Is this supposed to be Haskell code or Haskell semantics?

Perl-Code to show the general idea of how to apply lazy evaluation, not restricted to Haskell (which is just one of the languages having lazy evaluation builtin).

Using Tie::Array you could try to tie the iterator to an array @even such that you can subsequently write something like this, _with_ lazy evaluation.

Good point!

show me what you like about Haskell - eg list comprehensions (???) - and I will see whats feasible.

I'm not an advocate for a particular language (Haskell was just given as an example, because several implementations are available), but I like the idea in FP that you don't have side effects. This is nice for the programmer, but can be hairy for the implementation. List comprehension helps if you transform arrays, but I think array update is more tricky. Where you write in an imperative language something like $a->[$n]+=$k to update the array element at postition $n, you would in FP (conceptionally) return a copy of the array, which differs from the original one only in the element with index $n. Of course you don't want to actually copy the whole array, unless it is really necessary. For instance, if you have an array $a and a function &fa expecting an array (I stay with Perl syntax here), and you want to invoke the function with $a, but set the first element of $a to zero, the non-functional solution would be simply

$a->[0]=0; 
fa($a);
[download]

while a functional solution would need something like

fa(update($a,0,0))
[download]

and need to be clever enough to not make a copy of the array until this is becomes necessary. But a guess with a clever usage of tied variables and Perl objects, you might indeed be able to come up with a FP framework for Perl...

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