Such programs are built up by putting a series of function and type definitions after each other like they were dominoes and that is going to be eventually pushed over from the top, usually called a start expression. next() must return the same type, but with HKTs for the item type, you are allowed to vary the generic lifetime parameter from call to call.Module Tutorial where import Prelude hiding (abs, const, head, length, map, negate, foldr, map, repeat, replicate, sum, tail, take, uncurry, (.))įunctional programming is a branch of the declarative paradigm. With an ordinary iterator, every call to. Then a and b have different lifetimes, and thus, despite both being a &str, have different types. If we now take the example let a = iter.next() If you want a specific type, you need to say &'a str for some specific lifetime 'a, and given two different lifetimes 'a and 'b, there are two distinct types &'a str and &'b str. To see how this ties into HKTs, the technicality you need to understand is that &str is not a type because it's missing the lifetime.

Such an alternate Lines iterator would have item type &str, with a String buffer stored inside the iterator, whose allocation is reused on every call to. next(), but there's no reason you have to do that (and in fact you can avoid it with read_line instead). The Lines iterator currently allocates a new String every time you call. One feature you might imagine using this for would be an alternative to std::io::Lines. next() mutates the iterator, the second call to next might invalidate the reference stored in a.

However imagine what would happen if a held a reference into iter. With the current Iterator trait, the following code must be valid no matter what the item type is: let a = iter.next() an iterator that produces values that may contain a reference into the iterator itself. The primary feature enabled by HKTs is to create streaming iterators (and streaming streams in async world), i.e.

Haskell functional programming tutorial