Haskell Program to print factorial of a number

Program

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..8]

Haskell Program to Print Factorial of a Number

This Haskell program computes and prints the factorial of numbers from 0 to 8 using recursion.

1. module Main where

• Defines the Main module, which is required for a standalone Haskell program.

2. import Text.Printf

• Imports the printf function from Text.Printf for formatted output.

3. factorial :: Integer -> Integer

• Declares factorial as a function that takes an Integer and returns an Integer.

4. factorial 0 = 1

• The base case of the recursive function: 0! = 1.

5. factorial n = n * factorial (n - 1)

• The recursive case: calculates n! as n * (n-1)!.

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

• Defines a function line that:
  • Computes factorial x.
  • Formats and prints x! = factorial(x).

7. main = mapM_ line [0..8]

• Calls line for numbers from 0 to 8 using mapM_, which ensures all print statements execute sequentially.

Key Takeaways

• Uses recursion to compute the factorial.
• Uses pattern matching to define the base case factorial 0 = 1.
• Uses formatted printing via printf.
• Uses mapM_ to iterate over a list and print factorials.

Output

$ ghc factorial.hs 
[1 of 1] Compiling Main             ( factorial.hs, factorial.o )
Linking factorial ...
$ ./factorial 
0! = 1
1! = 1
2! = 2
3! = 6
4! = 24
5! = 120
6! = 720
7! = 5040
8! = 40320