Function definition by special cases in GHCi

Question

From a Haskell tutorial:

We can write functions on integers by cases.

-- Compute the sum of         


        

Answer 1

ghci is an interactive tool and as such allows to redefine a function when it's already be define. In your case, it doesn't see it as a two lines function definition but as two attempts of defining it. So f n = print n overrides f 0 = print 999 instead of completing it.

To enter multiple lines statement in ghci there is a special syntax. You need to do

Prelude> :{
Prelude> let foo 0 = print 999
Prelude>     foo n = print n
Prelude> :}