Factorial in Haskell

Example for versions GHC 6.10.4

This example uses recursive factorial definition and consists of three major parts:

  • definition of factorial function, which takes one argument of Integer type (integer number of unlimited precision) and returns the same type. The function is defined recursively, and types of argument and return are given explicitly to avoid ambiguity.
  • definition of line function, which prints the number and its factorial in required format. Use of
    printf command is the same as in C++.
  • printing the numbers and their factorials themselves. [0..16] creates a list of numbers from 0 to 16, inclusive. Function map applies first argument (line function) to each element of second argument ([0..16] list) and as a result creates a list of so-called “output actions” (which can be used as values in Haskell). To combine these actions in one we use sequence_ command, which is applied to a list of actions, executes first action from the list and recursively applies it to the tail of the list. But for simplicity, we use mapM_, which combines map and sequence_. 
module Main where

import Text.Printf

factorial :: Integer -> Integer
factorial 0 = 1
factorial n = n * factorial (n - 1)

line x = printf "%d! = %d\n" x $ factorial x

main = mapM_ line [0..16]