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• Unsure of how to get the right evaluation order

  - by Matt Fenwick

  I'm not sure what the difference between these two pieces of code is (with respect to x), but the first one completes: $ foldr (\x y -> if x == 4 then x else x + y) 0 [1,2 .. ] 10 and the second one doesn't (at least in GHCi): $ foldr (\x (y, n) -> if x == 4 then (x, n) else (x + y, n + 1)) (0, 0) [1,2 .. ] ....... What am I doing wrong…

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• Use QuickCheck by generating primes

  - by Dan

  Background For fun, I'm trying to write a property for quick-check that can test the basic idea behind cryptography with RSA. Choose two distinct primes, p and q. Let N = p*q e is some number relatively prime to (p-1)(q-1) (in practice, e is usually 3 for fast encoding) d is the modular inverse of e modulo (p-1)(q-1) For all x such that 1 <…

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• Binding type variables that only occur in assertions

  - by Giuseppe Maggiore

  Hi! I find it extremely difficult to describe my problem, so here goes nothing: I have a bunch of assertions on the type of a function. These assertions rely on a type variable that is not used for any parameter of the function, but is only used for internal bindings. Whenever I use this function it does not compile because, of course, the…

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• When should I use $ (and can it always be replaced with parentheses)?

  - by J Cooper

  From what I'm reading, $ is described as "applies a function to its arguments." However, it doesn't seem to work quite like (apply ...) in Lisp, because it's a binary operator, so really the only thing it looks like it does is help to avoid parentheses sometimes, like foo $ bar quux instead of foo (bar quux). Am I understanding it right? Is…

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• Asymptotic runtime of list-to-tree function

  - by Deestan

  I have a merge function which takes time O(log n) to combine two trees into one, and a listToTree function which converts an initial list of elements to singleton trees and repeatedly calls merge on each successive pair of trees until only one tree remains. Function signatures and relevant implementations are as follows: merge :: Tree a…

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• Why this Either-monad code does not type check?

  - by pf_miles

  instance Monad (Either a) where return = Left fail = Right Left x >>= f = f x Right x >>= _ = Right x this code frag in 'baby.hs' caused the horrible compilation error: Prelude> :l baby [1 of 1] Compiling Main ( baby.hs, interpreted ) baby.hs:2:18: Couldn't match expected type `a1'…

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• Can anyone explain Monads?

  - by Steve Willard

  I think I understand what 'Maybe Monads' are, but I'm not sure about the other types.

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• Complete Haskore example

  - by Bill

  Does anyone know of a complete Haskore example that will take a small example and output a MIDI file? I'm looking for a starting point to start using Haskore and Google isn't turning up much. Thanks!

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• Is do-notation specific to "base:GHC.Base.Monad"?

  - by yairchu

  The idea that the standard Monad class is flawed and that it should actually extend Functor or Pointed is floating around. I'm not necessarily claiming that it is the right thing to do, but suppose that one was trying to do it: import Prelude hiding (Monad(..)) class Functor m => Monad m where return :: a -> m a …

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• Weird cabal error: "inappropriate type"

  - by Bill

  ~ % cabal install --reinstall time Resolving dependencies... [1 of 1] Compiling Main ( /var/folders/0D/0D3du+YyGzuRETgUJZ5m8U+++TI/-Tmp-/time-1.2.0.251774/time-1.2.0.2/Setup.hs, /var/folders/0D/0D3du+YyGzuRETgUJZ5m8U+++TI/-Tmp-/time-1.2.0.251774/time-1.2.0.2/dist/setup/Main.o ) Linking…

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• wxHaskell scrollable grid

  - by snorlaks

  Hello, How can I make scrollable Grid in wxHaskell ? thanks

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• Nested parsers in happy / infinite loop?

  - by McManiaC

  I'm trying to write a parser for a simple markup language with happy. Currently, I'm having some issues with infinit loops and nested elements. My markup language basicly consists of two elements, one for "normal" text and one for bold/emphasized text. data Markup = MarkupText String |…

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• How to redirect within a monad in Yesod?

  - by Squazic

  I'm currently using the fb package to write a Yesod app that takes data from Facebook. In my Handler, I've managed to get the first step of the authentication to work, but I need to redirect to the url that getUserAccessTokenStep1 returns, which I've defined as fbRedirUrl. I'm having trouble with…

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• Better exception for non-exhaustive patterns in case

  - by toofarsideways

  Is there a way to get GHCi to produce better exception messages when it finds at runtime that a call has produced value that does not match the function's pattern matching? It currently gives the line numbers of the function which produced the non-exhaustive pattern match which though helpful at…

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• Are monads Writer m and Either e categorically dual?

  - by sdcvvc

  I noticed there is a dual relation between Writer m and Either e monads. If m is a monoid, then unit :: () -> m join :: (m,m) -> m can be used to form a monad: return is composition: a -> ((),a) -> (m,a) join is composition: (m,(m,a)) -> ((m,m),a) -> (m,a) The dual of ()…

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• Kernighan & Ritchie word count example program in a functional language

  - by Frank

  I have been reading a little bit about functional programming on the web lately and I think I got a basic idea about the concepts behind it. I'm curious how everyday programming problems which involve some kind of state are solved in a pure functional programing language. For example: how…

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• Safe and polymorphic toEnum

  - by jetxee

  I'd like to write a safe version of toEnum: safeToEnum :: (Enum t, Bounded t) => Int -> Maybe t A naive implementation: safeToEnum :: (Enum t, Bounded t) => Int -> Maybe t safeToEnum i = if (i >= fromEnum (minBound :: t)) && (i <= fromEnum (maxBound :: t)) …

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• Applying a function to an arbitrarily long list of arguments

  - by alphomega

  I want to create a function apply that takes a function with an arbitrary amount of arguments as well as a list of integers, and returns the result of the function (Where each integer in the list is an argument in order. I was thinking something like: apply :: ([Int] -> Int) ->…

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• problem with Double and Rational Number

  - by altair211

  Hi, I am writing a function in which I need to read a string contains floating point number and turn it back to Rational. But When I do toRational (read input :: Double), it will not turn for eg: 0.9 into 9 % 10 as expected, but instead 81..... % 9007... Thx

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• Existentials and Scrap your Boilerplate

  - by finnsson

  I'm writing a XML (de)serializer using Text.XML.Light and Scrap your Boilerplate (at http://github.com/finnsson/Text.XML.Generic) and so far I got working code for "normal" ADTs but I'm stuck at deserializing existentials. I got the existential data type data DataBox where DataBox…

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• Using MonadPlus in FRP.Reactive.FieldTrip

  - by ony

  I'm studying FRP at this moment through FieldTrip adaptor. And hit the problem with strange way of frames scheduling and integration. So now I'm trying to build own marker Event for aligning Behaviour stepping. So... flipflop :: Behavior String flipflop = stepper "none" (xflip 2)…

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• How do I use multiple where clauses in GHCi?

  - by T.R.

  I'm playing around with GHCi for the first time, and I'm having some trouble writing multi-line functions. My code is as follows: Prelude> :{ Prelude| let diffSquares lst = abs $ squareOfSums lst - sumOfSquares lst Prelude| where Prelude| squareOfSums lst = (fst…

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• Program to find the result of primitive recursive functions

  - by alphomega

  I'm writing a program to solve the result of primitive recursive functions: 1 --Basic functions------------------------------ 2 3 --Zero function 4 z :: Int -> Int 5 z = \_ -> 0 6 7 --Successor function 8 s :: Int -> Int 9 s = \x -> (x + 1) …

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• What is being passed in?

  - by Delirium tremens

  In the code: oneChar :: Char -> Doc oneChar c = case lookup c simpleEscapes of Just r -> text r Nothing | mustEscape c -> hexEscape c | otherwise -> char c where mustEscape c = c < ' ' || c == '\x7f'…

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• Which is your favorite "hidden gem" package on Hackage?

  - by finnsson

  There are a lot of packages on Hackage, some well known (such as HUnit) and some less known (such as AspectAG). I'm wondering which package you think is a hidden gem that deserves more users. Maybe a useful data structure, helpers for monads, networking, test, ...? Which…

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