 06 Jan '09

06 Jan '09

Hello everyone I am a very eclectic person when it comes to computer languages, and I found this monad tutorial for OCaml programmers: http://enfranchisedmind.com/blog/2007/08/06/a-monad-tutorial-for-ocaml/ I found it to be very interesting, since it shows those "warm, fuzzy things" implemented in another functional language with very clear explanations of what he is doing in each step. Best regards, and a happy 2k9 for you all! Rafael -- Rafael Gustavo da Cunha Pereira Pinto Electronic Engineer, MSc.

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Problems inferring instances
by dcmorse+haskell＠gmail.com 05 Jan '09

05 Jan '09

Learning from the example of "read" and also Real World Haskell, I come across the idea to overload my function's return types. Trying to think of an application for this, I've always wanted to write == applications like in Icon, that is a === b === c means a == b && b == c. This requires === to sense what context it is called in. If it's being called for a Boolean value, it needs to return a Boolean value. If it's being called as a parameter to another === application, then it needs to somehow remember both it's truthiness and if true what value its already seen. The idea is to eventually expand this to be able to write a >== b >== c ...but one thing at a time! My plan of attack is to write a typeclass: class Chain a b c where (==) :: a -> b -> c First check: Turned on -fglasgowexts to allow multiple parameters to typeclasses. Then write instances for the various contexts in which == might be applied. For boolean return values there are three instances: instance Eq a => Chain a a Bool ... example: 5 == 4 instance Eq a => Chain (Valid a) a Bool example: rightmost == in (5==4)==3 instance Eq a => Chain a (Valid a) Bool example: leftmost == in 5==(4==3) Sidebar: Valid is just an imitation of Maybe: data Valid a = Value a | Fail deriving (Show) But back to ==, the interesting part is the times when one senses we're in a context of comparing more values, for example, the left == in (x==y)==z. instance Eq a => Chain a a (Valid a) instance Eq a => Chain (Valid a) a (Valid a) instance Eq a => Chain a (Valid a) (Valid a) To test out this implementation I write a test function: test2 :: Eq a => a -> a -> Bool test2 a b = a === b and this works as expected. The problem comes when chainging the ===s together, I have to spoon-feed the compiler the inferred type: -- compiling this causes an error test3choke :: Eq a => a -> a -> a -> Bool test3choke a b c = a === b === c The error text: [1 of 1] Compiling ME ( ME.hs, interpreted ) ME.hs:63:19: Could not deduce (Chain a a c) from the context (Eq a) arising from use of `===' at ME.hs:63:19-25 Possible fix: add (Chain a a c) to the type signature(s) for `test3choke' In the first argument of `(===)', namely `a === b' In the expression: (a === b) === c In the definition of `test3choke': test3choke a b c = (a === b) === c ME.hs:63:19: Could not deduce (Chain c a Bool) from the context (Eq a) arising from use of `===' at ME.hs:63:19-31 Possible fix: add (Chain c a Bool) to the type signature(s) for `test3choke' or add an instance declaration for (Chain c a Bool) In the expression: (a === b) === c In the definition of `test3choke': test3choke a b c = (a === b) === c Failed, modules loaded: none. -- but spoon-feeding it the types will work test3Int :: Int -> Int -> Int -> Bool test3Int a b c = ((a === b) :: Valid Int) === c So it seems that the compiler is not doing instance inference the same way it does type inference. This is frustrating because the output of the parenthesiszed a === b can only be either of type Bool or Valid a, and the second argument of the outer === has to have the same type, which will force it to Valid a in most cases (Bool being an interesting exception). Is there some way to goad the compiler forward on this one, or is this the wrong approach altogether? Attachment: http://www.osaurus.us/~dm/tmp/ME.hs

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Type question
by Paul Johnston 05 Jan '09

05 Jan '09

Hi, I was playing around after getting the new O'Reilly book with lists and their operators! Not sure what the below actually means? Prelude> [] : [] [[]] it :: [[a]] (Got :set +t on ) I thought the first argument of ':' must be an element, so is the empty list an element of the same type of the contents of the empty list? Yours confused Paul :-)

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