hy/hy/core/macros.hy

;;; Hy core macros
;; Copyright 2017 the authors.
;; This file is part of Hy, which is free software licensed under the Expat
;; license. See the LICENSE.

;;; These macros form the hy language
;;; They are automatically required in every module, except inside hy.core


(import [hy.models [HyList HySymbol]])

(defmacro as-> [head name &rest rest]
  "Expands to sequence of assignments to the provided name, starting with head.
  The previous result is thus available in the subsequent form. Returns the
  final result, and leaves the name bound to it in the local scope. This behaves
  much like the other threading macros, but requires you to specify the threading
  point per form via the name instead of always the first or last argument."
  `(do (setv
         ~name ~head
         ~@(interleave (repeat name) rest))
     ~name))

(defmacro with [args &rest body]
  "shorthand for nested with* loops:
  (with [x foo y bar] baz) ->
  (with* [x foo]
    (with* [y bar]
      baz))"

  (if (not (empty? args))
    (do
     (if (>= (len args) 2)
       (do
        (setv p1 (.pop args 0)
              p2 (.pop args 0)
              primary [p1 p2])
        `(with* [~@primary] (with ~args ~@body)))
       `(with* [~@args] ~@body)))
    `(do ~@body)))


(defmacro cond [&rest branches]
  "shorthand for nested ifs:
   (cond [foo bar] [baz quux]) ->
   (if foo
     bar
     (if baz
       quux))"
  (if (empty? branches)
    None
    (do
     (setv branches (iter branches))
     (setv branch (next branches))
     (defn check-branch [branch]
       "check `cond` branch for validity, return the corresponding `if` expr"
       (if (not (= (type branch) HyList))
         (macro-error branch "cond branches need to be a list"))
       (if (< (len branch) 2)
         (do
           (setv g (gensym))
           `(if (do (setv ~g ~(first branch)) ~g) ~g))
         `(if ~(first branch) (do ~@(cut branch 1)))))

     (setv root (check-branch branch))
     (setv latest-branch root)

     (for* [branch branches]
       (setv cur-branch (check-branch branch))
       (.append latest-branch cur-branch)
       (setv latest-branch cur-branch))
     root)))


(defmacro for [args &rest body]
  "shorthand for nested for loops:
  (for [x foo
        y bar]
    baz) ->
  (for* [x foo]
    (for* [y bar]
      baz))"
  (setv body (list body))
  (if (empty? body)
    (macro-error None "`for' requires a body to evaluate"))
  (setv lst (get body -1))
  (setv belse (if (and (isinstance lst HyExpression) (= (get lst 0) "else"))
                [(body.pop)]
                []))
  (cond
   [(odd? (len args))
    (macro-error args "`for' requires an even number of args.")]
   [(empty? body)
    (macro-error None "`for' requires a body to evaluate")]
   [(empty? args) `(do ~@body ~@belse)]
   [(= (len args) 2) `(for* [~@args] (do ~@body) ~@belse)]
   [True
    (setv alist (cut args 0 None 2))
    `(for* [(, ~@alist) (genexpr (, ~@alist) [~@args])] (do ~@body) ~@belse)]))


(defmacro -> [head &rest rest]
  "Threads the head through the rest of the forms. Inserts
   head as the second item in the first form of rest. If
   there are more forms, inserts the first form as the
   second item in the second form of rest, etc."
  (setv ret head)
  (for* [node rest]
    (if (not (isinstance node HyExpression))
      (setv node `(~node)))
    (.insert node 1 ret)
    (setv ret node))
  ret)


(defmacro doto [form &rest expressions]
  "Performs a sequence of potentially mutating actions
   on an initial object, returning the resulting object"
  (setv f (gensym))
  (defn build-form [expression]
    (if (isinstance expression HyExpression)
      `(~(first expression) ~f ~@(rest expression))
      `(~expression ~f)))
  `(do
     (setv ~f ~form)
     ~@(map build-form expressions)
     ~f))

(defmacro ->> [head &rest rest]
  "Threads the head through the rest of the forms. Inserts
   head as the last item in the first form of rest. If there
   are more forms, inserts the first form as the last item
   in the second form of rest, etc."
  (setv ret head)
  (for* [node rest]
    (if (not (isinstance node HyExpression))
      (setv node `(~node)))
    (.append node ret)
    (setv ret node))
  ret)


(defmacro if-not [test not-branch &optional yes-branch]
  "Like `if`, but execute the first branch when the test fails"
  `(if* (not ~test) ~not-branch ~yes-branch))


(defmacro lif [&rest args]
  "Like `if`, but anything that is not None is considered true."
  (setv n (len args))
  (if* n
       (if* (= n 1)
            (get args 0)
            `(if* (is-not ~(get args 0) None)
                  ~(get args 1)
                  (lif ~@(cut args 2))))))


(defmacro lif-not [test not-branch &optional yes-branch]
  "Like `if-not`, but anything that is not None is considered true."
  `(if* (is ~test None) ~not-branch ~yes-branch))


(defmacro when [test &rest body]
  "Execute `body` when `test` is true"
  `(if ~test (do ~@body)))


(defmacro unless [test &rest body]
  "Execute `body` when `test` is false"
  `(if-not ~test (do ~@body)))


(defmacro with-gensyms [args &rest body]
  (setv syms [])
  (for* [arg args]
    (.extend syms [arg `(gensym '~arg)]))
  `(do
    (setv ~@syms)
    ~@body))

(defmacro defmacro/g! [name args &rest body]
  (setv syms (list
              (distinct
               (filter (fn [x]
                         (and (hasattr x "startswith")
                              (.startswith x "g!")))
                       (flatten body))))
        gensyms [])
  (for* [sym syms]
    (.extend gensyms [sym `(gensym ~(cut sym 2))]))
  `(defmacro ~name [~@args]
     (setv ~@gensyms)
     ~@body))

(defmacro defmacro! [name args &rest body]
  "Like defmacro/g! plus automatic once-only evaluation for o!
   parameters, which are available as the equivalent g! symbol."
  (setv os (list-comp s [s args] (.startswith s "o!"))
        gs (list-comp (HySymbol (+ "g!" (cut s 2))) [s os]))
  `(defmacro/g! ~name ~args
     `(do (setv ~@(interleave ~gs ~os))
          ~@~body)))

(if-python2
  (defmacro/g! yield-from [expr]
    `(do (import types)
         (setv ~g!iter (iter ~expr))
         (setv ~g!return None)
         (setv ~g!message None)
         (while True
           (try (if (isinstance ~g!iter types.GeneratorType)
                  (setv ~g!message (yield (.send ~g!iter ~g!message)))
                  (setv ~g!message (yield (next ~g!iter))))
           (except [~g!e StopIteration]
             (do (setv ~g!return (if (hasattr ~g!e "value")
                                     (. ~g!e value)
                                     None))
               (break)))))
           ~g!return))
  None)


(defmacro defmain [args &rest body]
  "Write a function named \"main\" and do the if __main__ dance"
  (setv retval (gensym))
  `(when (= --name-- "__main__")
     (import sys)
     (setv ~retval (apply (fn [~@args] ~@body) sys.argv))
     (if (integer? ~retval)
       (sys.exit ~retval))))


(deftag @ [expr]
  (setv decorators (cut expr None -1)
        fndef (get expr -1))
  `(with-decorator ~@decorators ~fndef))