Termination

Once we start writing recursive functions, we need to worry about the termination of our programs (i.e. do they return a result in a finite amount of time?

Non-termination

It is possible to write recursive functions that do not terminate.

loop :: Int -> Int
loop x = loop x

Here, the right hand side of the only equation of loop is identical to its left hand side. So, the evaluation of an expression like loop 1 will make no progress:

loop 1 
 = loop 1
 = loop 1
 = ...

If we try to evaluate an expression like loop 1 in ghci, it will loop forever. You can cancel computations in ghci by hitting CTRL + C on ...