[Haskell-cafe] Memoization/call-by-need

I feel that there is something that I don't understand completely: I have been told that Haskell does not memoize function call, e.g.

slowFib 50 will run just as slowly each time it is called. However, I have read that Haskell has call-by-need semantics, which were described as "lazy evaluation with memoization"

I understand that

fib50 = slowFib 50 will take a while to run the first time but be instant each subsequent call; does this count as memoization?

(I'm trying to understand "Purely Functional Data Structures", hence this question) -- Alex R