[pipo234]

To some extent for while loops are a matter of taste. But the issue is that unless you have mutable variables, you cannot use typical imperative control variables.

So something like:

  bool done = false;
  while (!done) {
     ...
  }

makes no sense unless you have some way of setting `done = true`; which is not possible in purely functional languages like Haskell.

[chongli]

This construct is totally possible in Haskell, one common way of achieving it is to store the 'done' boolean in a State monad.

The really funny thing is that the while loop itself is not built-in to Haskell but you can write it yourself as a function. In other languages (non-lazy) this sort of thing is not possible without resorting to macros or other tricks, because laziness is required in order to define the correct semantics for conditionals.

[bkallus]

Functional languages support this type of operation in a few different ways.

One is through constructs like Clojure's `take-while`. Instead of setting a vairable when you want to exit the loop, you define a predicate that is false when the loop should exit.

Another is through `any` and `all` (aka `some` and `every`), which work in a similar way to `take-while`, except they reduce OR or AND across the returned values from the predicate as they go.

EDIT: grammar and clarity