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I don't understand how this program can ever terminate, since there is no base case. Every invocation of find_i starts by calling forsome, and every invocation
by brianberns 11y ago
I don't understand how this program can ever terminate, since there is no base case. Every invocation of find_i starts by calling forsome, and every invocation of forsome calls find_i. How can it possibly end?
Perhaps this relies subtly on Haskell's laziness? If so, I don't think there is a "quick translation" to ML or OCaml.
- cousin_it 11y agoNot every invocation of forsome calls find_i. It calls p(find_i(...)), but p can return a value without actually evaluating the inner find_i. So yeah, it relies on laziness, though you can translate it into a strict language by inserting lambdas in a bunch of places.
- brianberns 11y agoThank you. How does Haskell manage to avoid evaluating find_i(...) before invoking p? Since the inner value is the argument to p, it's hard to imagine how p can do anything until it has that value. EDIT: I think I see the light. The result of find_i(...) is not what's sent to p. Instead, Haskell passes a thunk that might never need to be evaluated.
- tome 11y agoExactly. If I write let f x = 1 print (f (g somethingelse)) then the call to g is never evaluated. f returns 1 immediately.