Fibonacci in Haskell

Published on 15 October 2018 (Updated: 07 April 2019)

Welcome to the Fibonacci in Haskell page! Here, you'll find the source code for this program as well as a description of how the program works.

Current Solution

module Main where

import System.Environment
import Text.Read

fibonacci :: Int -> [Integer]
fibonacci = flip (take) fibs

fibs :: [Integer]
fibs = 1 : 1 : zipWith (+) fibs (tail fibs)

headMaybe :: [a] -> Maybe a
headMaybe []     = Nothing
headMaybe (x:xs) = Just x

-- Takes a list of values and returns a list of strings in the format "ONE_BASED_INDEX: VALUE"
printWithIndex :: (Show a) => [a] -> [[Char]]
printWithIndex = zipWith (\i x -> (show i) ++ ": " ++ (show x)) [1..]

-- Prints out the first N numbers from the fibonacci sequence
-- where N equals to the first command line argument.
main :: IO ()
main = do
  args <- getArgs
  let n = headMaybe args
  case n >>= readMaybe of
    Nothing -> putStrLn "Usage: please input the count of fibonacci numbers to output"
    Just n  -> mapM_ (putStrLn) $ (printWithIndex . fibonacci) n

Fibonacci in Haskell was written by:

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How to Implement the Solution

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How to Run the Solution

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