Fibonacci Numbers in Haskell

Question

Hi everbody I need to change my F# code to Haskell code but I am so new in Haskell and I can not this My code simply read data from keyboard if data not an integer return an error message then calculate the n fibonacci number then writes to a list after that writes the list into a txt file Here is my code

open System
let rec fib n = 
    match n with
    |0->0
    |1->1
    |2->1
    |n->fib(n-1)+fib(n-2);;

let printFibonacci list = 
    for i=0 to (List.length list)-1 do
        printf "%d " (list.Item(i));;

let writeToFile list = 
    let file = System.IO.File.Create("C:\out2.txt")
    let mutable s =""
    let writer = new System.IO.StreamWriter(file)
    try
        for i=0 to (List.length list)-1 do
        s <- list.Item(i).ToString()
        writer.Write(s+" ")

    finally
        writer.Close()
        file.Dispose()
        printfn "Writed To File"


let mutable control = true
let mutable num = 0
while control do 
    try
    printfn "Enter a Number:" 

    num <- Convert.ToInt32(stdin.ReadLine()) 
    let listFibonacci = [for i in 0 .. num-1->fib(i)]
    printFibonacci(listFibonacci)
    printfn "\n%A"(listFibonacci)
    writeToFile(listFibonacci)
    control<-false
    with
        | :? System.FormatException->printfn "Number Format Exception";

Console.ReadKey true|>ignore

Answer 1

fib 0 = 0
fib 1 = 1
fib n = fib (n-1) + fib (n-2)

main = do putStrLn "Enter a number:"
          num <- readLn
          fibs = map fib [0..n]
          mapM' print fibs

However since haskell is lazy there is a clever way to define the list of all fibonacci numbers. And since you want a prefix of that list, it's more natural to use this definition (also more efficient):

fibs = 0 : 1 : zipWith (+) fibs (tail fibs)

main = do putStrLn "Enter a number:"
          num <- readLn
          mapM' print (take n fibs)

Edit: To write to a file instead of stdout replace print with (\num -> appendFile "filename" (show num)) or (appendFile "filename" . show).

Answer 2

That is basically the most common implementation of the sequence itself:

fib 0 = 0
fib 1 = 1
fib n = fib (n-1) + fib (n-2)

If you want the number from keyboard:

main :: IO ()
main = catch doFib handle
   where doFib = do
            num <- readLn
            putStrLn $ "Fib of " ++ show num ++ " = " ++ (show . fib $ num)
         handle _ -> putStrLn "Malformed input!"

Done!