Simple Haskell Fibonacci function gets stuck

Question

I am running the GHCi console, and typing

fib 0 = 0
fib 1 = 1
fib n = fib (n-1) + fib (n-2)

and then

fib 5

and the console hangs and dies with:

: Out of memory

What am I doing wrong ?

Answer 1

By typing the declarations separately into ghci, each one is read as a separate definition of fib, shadowing its predecessors. In other words, you're effectively running

fib n = fib (n-1) + fib (n-2)
fib 5  -- infinite recursion here

The easiest way to avoid this problem is to put the definition of fib into a file.

Answer 2

At the console, you haven't defined a single function fib with 3 different cases; you first defined fib 0 = 0, then overwrite that with a new function fib 1 = 1, then finally with a third function fib n = fib (n-1) + fib (n-2) that has no base case. You can use

> fib 0 = 1; fib 1 = 1; fib n = fib (n-1) + fib (n-2)

to correctly define a single, 3-case function.

Note this is primarily a problem starting with GHCi 8, since in previous versions you would have to use let to start the definition and would get a parse error on the second line:

> let fib 0 = 0
> fib 1 = 1

<interactive>:3:7: parse error on input '='