[Haskell-cafe] GHC predictability

[Abhay Parvate]

I don't know why, but perhaps beginners may expect too much from the
laziness, almost to the level of magic (me too, in the beginning!). In an
eager language, a function like

mean :: (Fractional a) => [a] -> a

expects the *whole* list before it can calculate the mean, and the question
of the 'mean' function consuming memory does not arise. We look for other
methods of finding the mean of very long lists. We do not expect such a
function in C or Scheme to succeed when the number of numbers is more than
that can fit the memory. (It will not even be called; the list creation
itself will not succeed.) Lazy languages allow us to use the same
abstraction while allowing doing more. But it is not magic, it is plain
normal order evaluation. Just as every Scheme programmer or C programmer
must understand the consequences of the fact that the arguments to a
function will be evaluated first, a Haskell programmer must understand the
consequences of the fact that the arguments to a function will be evaluated
only when needed/forced. Perhaps an early emphasis on an understanding of
normal order evaluation is needed while learning Haskell in order to stop
expecting magic, especially when one comes prejudiced from eager languages.

Regards
Abhay
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