Simple pattern matching in Haskell

Question

I'm a beginner in Haskell, and I tried entering the following in WinGHCi:

Prelude> factorial 0=1
Prelude> factorial n=n*factorial (n-1)
Prelude> factorial 5

But when I did this, WinGHCi got stuck and didn't do anything. Why didn't it print out the factorial of 5?

Answer 1

When you write in GHCi

> let x = 4
> let x = 5

the second definition overrides the first one, removing it from the environment. This also holds for functions.

> let f 0 = 1
> let f n = 1 + f (n-1)

is equivalent to

> let f n = 1 + f (n-1)

which will recurse forever on any input.

In GHCi, you can have both using

> let f 0 = 1 ; f n = 1 + f (n-1)

but the best approach is to edit a .hs file, write your definitions there, and then load it in GHCi.