boot.janet on line 484, column 1
(loop head & body)
A general purpose loop macro. This macro is similar to the Common Lisp
loop macro, although intentionally much smaller in scope. The head of
the loop should be a tuple that contains a sequence of either bindings
or conditionals. A binding is a sequence of three values that define
something to loop over. They are formatted like:
binding :verb object/expression
Where binding is a binding as passed to def, :verb is one of a set of
keywords, and object is any janet expression. The available verbs are:
:iterate - repeatedly evaluate and bind to the expression while it
:range - loop over a range. The object should be two element tuple
with a start and end value, and an optional positive step. The range is
half open, [start, end).
:range-to - same as :range, but the range is inclusive [start, end].
:down - loop over a range, stepping downwards. The object should be
two element tuple with a start and (exclusive) end value, and an
optional (positive!) step size.
:down-to - same :as down, but the range is inclusive [start, end].
:keys - iterate over the keys in a data structure.
:pairs - iterate over the keys value pairs as tuples in a data
:in - iterate over the values in a data structure.
:generate - iterate over values yielded from a fiber. Can be paired
with the generator function for the producer/consumer pattern.
loop also accepts conditionals to refine the looping further.
Conditionals are of the form:
where :modifier is one of a set of keywords, and argument is keyword
dependent. :modifier can be one of:
:while expression - breaks from the loop if expression is falsey.
:until expression - breaks from the loop if expression is truthy.
:let bindings - defines bindings inside the loop as passed to the
:before form - evaluates a form for a side effect before of the next
:after form - same as :before, but the side effect happens after the
next inner loop.
:repeat n - repeats the next inner loop n times.
:when condition - only evaluates the loop body when condition is
The loop macro always evaluates to nil.