Iteration

The usual way to iterate a collection is with a for loop.

A for loop is an expression. It can be void, or return an output collection.

A type annotation after the for keyword types the entire for expression.

A for loop can be squeezed onto one line using then.

for loops are a convenient syntax for calling a function named for-loop with a lambda.

If you want the collection indices too, use with-index.

for? loops work like for loops, but the body can return an option. An empty option value will be ignored.
These call a function named for-option.

There are also for* loops, which work like for loops, but the body returns an array that is flattened into the result.
These call a function named for-star.

nat types are iterable too. The number acts like a collection of all the numbers less than it.

ranges are also iterable (if the type in the range has a next function).

You can reverse most iterable things.

There is no early return from a for loop; it always iterates every element of the collection.

The most primitive iteration function is some which stops when the lambda returns true.

A more general function is first. Here, the callback returns an option, and the first non-empty result is returned.

first works a lot like an until loop with a condition-body (see Loops); it keeps running the body until it gets a result.
Unlike an until loop, it can also stop running if it reaches the end of the collection, so it returns an option.