Surprise!
...You'd be surprised what's grammatically valid in Hy.
...(Keep delimiters in balance, and you're mostly good to go.)
) ... "Hy") None Hy => (print #_(commentSurprise!
...You'd be surprised what's grammatically valid in Hy.
...(Keep delimiters in balance, and you're mostly good to go.)
)) ... "Hy") Hy .. _cond: cond ---- ``cond`` can be used to build nested ``if`` statements. The following example shows the relationship between the macro and its expansion: .. code-block:: clj (cond [condition-1 result-1] [condition-2 result-2]) (if condition-1 result-1 (if condition-2 result-2)) If only the condition is given in a branch, then the condition is also used as the result. The expansion of this single argument version is demonstrated below: .. code-block:: clj (cond [condition-1] [condition-2]) (if condition-1 condition-1 (if condition-2 condition-2)) As shown below, only the first matching result block is executed. .. code-block:: clj => (defn check-value [value] ... (cond [(< value 5) (print "value is smaller than 5")] ... [(= value 5) (print "value is equal to 5")] ... [(> value 5) (print "value is greater than 5")] ... [True (print "value is something that it should not be")])) => (check-value 6) value is greater than 5 continue -------- ``continue`` returns execution to the start of a loop. In the following example, ``(side-effect1)`` is called for each iteration. ``(side-effect2)``, however, is only called on every other value in the list. .. code-block:: clj ;; assuming that (side-effect1) and (side-effect2) are functions and ;; collection is a list of numerical values (for [x collection] (side-effect1 x) (if (% x 2) (continue)) (side-effect2 x)) .. _do: do ---------- ``do`` (called ``progn`` in some Lisps) takes any number of forms, evaluates them, and returns the value of the last one, or ``None`` if no forms were provided. :: => (+ 1 (do (setv x (+ 1 1)) x)) 3 doc / #doc ---------- Documentation macro and tag macro. Gets help for macros or tag macros, respectively. .. code-block:: clj => (doc doc) Help on function (doc) in module hy.core.macros: (doc)(symbol) macro documentation Gets help for a macro function available in this module. Use ``require`` to make other macros available. Use ``#doc foo`` instead for help with tag macro ``#foo``. Use ``(help foo)`` instead for help with runtime objects. => (doc comment) Help on function (comment) in module hy.core.macros: (comment)(*body) Ignores body and always expands to None => #doc doc Help on function #doc in module hy.core.macros: #doc(symbol) tag macro documentation Gets help for a tag macro function available in this module. dfor ---- ``dfor`` creates a :ref:`dictionary comprehension
=> (render-html-tag "img" :empty True)
'![]()
'
=> (render-html-tag "img" :id "china" :class "big-image" :empty True)
'![]()
'
=> (render-html-tag "p" " --- " (render-html-tag "span" "" :class "fancy" "I'm fancy!") "I'm to the right of fancy" "I'm alone :(")
' at 0x7f76d0271158>
=> #♣ 5
[5, 5]
=> (setv x 0)
=> #♣(+= x 1)
[None, None]
=> x
2
In this example, if you used ``(defmacro ♣ ...)`` instead of ``(deftag
♣ ...)``, you would call the macro as ``(♣ 5)`` or ``(♣ (+= x 1))``.
The syntax for calling tag macros is similar to that of reader macros a la
Common Lisp's ``SET-MACRO-CHARACTER``. In fact, before Hy 0.13.0, tag macros
were called "reader macros", and defined with ``defreader`` rather than
``deftag``. True reader macros are not (yet) implemented in Hy.
del
---
.. versionadded:: 0.9.12
``del`` removes an object from the current namespace.
.. code-block:: clj
=> (setv foo 42)
=> (del foo)
=> foo
Traceback (most recent call last):
File "", line 1, in
NameError: name 'foo' is not defined
``del`` can also remove objects from mappings, lists, and more.
.. code-block:: clj
=> (setv test (list (range 10)))
=> test
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
=> (del (cut test 2 4)) ;; remove items from 2 to 4 excluded
=> test
[0, 1, 4, 5, 6, 7, 8, 9]
=> (setv dic {"foo" "bar"})
=> dic
{"foo": "bar"}
=> (del (get dic "foo"))
=> dic
{}
doto
----
.. versionadded:: 0.10.1
``doto`` is used to simplify a sequence of method calls to an object.
.. code-block:: clj
=> (doto [] (.append 1) (.append 2) .reverse)
[2, 1]
.. code-block:: clj
=> (setv collection [])
=> (.append collection 1)
=> (.append collection 2)
=> (.reverse collection)
=> collection
[2, 1]
eval-and-compile
----------------
``eval-and-compile`` is a special form that takes any number of forms. The input forms are evaluated as soon as the ``eval-and-compile`` form is compiled, instead of being deferred until run-time. The input forms are also left in the program so they can be executed at run-time as usual. So, if you compile and immediately execute a program (as calling ``hy foo.hy`` does when ``foo.hy`` doesn't have an up-to-date byte-compiled version), ``eval-and-compile`` forms will be evaluated twice.
One possible use of ``eval-and-compile`` is to make a function available both at compile-time (so a macro can call it while expanding) and run-time (so it can be called like any other function)::
(eval-and-compile
(defn add [x y]
(+ x y)))
(defmacro m [x]
(add x 2))
(print (m 3)) ; prints 5
(print (add 3 6)) ; prints 9
Had the ``defn`` not been wrapped in ``eval-and-compile``, ``m`` wouldn't be able to call ``add``, because when the compiler was expanding ``(m 3)``, ``add`` wouldn't exist yet.
eval-when-compile
-----------------
``eval-when-compile`` is like ``eval-and-compile``, but the code isn't executed at run-time. Hence, ``eval-when-compile`` doesn't directly contribute any code to the final program, although it can still change Hy's state while compiling (e.g., by defining a function).
.. code-block:: clj
(eval-when-compile
(defn add [x y]
(+ x y)))
(defmacro m [x]
(add x 2))
(print (m 3)) ; prints 5
(print (add 3 6)) ; raises NameError: name 'add' is not defined
first
-----
``first`` is a function for accessing the first element of a collection.
.. code-block:: clj
=> (first (range 10))
0
It is implemented as ``(next (iter coll) None)``, so it works with any
iterable, and if given an empty iterable, it will return ``None`` instead of
raising an exception.
.. code-block:: clj
=> (first (repeat 10))
10
=> (first [])
None
.. _for:
for
---
``for`` is used to evaluate some forms for each element in an iterable
object, such as a list. The return values of the forms are discarded and
the ``for`` form returns ``None``.
::
=> (for [x [1 2 3]]
... (print "iterating")
... (print x))
iterating
1
iterating
2
iterating
3
In its square-bracketed first argument, ``for`` allows the same types of
clauses as lfor_.
::
=> (for [x [1 2 3] :if (!= x 2) y [7 8]]
... (print x y))
1 7
1 8
3 7
3 8
Furthermore, the last argument of ``for`` can be an ``(else …)`` form.
This form is executed after the last iteration of the ``for``\'s
outermost iteration clause, but only if that outermost loop terminates
normally. If it's jumped out of with e.g. ``break``, the ``else`` is
ignored.
.. code-block:: clj
=> (for [element [1 2 3]] (if (< element 3)
... (print element)
... (break))
... (else (print "loop finished")))
1
2
=> (for [element [1 2 3]] (if (< element 4)
... (print element)
... (break))
... (else (print "loop finished")))
1
2
3
loop finished
.. _gensym:
gensym
------
.. versionadded:: 0.9.12
``gensym`` is used to generate a unique symbol that allows macros to be
written without accidental variable name clashes.
.. code-block:: clj
=> (gensym)
HySymbol('_G\uffff1')
=> (gensym "x")
HySymbol('_x\uffff2')
.. seealso::
Section :ref:`using-gensym`
.. _get:
get
---
``get`` is used to access single elements in collections. ``get`` takes at
least two parameters: the *data structure* and the *index* or *key* of the
item. It will then return the corresponding value from the collection. If
multiple *index* or *key* values are provided, they are used to access
successive elements in a nested structure. Example usage:
.. code-block:: clj
=> (do
... (setv animals {"dog" "bark" "cat" "meow"}
... numbers (, "zero" "one" "two" "three")
... nested [0 1 ["a" "b" "c"] 3 4])
... (print (get animals "dog"))
... (print (get numbers 2))
... (print (get nested 2 1)))
bark
two
b
.. note:: ``get`` raises a KeyError if a dictionary is queried for a
non-existing key.
.. note:: ``get`` raises an IndexError if a list or a tuple is queried for an
index that is out of bounds.
.. _gfor:
gfor
----
``gfor`` creates a :ref:`generator expression `. Its syntax
is the same as that of `lfor`_. The difference is that ``gfor`` returns
an iterator, which evaluates and yields values one at a time.
::
=> (setv accum [])
=> (list (take-while
... (fn [x] (< x 5))
... (gfor x (count) :do (.append accum x) x)))
[0, 1, 2, 3, 4]
=> accum
[0, 1, 2, 3, 4, 5]
global
------
``global`` can be used to mark a symbol as global. This allows the programmer to
assign a value to a global symbol. Reading a global symbol does not require the
``global`` keyword -- only assigning it does.
The following example shows how the global symbol ``a`` is assigned a value in a
function and is later on printed in another function. Without the ``global``
keyword, the second function would have raised a ``NameError``.
.. code-block:: clj
(defn set-a [value]
(global a)
(setv a value))
(defn print-a []
(print a))
(set-a 5)
(print-a)
.. _if:
if / if* / if-not
-----------------
.. versionadded:: 0.10.0
if-not
``if / if* / if-not`` respect Python *truthiness*, that is, a *test* fails if it
evaluates to a "zero" (including values of ``len`` zero, ``None``, and
``False``), and passes otherwise, but values with a ``__bool__`` method
can override this.
The ``if`` macro is for conditionally selecting an expression for evaluation.
The result of the selected expression becomes the result of the entire ``if``
form. ``if`` can select a group of expressions with the help of a ``do`` block.
``if`` takes any number of alternating *test* and *then* expressions, plus an
optional *else* expression at the end, which defaults to ``None``. ``if`` checks
each *test* in turn, and selects the *then* corresponding to the first passed
test. ``if`` does not evaluate any expressions following its selection, similar
to the ``if/elif/else`` control structure from Python. If no tests pass, ``if``
selects *else*.
The ``if*`` special form is restricted to 2 or 3 arguments, but otherwise works
exactly like ``if`` (which expands to nested ``if*`` forms), so there is
generally no reason to use it directly.
``if-not`` is similar to ``if*`` but the second expression will be executed
when the condition fails while the third and final expression is executed when
the test succeeds -- the opposite order of ``if*``. The final expression is
again optional and defaults to ``None``.
Example usage:
.. code-block:: clj
(print (if (< n 0.0) "negative"
(= n 0.0) "zero"
(> n 0.0) "positive"
"not a number"))
(if* (money-left? account)
(print "let's go shopping")
(print "let's go and work"))
(if-not (money-left? account)
(print "let's go and work")
(print "let's go shopping"))
lif and lif-not
---------------------------------------
.. versionadded:: 0.10.0
.. versionadded:: 0.11.0
lif-not
For those that prefer a more Lispy ``if`` clause, we have
``lif``. This *only* considers ``None`` to be false! All other
"false-ish" Python values are considered true. Conversely, we have
``lif-not`` in parallel to ``if`` and ``if-not`` which
reverses the comparison.
.. code-block:: clj
=> (lif True "true" "false")
"true"
=> (lif False "true" "false")
"true"
=> (lif 0 "true" "false")
"true"
=> (lif None "true" "false")
"false"
=> (lif-not None "true" "false")
"true"
=> (lif-not False "true" "false")
"false"
.. _import:
import
------
``import`` is used to import modules, like in Python. There are several ways
that ``import`` can be used.
.. code-block:: clj
;; Imports each of these modules
;;
;; Python:
;; import sys
;; import os.path
(import sys os.path)
;; Import from a module
;;
;; Python: from os.path import exists, isdir, isfile
(import [os.path [exists isdir isfile]])
;; Import with an alias
;;
;; Python: import sys as systest
(import [sys :as systest])
;; You can list as many imports as you like of different types.
;;
;; Python:
;; from tests.resources import kwtest, function_with_a_dash
;; from os.path import exists, isdir as is_dir, isfile as is_file
;; import sys as systest
(import [tests.resources [kwtest function-with-a-dash]]
[os.path [exists
isdir :as dir?
isfile :as file?]]
[sys :as systest])
;; Import all module functions into current namespace
;;
;; Python: from sys import *
(import [sys [*]])
.. _fn:
fn
-----------
``fn``, like Python's ``lambda``, can be used to define an anonymous function.
Unlike Python's ``lambda``, the body of the function can comprise several
statements. The parameters are similar to ``defn``: the first parameter is
vector of parameters and the rest is the body of the function. ``fn`` returns a
new function. In the following example, an anonymous function is defined and
passed to another function for filtering output.
.. code-block:: clj
=> (setv people [{:name "Alice" :age 20}
... {:name "Bob" :age 25}
... {:name "Charlie" :age 50}
... {:name "Dave" :age 5}])
=> (defn display-people [people filter]
... (for [person people] (if (filter person) (print (:name person)))))
=> (display-people people (fn [person] (< (:age person) 25)))
Alice
Dave
Just as in normal function definitions, if the first element of the
body is a string, it serves as a docstring. This is useful for giving
class methods docstrings.
.. code-block:: clj
=> (setv times-three
... (fn [x]
... "Multiplies input by three and returns the result."
... (* x 3)))
This can be confirmed via Python's built-in ``help`` function::
=> (help times-three)
Help on function times_three:
times_three(x)
Multiplies input by three and returns result
(END)
fn/a
----
``fn/a`` is a variant of ``fn`` than defines an anonymous coroutine.
The parameters are similar to ``defn/a``: the first parameter is
vector of parameters and the rest is the body of the function. ``fn/a`` returns a
new coroutine.
last
-----------
.. versionadded:: 0.11.0
``last`` can be used for accessing the last element of a collection:
.. code-block:: clj
=> (last [2 4 6])
6
.. _lfor:
lfor
----
The comprehension forms ``lfor``, `sfor`_, `dfor`_, `gfor`_, and `for`_
are used to produce various kinds of loops, including Python-style
:ref:`comprehensions `. ``lfor`` in particular
creates a list comprehension. A simple use of ``lfor`` is::
=> (lfor x (range 5) (* 2 x))
[0, 2, 4, 6, 8]
``x`` is the name of a new variable, which is bound to each element of
``(range 5)``. Each such element in turn is used to evaluate the value
form ``(* 2 x)``, and the results are accumulated into a list.
Here's a more complex example::
=> (lfor
... x (range 3)
... y (range 3)
... :if (!= x y)
... :setv total (+ x y)
... [x y total])
[[0, 1, 1], [0, 2, 2], [1, 0, 1], [1, 2, 3], [2, 0, 2], [2, 1, 3]]
When there are several iteration clauses (here, the pairs of forms ``x
(range 3)`` and ``y (range 3)``), the result works like a nested loop or
Cartesian product: all combinations are considered in lexicographic
order.
The general form of ``lfor`` is::
(lfor CLAUSES VALUE)
where the ``VALUE`` is an arbitrary form that is evaluated to produce
each element of the result list, and ``CLAUSES`` is any number of
clauses. There are several types of clauses:
- Iteration clauses, which look like ``LVALUE ITERABLE``. The ``LVALUE``
is usually just a symbol, but could be something more complicated,
like ``[x y]``.
- ``:async LVALUE ITERABLE``, which is an
:ref:`asynchronous ` form of iteration clause.
- ``:do FORM``, which simply evaluates the ``FORM``. If you use
``(continue)`` or ``(break)`` here, they will apply to the innermost
iteration clause before the ``:do``.
- ``:setv LVALUE RVALUE``, which is equivalent to ``:do (setv LVALUE
RVALUE)``.
- ``:if CONDITION``, which is equivalent to ``:do (unless CONDITION
(continue))``.
For ``lfor``, ``sfor``, ``gfor``, and ``dfor``, variables are scoped as
if the comprehension form were its own function, so variables defined by
an iteration clause or ``:setv`` are not visible outside the form. In
fact, these forms are implemented as generator functions whenever they
contain Python statements, with the attendant consequences for calling
``return``. By contrast, ``for`` shares the caller's scope.
nonlocal
--------
.. versionadded:: 0.11.1
``nonlocal`` can be used to mark a symbol as not local to the current scope.
The parameters are the names of symbols to mark as nonlocal. This is necessary
to modify variables through nested ``fn`` scopes:
.. code-block:: clj
(defn some-function []
(setv x 0)
(register-some-callback
(fn [stuff]
(nonlocal x)
(setv x stuff))))
Without the call to ``(nonlocal x)``, the inner function would redefine ``x`` to
``stuff`` inside its local scope instead of overwriting the ``x`` in the outer
function.
See `PEP3104 `. If you want to evaluate some
Python code that's only defined at run-time, try the standard Python function
:func:`eval`.
Python code need not syntactically round-trip if you use ``hy2py`` on a Hy
program that uses ``py`` or ``pys``. For example, comments will be removed.
.. _pys-specialform:
pys
---
As :ref:`py-specialform`, but the code can consist of zero or more statements,
including compound statements such as ``for`` and ``def``. ``pys`` always
returns ``None``. Also, the code string is dedented with
:func:`textwrap.dedent` before parsing, which allows you to intend the code to
match the surrounding Hy code, but significant leading whitespace in embedded
string literals will be removed. ::
(pys "myvar = 5")
(print "myvar is" myvar)
.. _quasiquote:
quasiquote
----------
``quasiquote`` allows you to quote a form, but also selectively evaluate
expressions. Expressions inside a ``quasiquote`` can be selectively evaluated
using ``unquote`` (``~``). The evaluated form can also be spliced using
``unquote-splice`` (``~@``). Quasiquote can be also written using the backquote
(`````) symbol.
.. code-block:: clj
;; let `qux' be a variable with value (bar baz)
`(foo ~qux)
; equivalent to '(foo (bar baz))
`(foo ~@qux)
; equivalent to '(foo bar baz)
.. _quote:
quote
-----
``quote`` returns the form passed to it without evaluating it. ``quote`` can
alternatively be written using the apostrophe (``'``) symbol.
.. code-block:: clj
=> (setv x '(print "Hello World"))
=> x ; variable x is set to unevaluated expression
HyExpression([
HySymbol('print'),
HyString('Hello World')])
=> (eval x)
Hello World
.. _require:
require
-------
``require`` is used to import macros from one or more given modules. It allows
parameters in all the same formats as ``import``. The ``require`` form itself
produces no code in the final program: its effect is purely at compile-time, for
the benefit of macro expansion. Specifically, ``require`` imports each named
module and then makes each requested macro available in the current module.
The following are all equivalent ways to call a macro named ``foo`` in the module ``mymodule``:
.. code-block:: clj
(require mymodule)
(mymodule.foo 1)
(require [mymodule :as M])
(M.foo 1)
(require [mymodule [foo]])
(foo 1)
(require [mymodule [*]])
(foo 1)
(require [mymodule [foo :as bar]])
(bar 1)
Macros that call macros
~~~~~~~~~~~~~~~~~~~~~~~
One aspect of ``require`` that may be surprising is what happens when one
macro's expansion calls another macro. Suppose ``mymodule.hy`` looks like this:
.. code-block:: clj
(defmacro repexpr [n expr]
; Evaluate the expression n times
; and collect the results in a list.
`(list (map (fn [_] ~expr) (range ~n))))
(defmacro foo [n]
`(repexpr ~n (input "Gimme some input: ")))
And then, in your main program, you write:
.. code-block:: clj
(require [mymodule [foo]])
(print (mymodule.foo 3))
Running this raises ``NameError: name 'repexpr' is not defined``, even though
writing ``(print (foo 3))`` in ``mymodule`` works fine. The trouble is that your
main program doesn't have the macro ``repexpr`` available, since it wasn't
imported (and imported under exactly that name, as opposed to a qualified name).
You could do ``(require [mymodule [*]])`` or ``(require [mymodule [foo
repexpr]])``, but a less error-prone approach is to change the definition of
``foo`` to require whatever sub-macros it needs:
.. code-block:: clj
(defmacro foo [n]
`(do
(require mymodule)
(mymodule.repexpr ~n (input "Gimme some input: "))))
It's wise to use ``(require mymodule)`` here rather than ``(require [mymodule
[repexpr]])`` to avoid accidentally shadowing a function named ``repexpr`` in
the main program.
Qualified macro names
~~~~~~~~~~~~~~~~~~~~~
Note that in the current implementation, there's a trick in qualified macro
names, like ``mymodule.foo`` and ``M.foo`` in the above example. These names
aren't actually attributes of module objects; they're just identifiers with
periods in them. In fact, ``mymodule`` and ``M`` aren't defined by these
``require`` forms, even at compile-time. None of this will hurt you unless try
to do introspection of the current module's set of defined macros, which isn't
really supported anyway.
rest
----
``rest`` takes the given collection and returns an iterable of all but the
first element.
.. code-block:: clj
=> (list (rest (range 10)))
[1, 2, 3, 4, 5, 6, 7, 8, 9]
Given an empty collection, it returns an empty iterable.
.. code-block:: clj
=> (list (rest []))
[]
return
-------
``return`` compiles to a :py:keyword:`return` statement. It exits the
current function, returning its argument if provided with one or
``None`` if not.
.. code-block:: hy
=> (defn f [x] (for [n (range 10)] (when (> n x) (return n))))
=> (f 3.9)
4
Note that in Hy, ``return`` is necessary much less often than in Python,
since the last form of a function is returned automatically. Hence, an
explicit ``return`` is only necessary to exit a function early.
.. code-block:: hy
=> (defn f [x] (setv y 10) (+ x y))
=> (f 4)
14
To get Python's behavior of returning ``None`` when execution reaches
the end of a function, put ``None`` there yourself.
.. code-block:: hy
=> (defn f [x] (setv y 10) (+ x y) None)
=> (print (f 4))
None
sfor
----
``sfor`` creates a set comprehension. ``(sfor CLAUSES VALUE)`` is
equivalent to ``(set (lfor CLAUSES VALUE))``. See `lfor`_.
.. _cut:
cut
-----
``cut`` can be used to take a subset of a list and create a new list from it.
The form takes at least one parameter specifying the list to cut. Two
optional parameters can be used to give the start and end position of the
subset. If they are not supplied, the default value of ``None`` will be used
instead. The third optional parameter is used to control step between the elements.
``cut`` follows the same rules as its Python counterpart. Negative indices are
counted starting from the end of the list. Some example usage:
.. code-block:: clj
=> (setv collection (range 10))
=> (cut collection)
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
=> (cut collection 5)
[5, 6, 7, 8, 9]
=> (cut collection 2 8)
[2, 3, 4, 5, 6, 7]
=> (cut collection 2 8 2)
[2, 4, 6]
=> (cut collection -4 -2)
[6, 7]
raise
-------------
The ``raise`` form can be used to raise an ``Exception`` at
runtime. Example usage:
.. code-block:: clj
(raise)
; re-rase the last exception
(raise IOError)
; raise an IOError
(raise (IOError "foobar"))
; raise an IOError("foobar")
``raise`` can accept a single argument (an ``Exception`` class or instance)
or no arguments to re-raise the last ``Exception``.
try
---
The ``try`` form is used to catch exceptions (``except``) and run cleanup
actions (``finally``).
.. code-block:: clj
(try
(error-prone-function)
(another-error-prone-function)
(except [ZeroDivisionError]
(print "Division by zero"))
(except [[IndexError KeyboardInterrupt]]
(print "Index error or Ctrl-C"))
(except [e ValueError]
(print "ValueError:" (repr e)))
(except [e [TabError PermissionError ReferenceError]]
(print "Some sort of error:" (repr e)))
(else
(print "No errors"))
(finally
(print "All done")))
The first argument of ``try`` is its body, which can contain one or more forms.
Then comes any number of ``except`` clauses, then optionally an ``else``
clause, then optionally a ``finally`` clause. If an exception is raised with a
matching ``except`` clause during the execution of the body, that ``except``
clause will be executed. If no exceptions are raised, the ``else`` clause is
executed. The ``finally`` clause will be executed last regardless of whether an
exception was raised.
The return value of ``try`` is the last form of the ``except`` clause that was
run, or the last form of ``else`` if no exception was raised, or the ``try``
body if there is no ``else`` clause.
unless
------
The ``unless`` macro is a shorthand for writing an ``if`` statement that checks if
the given conditional is ``False``. The following shows the expansion of this macro.
.. code-block:: clj
(unless conditional statement)
(if conditional
None
(do statement))
.. _unpack-iterable:
unpack-iterable, unpack-mapping
-------------------------------
(Also known as the splat operator, star operator, argument expansion, argument
explosion, argument gathering, and varargs, among others...)
``unpack-iterable`` and ``unpack-mapping`` allow an iterable or mapping
object (respectively) to provide positional or keywords arguments
(respectively) to a function.
.. code-block:: clj
=> (defn f [a b c d] [a b c d])
=> (f (unpack-iterable [1 2]) (unpack-mapping {"c" 3 "d" 4}))
[1, 2, 3, 4]
``unpack-iterable`` is usually written with the shorthand ``#*``, and
``unpack-mapping`` with ``#**``.
.. code-block:: clj
=> (f #* [1 2] #** {"c" 3 "d" 4})
[1, 2, 3, 4]
Unpacking is allowed in a variety of contexts, and you can unpack
more than once in one expression (:pep:`3132`, :pep:`448`).
.. code-block:: clj
=> (setv [a #* b c] [1 2 3 4 5])
=> [a b c]
[1, [2, 3, 4], 5]
=> [#* [1 2] #* [3 4]]
[1, 2, 3, 4]
=> {#** {1 2} #** {3 4}}
{1: 2, 3: 4}
=> (f #* [1] #* [2] #** {"c" 3} #** {"d" 4})
[1, 2, 3, 4]
.. _unquote:
unquote
-------
Within a quasiquoted form, ``unquote`` forces evaluation of a symbol. ``unquote``
is aliased to the tilde (``~``) symbol.
.. code-block:: clj
=> (setv nickname "Cuddles")
=> (quasiquote (= nickname (unquote nickname)))
HyExpression([
HySymbol('='),
HySymbol('nickname'),
'Cuddles'])
=> `(= nickname ~nickname)
HyExpression([
HySymbol('='),
HySymbol('nickname'),
'Cuddles'])
unquote-splice
--------------
``unquote-splice`` forces the evaluation of a symbol within a quasiquoted form,
much like ``unquote``. ``unquote-splice`` can be used when the symbol
being unquoted contains an iterable value, as it "splices" that iterable into
the quasiquoted form. ``unquote-splice`` can also be used when the value
evaluates to a false value such as ``None``, ``False``, or ``0``, in which
case the value is treated as an empty list and thus does not splice anything
into the form. ``unquote-splice`` is aliased to the ``~@`` syntax.
.. code-block:: clj
=> (setv nums [1 2 3 4])
=> (quasiquote (+ (unquote-splice nums)))
HyExpression([
HySymbol('+'),
1,
2,
3,
4])
=> `(+ ~@nums)
HyExpression([
HySymbol('+'),
1,
2,
3,
4])
=> `[1 2 ~@(if (neg? (first nums)) nums)]
HyList([
HyInteger(1),
HyInteger(2)])
Here, the last example evaluates to ``('+' 1 2)``, since the condition
``(< (nth nums 0) 0)`` is ``False``, which makes this ``if`` expression
evaluate to ``None``, because the ``if`` expression here does not have an
else clause. ``unquote-splice`` then evaluates this as an empty value,
leaving no effects on the list it is enclosed in, therefore resulting in
``('+' 1 2)``.
when
----
``when`` is similar to ``unless``, except it tests when the given conditional is
``True``. It is not possible to have an ``else`` block in a ``when`` macro. The
following shows the expansion of the macro.
.. code-block:: clj
(when conditional statement)
(if conditional (do statement))
.. _while:
while
-----
``while`` compiles to a :py:keyword:`while` statement. It is used to execute a
set of forms as long as a condition is met. The first argument to ``while`` is
the condition, and any remaining forms constitute the body. The following
example will output "Hello world!" to the screen indefinitely:
.. code-block:: clj
(while True (print "Hello world!"))
The last form of a ``while`` loop can be an ``else`` clause, which is executed
after the loop terminates, unless it exited abnormally (e.g., with ``break``). So,
.. code-block:: clj
(setv x 2)
(while x
(print "In body")
(-= x 1)
(else
(print "In else")))
prints
::
In body
In body
In else
If you put a ``break`` or ``continue`` form in the condition of a ``while``
loop, it will apply to the very same loop rather than an outer loop, even if
execution is yet to ever reach the loop body. (Hy compiles a ``while`` loop
with statements in its condition by rewriting it so that the condition is
actually in the body.) So,
.. code-block:: clj
(for [x [1]]
(print "In outer loop")
(while
(do
(print "In condition")
(break)
(print "This won't print.")
True)
(print "This won't print, either."))
(print "At end of outer loop"))
prints
::
In outer loop
In condition
At end of outer loop
.. _with:
with
----
``with`` is used to wrap the execution of a block within a context manager. The
context manager can then set up the local system and tear it down in a controlled
manner. The archetypical example of using ``with`` is when processing files.
``with`` can bind context to an argument or ignore it completely, as shown below:
.. code-block:: clj
(with [arg (expr)] block)
(with [(expr)] block)
(with [arg (expr) (expr)] block)
The following example will open the ``NEWS`` file and print its content to the
screen. The file is automatically closed after it has been processed.
.. code-block:: clj
(with [f (open "NEWS")] (print (.read f)))
``with`` returns the value of its last form, unless it suppresses an exception
(because the context manager's ``__exit__`` method returned true), in which
case it returns ``None``. So, the previous example could also be written
.. code-block:: clj
(print (with [f (open "NEWS")] (.read f)))
with/a
------
``with/a`` behaves like ``with``, but is used to wrap the execution of
a block within an asynchronous context manager. The context manager can
then set up the local system and tear it down in a controlled manner
asynchronously.
.. code-block:: clj
(with/a [arg (expr)] block)
(with/a [(expr)] block)
(with/a [arg (expr) (expr)] block)
``with/a`` returns the value of its last form, unless it suppresses an exception
(because the context manager's ``__aexit__`` method returned true), in which
case it returns ``None``.
with-decorator
--------------
``with-decorator`` is used to wrap a function with another. The function
performing the decoration should accept a single value: the function being
decorated, and return a new function. ``with-decorator`` takes a minimum
of two parameters: the function performing decoration and the function
being decorated. More than one decorator function can be applied; they
will be applied in order from outermost to innermost, ie. the first
decorator will be the outermost one, and so on. Decorators with arguments
are called just like a function call.
.. code-block:: clj
(with-decorator decorator-fun
(defn some-function [] ...)
(with-decorator decorator1 decorator2 ...
(defn some-function [] ...)
(with-decorator (decorator arg) ..
(defn some-function [] ...)
In the following example, ``inc-decorator`` is used to decorate the function
``addition`` with a function that takes two parameters and calls the
decorated function with values that are incremented by 1. When
the decorated ``addition`` is called with values 1 and 1, the end result
will be 4 (``1+1 + 1+1``).
.. code-block:: clj
=> (defn inc-decorator [func]
... (fn [value-1 value-2] (func (+ value-1 1) (+ value-2 1))))
=> (defn inc2-decorator [func]
... (fn [value-1 value-2] (func (+ value-1 2) (+ value-2 2))))
=> (with-decorator inc-decorator (defn addition [a b] (+ a b)))
=> (addition 1 1)
4
=> (with-decorator inc2-decorator inc-decorator
... (defn addition [a b] (+ a b)))
=> (addition 1 1)
8
#@
~~
.. versionadded:: 0.12.0
The tag macro ``#@`` can be used as a shorthand for ``with-decorator``. With
``#@``, the previous example becomes:
.. code-block:: clj
=> #@(inc-decorator (defn addition [a b] (+ a b)))
=> (addition 1 1)
4
=> #@(inc2-decorator inc-decorator
... (defn addition [a b] (+ a b)))
=> (addition 1 1)
8
.. _with-gensyms:
with-gensyms
-------------
.. versionadded:: 0.9.12
``with-gensym`` is used to generate a set of :ref:`gensym` for use in a macro.
The following code:
.. code-block:: hy
(with-gensyms [a b c]
...)
expands to:
.. code-block:: hy
(do
(setv a (gensym)
b (gensym)
c (gensym))
...)
.. seealso::
Section :ref:`using-gensym`
xor
---
.. versionadded:: 0.12.0
``xor`` performs the logical operation of exclusive OR. It takes two arguments.
If exactly one argument is true, that argument is returned. If neither is true,
the second argument is returned (which will necessarily be false). Otherwise,
when both arguments are true, the value ``False`` is returned.
.. code-block:: clj
=> [(xor 0 0) (xor 0 1) (xor 1 0) (xor 1 1)]
[0, 1, 1, False]
yield
-----
``yield`` is used to create a generator object that returns one or more values.
The generator is iterable and therefore can be used in loops, list
comprehensions and other similar constructs.
The function ``random-numbers`` shows how generators can be used to generate
infinite series without consuming infinite amount of memory.
.. code-block:: clj
=> (defn multiply [bases coefficients]
... (for [(, base coefficient) (zip bases coefficients)]
... (yield (* base coefficient))))
=> (multiply (range 5) (range 5))
=> (list (multiply (range 10) (range 10)))
[0, 1, 4, 9, 16, 25, 36, 49, 64, 81]
=> (import random)
=> (defn random-numbers [low high]
... (while True (yield (.randint random low high))))
=> (list (take 15 (random-numbers 1 50)))
[7, 41, 6, 22, 32, 17, 5, 38, 18, 38, 17, 14, 23, 23, 19]
yield-from
----------
.. versionadded:: 0.9.13
``yield-from`` is used to call a subgenerator. This is useful if you
want your coroutine to be able to delegate its processes to another
coroutine, say, if using something fancy like
`asyncio `_.
I\'m fancy! --- I\'m to right right of fancy --- I\'m alone :(
' defn/a ------ ``defn/a`` macro is a variant of ``defn`` that instead defines coroutines. It takes three parameters: the *name* of the function to define, a vector of *parameters*, and the *body* of the function: .. code-block:: clj (defn/a name [params] body) defmain ------- .. versionadded:: 0.10.1 The ``defmain`` macro defines a main function that is immediately called with ``sys.argv`` as arguments if and only if this file is being executed as a script. In other words, this: .. code-block:: clj (defmain [&rest args] (do-something-with args)) is the equivalent of:: def main(*args): do_something_with(args) return 0 if __name__ == "__main__": import sys retval = main(*sys.argv) if isinstance(retval, int): sys.exit(retval) Note that as you can see above, if you return an integer from this function, this will be used as the exit status for your script. (Python defaults to exit status 0 otherwise, which means everything's okay!) Since ``(sys.exit 0)`` is not run explicitly in the case of a non-integer return from ``defmain``, it's a good idea to put ``(defmain)`` as the last piece of code in your file. If you want fancy command-line arguments, you can use the standard Python module ``argparse`` in the usual way: .. code-block:: clj (import argparse) (defmain [&rest _] (setv parser (argparse.ArgumentParser)) (.add-argument parser "STRING" :help "string to replicate") (.add-argument parser "-n" :type int :default 3 :help "number of copies") (setv args (parser.parse_args)) (print (* args.STRING args.n)) 0) .. _defmacro: defmacro -------- ``defmacro`` is used to define macros. The general format is ``(defmacro name [parameters] expr)``. The following example defines a macro that can be used to swap order of elements in code, allowing the user to write code in infix notation, where operator is in between the operands. .. code-block:: clj => (defmacro infix [code] ... (quasiquote ( ... (unquote (get code 1)) ... (unquote (get code 0)) ... (unquote (get code 2))))) => (infix (1 + 1)) 2 .. _defmacro/g!: defmacro/g! ------------ .. versionadded:: 0.9.12 ``defmacro/g!`` is a special version of ``defmacro`` that is used to automatically generate :ref:`gensym` for any symbol that starts with ``g!``. For example, ``g!a`` would become ``(gensym "a")``. .. seealso:: Section :ref:`using-gensym` .. _defmacro!: defmacro! --------- ``defmacro!`` is like ``defmacro/g!`` plus automatic once-only evaluation for ``o!`` parameters, which are available as the equivalent ``g!`` symbol. For example, .. code-block:: clj => (defn expensive-get-number [] (print "spam") 14) => (defmacro triple-1 [n] `(+ ~n ~n ~n)) => (triple-1 (expensive-get-number)) ; evals n three times spam spam spam 42 => (defmacro/g! triple-2 [n] `(do (setv ~g!n ~n) (+ ~g!n ~g!n ~g!n))) => (triple-2 (expensive-get-number)) ; avoid repeats with a gensym spam 42 => (defmacro! triple-3 [o!n] `(+ ~g!n ~g!n ~g!n)) => (triple-3 (expensive-get-number)) ; easier with defmacro! spam 42 deftag -------- .. versionadded:: 0.13.0 ``deftag`` defines a tag macro. A tag macro is a unary macro that has the same semantics as an ordinary macro defined with ``defmacro``. It is called with the syntax ``#tag FORM``, where ``tag`` is the name of the macro, and ``FORM`` is any form. The ``tag`` is often only one character, but it can be any symbol. .. code-block:: clj => (deftag ♣ [expr] `[~expr ~expr])