Something

sprawl$ more tmp.hs
fib :: Integer -> Integer
fib 0 = 1
fib 1 = 1
fib n = fib (n-2) + fib (n-1)
sprawl$ ghci
GHCi, version 7.0.4: http://www.haskell.org/ghc/  :? for help
Loading package ghc-prim ... linking ... done.
Loading package integer-gmp ... linking ... done.
Loading package base ... linking ... done.
Loading package ffi-1.0 ... linking ... done.
ghci> :l tmp.hs
[1 of 1] Compiling Main             ( tmp.hs, interpreted )
Ok, modules loaded: Main.
ghci> fib 0
1
ghci> fib 1
1
ghci> fib 2
2
ghci> fib 3
3
ghci> fib 30
1346269