Functions
=========

Function types
--------------

Unlike other Lisps, macros and language axioms are functions in Antilisp. The difference between a typical function and a special function like ``(if)`` is the execution mode. The execution mode of a function is a symbol that specifies if and how arguments should be evaluated. The available values are:

+----------+----------------------------------------------------+-------------------------------+
| Symbol   | What it does                                       | Example                       |
+==========+====================================================+===============================+
| function | Evaluate arguments and call the function with them | ``(+)``, ``(list)``           |
+----------+----------------------------------------------------+-------------------------------+
| macro    | Evaluate the result of the computation             | ``(def)``, ``(defclass)``     |
+----------+----------------------------------------------------+-------------------------------+
| no-eval  | Do not evaluate the arguments at all               | ``(if)``, ``(and)``, ``(or)`` |
+----------+----------------------------------------------------+-------------------------------+
| no-expand| Do not expand macros and do not evaluate arguments | ``(quote)``, ``(quasiquote)`` |
+----------+----------------------------------------------------+-------------------------------+



The execution mode of a function cannot be changed because functions are immutable. However, you can create a modified version of a function with ``(with-exec-mode)``.

.. highlight:: hy

This can be very useful when debugging macros. Here is an example with the ``(incr)`` macro, used to increment a variable: ::

    ; this macro has a bug and won't work
    (defmacro (incr x)
      `(set-symbol ,x (+ ,x 1)))
    (define counter 0)
    (incr counter)
    (print counter)  ; still 0

    ; print the code that would be generated by (incr counter)
    (print ((with-exec-mode 'function incr) 'counter))
    ; returns: (set-symbol counter (+ counter 1))
    ; the bug: (set-symbol) wants a quoted symbol as its first argument

    ; fixed implementation:
    (defmacro (incr x)
      `(set-symbol ',x (+ ,x 1))`)
    (incr counter)
    (print counter)  # 1



Rest arguments
--------------

Functions can accept any number of arguments using a rest parameter: ::

    ; f will accept 2 or more parameters:
    (def (f x y . args)
      (print x)
      (print y)
      (print args))

Applying arguments
------------------

Sometimes, you may need to call a function with a list of arguments. This is where you can use ``(apply)``: ::


    (apply print '(5))
    ; 5
    (def (my-sum lst)
      (apply + lst))