842 lines
20 KiB
ReStructuredText
842 lines
20 KiB
ReStructuredText
=================
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Hy (the language)
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=================
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.. warning::
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This is incomplete; please consider contributing to the documentation
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effort.
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Theory of Hy
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============
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Hy maintains, over everything else, 100% compatibility in both directions
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with Python itself. All Hy code follows a few simple rules. Memorize
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this, it's going to come in handy.
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These rules help make sure code is idiomatic and interface-able in both
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languages.
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* Symbols in earmufs will be translated to the uppercased version of that
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string. For example, `*foo*` will become `FOO`.
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* UTF-8 entities will be encoded using
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`punycode <http://en.wikipedia.org/wiki/Punycode>`_ and prefixed with
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`hy_`. For instance, `⚘` will become `hy_w7h`, and `♥` will become
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`hy_g6h`.
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* Symbols that contain dashes will have them replaced with underscores. For
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example, `render-template` will become `render_template`.
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Builtins
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========
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Hy features a number special forms that are used to help generate
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correct Python AST. The following are "special" forms, which may have
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behavior that's slightly unexpected in some situations.
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->
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--
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`->` or `threading macro` is used to avoid nesting of expressions. The threading
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macro inserts each expression into the next expression’s first argument place.
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The following code demonstrates this:
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.. code-block:: clj
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=> (defn output [a b] (print a b))
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=> (-> (+ 5 5) (output 5))
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10 5
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->>
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---
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`->>` or `threading tail macro` is similar to `threading macro` but instead of
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inserting each expression into the next expression’s first argument place it
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appends it as the last argument. The following code demonstrates this:
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.. code-block:: clj
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=> (defn output [a b] (print a b))
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=> (->> (+ 5 5) (output 5))
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5 10
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and
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---
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`and` form is used in logical expressions. It takes at least two parameters. If
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all parameters evaluate to `True` the last parameter is returned. In any other
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case the first false value will be returned. Examples of usage:
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.. code-block:: clj
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=> (and True False)
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False
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=> (and True True)
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True
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=> (and True 1)
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1
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=> (and True [] False True)
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[]
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.. note:: `and` shortcuts and stops evaluating parameters as soon as the first
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false is encountered. However, in the current implementation of Hy
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statements are executed as soon as they are converted to expressions.
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The following two examples demonstrates the difference.
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.. code-block:: clj
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=> (and False (print "hello"))
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hello
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False
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=> (defn side-effects [x] (print "I can has" x) x)
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=> (and (side-effects false) (side-effects 42))
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I can has False
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False
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assert
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------
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`assert` is used to verify conditions while the program is running. If the
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condition is not met, an `AssertionError` is raised. The example usage:
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.. code-block:: clj
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(assert (= variable expected-value))
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Assert takes a single parameter, a conditional that evaluates to either `True`
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or `False`.
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assoc
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-----
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`assoc` form is used to associate a key with a value in a dictionary or to set
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an index of a list to a value. It takes three parameters: `datastructure` to be
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modified, `key` or `index` and `value`.
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Examples of usage:
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.. code-block:: clj
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=>(let [[collection ({})]]
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... (assoc collection "Dog" "Bark")
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... (print collection))
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{u'Dog': u'Bark'}
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=>(let [[collection [1 2 3 4]]]
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... (assoc collection 2 None)
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... (print collection))
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[1, 2, None, 4]
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.. note:: `assoc` modifies the datastructure in place and returns `None`.
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break
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-----
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`break` is used to break out from a loop. It terminates the loop immediately.
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The following example has an infinite while loop that is terminated as soon as
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the user enters `k`.
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.. code-block:: clj
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(while True (if (= "k" (raw-input "? "))
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(break)
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(print "Try again")))
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cond
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----
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`cond` macro can be used to build nested if-statements.
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The following example shows the relationship between the macro and the expanded
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code:
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.. code-block:: clj
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(cond (condition-1 result-1)
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(condition-2 result-2))
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(if condition-1 result-1
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(if condition-2 result-2))
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As shown below only the first matching result block is executed.
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.. code-block:: clj
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=> (defn check-value [value]
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... (cond ((< value 5) (print "value is smaller than 5"))
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... ((= value 5) (print "value is equal to 5"))
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... ((> value 5) (print "value is greater than 5"))
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... (True (print "value is something that it should not be"))))
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=> (check-value 6)
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value is greater than 5
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continue
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--------
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`continue` returns execution to the start of a loop. In the following example,
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function `(side-effect1)` is called for each iteration. `(side-effect2)`
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however is called only for every other value in the list.
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.. code-block:: clj
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;; assuming that (side-effect1) and (side-effect2) are functions and
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;; collection is a list of numerical values
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(for (x collection) (do
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(side-effect1 x)
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(if (% x 2)
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(continue))
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(side-effect2 x)))
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do / progn
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----------
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the `do` and `progn` forms are used to evaluate each of their arguments and
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return the last one. Return values from every other than the last argument are
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discarded. It can be used in `lambda` or `list-comp` to perform more complex
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logic as show by one of the examples.
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Some example usage:
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.. code-block:: clj
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=> (if true
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... (do (print "Side effects rock!")
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... (print "Yeah, really!")))
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Side effects rock!
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Yeah, really!
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;; assuming that (side-effect) is a function that we want to call for each
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;; and every value in the list, but which return values we do not care
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=> (list-comp (do (side-effect x)
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... (if (< x 5) (* 2 x)
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... (* 4 x)))
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... (x (range 10)))
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[0, 2, 4, 6, 8, 20, 24, 28, 32, 36]
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`do` can accept any number of arguments, from 1 to n.
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def / setv
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-----------------
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`def` and `setv` are used to bind value, object or a function to a symbol. For
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example:
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.. code-block:: clj
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=> (def names ["Alice" "Bob" "Charlie"]
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=> (print names)
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[u'Alice', u'Bob', u'Charlie']
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=> (setv counter (fn [collection item] (.count collection item)))
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=> (counter [1 2 3 4 5 2 3] 2)
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2
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defclass
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--------
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new classes are declared with `defclass`. It can takes two optional parameters:
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a vector defining a possible super classes and another vector containing
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attributes of the new class as two item vectors.
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.. code-block:: clj
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(defclass class-name [super-class-1 super-class-2]
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[[attribute value]])
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Both values and functions can be bound on the new class as shown by the example
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below:
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.. code-block:: clj
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=> (defclass Cat []
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... [[age None]
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... [colour "white"]
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... [speak (fn [self] (print "Meow"))]])
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=> (def spot (Cat))
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=> (setv spot.colour "Black")
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'Black'
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=> (.speak spot)
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Meow
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defn / defun
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------------
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defmacro
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--------
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`defmacro` is used to define macros. The general format is
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`(defmacro [parameters] expr)`.
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Following example defines a macro that can be used to swap order of elements in
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code, allowing the user to write code in infix notation, where operator is in
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between the operands.
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.. code-block:: clj
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=> (defmacro infix [code]
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... (quasiquote (
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... (unquote (get code 1))
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... (unquote (get code 0))
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... (unquote (get code 2)))))
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=> (infix (1 + 1))
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2
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eval
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----
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eval-and-compile
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----------------
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eval-when-compile
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-----------------
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first / car
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-----------
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`first` and `car` are macros for accessing the first element of a collection:
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.. code-block:: clj
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=> (first (range 10))
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0
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for
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---
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`for` macro is used to build nested `foreach` loops. The macro takes two
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parameters, first being a vector specifying collections to iterate over and
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variables to bind. The second parameter is a statement which is executed during
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each loop:
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.. code-block:: clj
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(for [x iter y iter] stmt)
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(foreach [x iter]
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(foreach [y iter] stmt))
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foreach
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-------
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`foreach` is used to call a function for each element in a list or vector.
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Results are discarded and None is returned instead. Example code iterates over
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collection and calls side-effect to each element in the collection:
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.. code-block:: clj
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;; assuming that (side-effect) is a function that takes a single parameter
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(foreach [element collection] (side-effect element))
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;; foreach can have an optional else block
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(foreach [element collection] (side-effect element)
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(else (side-effect-2)))
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The optional `else` block is executed only if the `foreach` loop terminates
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normally. If the execution is halted with `break`, the `else` does not execute.
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.. code-block:: clj
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=> (foreach [element [1 2 3]] (if (< element 3)
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... (print element)
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... (break))
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... (else (print "loop finished")))
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1
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2
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=> (foreach [element [1 2 3]] (if (< element 4)
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... (print element)
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... (break))
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... (else (print "loop finished")))
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1
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2
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3
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loop finished
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get
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---
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`get` form is used to access single elements in lists and dictionaries. `get`
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takes two parameters, the `datastructure` and the `index` or `key` of the item.
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It will then return the corresponding value from the dictionary or the list.
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Example usages:
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.. code-block:: clj
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=> (let [[animals {"dog" "bark" "cat" "meow"}]
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... [numbers ["zero" "one" "two" "three"]]]
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... (print (get animals "dog"))
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... (print (get numbers 2)))
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bark
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two
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.. note:: `get` raises a KeyError if a dictionary is queried for a non-existing
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key.
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.. note:: `get` raises an IndexError if a list is queried for an index that is
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out of bounds.
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global
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------
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`global` can be used to mark a symbol as global. This allows the programmer to
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assign a value to a global symbol. Reading a global symbol does not require the
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`global` keyword, just the assigning does.
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Following example shows how global `a` is assigned a value in a function and later
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on printed on another function. Without the `global` keyword, the second function
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would thrown a `NameError`.
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.. code-block:: clj
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(defn set-a [value]
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(global a)
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(setv a value))
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(defn print-a []
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(print a))
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(set-a 5)
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(print-a)
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if
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--
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the `if` form is used to conditionally select code to be executed. It has to
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contain the condition block and the block to be executed if the condition
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evaluates `True`. Optionally it may contain a block that is executed in case
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the evaluation of the condition is `False`.
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Example usage:
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.. code-block:: clj
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(if (money-left? account)
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(print "lets go shopping")
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(print "lets go and work"))
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Truth values of Python objects are respected. Values `None`, `False`, zero of
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any numeric type, empty sequence and empty dictionary are considered `False`.
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Everything else is considered `True`.
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import
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------
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`import` is used to import modules, like in Python. There are several forms
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of import you can use.
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.. code-block:: clj
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;; Imports each of these modules
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;;
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;; Python:
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;; import sys
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;; import os.path
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(import sys os.path)
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;; Import from a module
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;;
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;; Python: from os.path import exists, isdir, isfile
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(import [os.path [exists isdir isfile]])
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;; Import with an alias
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;;
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;; Python: import sys as systest
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(import [sys :as systest])
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;; You can list as many imports as you like of different types.
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(import [tests.resources [kwtest function-with-a-dash]]
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[os.path [exists isdir isfile]]
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[sys :as systest])
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kwapply
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-------
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`kwapply` can be used to supply keyword arguments to a function.
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For example:
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.. code-block:: clj
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=> (defn rent-car [&kwargs kwargs]
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... (cond ((in :brand kwargs) (print "brand:" (:brand kwargs)))
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... ((in :model kwargs) (print "model:" (:model kwargs)))))
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=> (kwapply (rent-car) {:model "T-Model"})
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model: T-Model
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=> (defn total-purchase [price amount &optional [fees 1.05] [vat 1.1]]
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... (* price amount fees vat))
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=> (total-purchase 10 15)
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173.25
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=> (kwapply (total-purchase 10 15) {"vat" 1.05})
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165.375
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lambda / fn
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-----------
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`lambda` and `fn` can be used to define an anonymous function. The parameters are
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similar to `defn`: first parameter is vector of parameters and the rest is the
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body of the function. lambda returns a new function. In the example an anonymous
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function is defined and passed to another function for filtering output.
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.. code-block:: clj
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=> (def people [{:name "Alice" :age 20}
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... {:name "Bob" :age 25}
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... {:name "Charlie" :age 50}
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... {:name "Dave" :age 5}])
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=> (defn display-people [people filter]
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... (foreach [person people] (if (filter person) (print (:name person)))))
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=> (display-people people (fn [person] (< (:age person) 25)))
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Alice
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Dave
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let
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---
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list-comp
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---------
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`list-comp` performs list comprehensions. It takes two or three parameters.
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The first parameter is the expression controlling the return value, while
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the second is used to select items from a list. The third and optional
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parameter can be used to filter out some of the items in the list based on a
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conditional expression. Some examples:
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.. code-block:: clj
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=> (def collection (range 10))
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=> (list-comp x [x collection])
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[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
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=> (list-comp (* x 2) [x collection])
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[0, 2, 4, 6, 8, 10, 12, 14, 16, 18]
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=> (list-comp (* x 2) [x collection] (< x 5))
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[0, 2, 4, 6, 8]
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not
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---
|
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|
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`not` form is used in logical expressions. It takes a single parameter and
|
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returns a reversed truth value. If `True` is given as a parameter, `False`
|
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will be returned and vice-versa. Examples for usage:
|
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.. code-block:: clj
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=> (not True)
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False
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=> (not False)
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True
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=> (not None)
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True
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or
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--
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`or` form is used in logical expressions. It takes at least two parameters. It
|
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will return the first non-false parameter. If no such value exist, the last
|
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parameter will be returned.
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.. code-block:: clj
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=> (or True False)
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True
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=> (and False False)
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False
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=> (and False 1 True False)
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1
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|
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.. note:: `or` shortcuts and stops evaluating parameters as soon as the first
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true is encountered. However, in the current implementation of Hy
|
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statements are executed as soon as they are converted to expressions.
|
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The following two examples demonstrates the difference.
|
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|
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.. code-block:: clj
|
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|
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=> (or True (print "hello"))
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hello
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True
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=> (defn side-effects [x] (print "I can has" x) x)
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=> (or (side-effects 42) (side-effects False))
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I can has 42
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42
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print
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-----
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the `print` form is used to output on screen. Example usage:
|
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|
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.. code-block:: clj
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(print "Hello world!")
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|
||
.. note:: `print` always returns None
|
||
|
||
|
||
require
|
||
-------
|
||
|
||
`require` is used to import macros from a given module. It takes at least one
|
||
parameter specifying the module which macros should be imported. Multiple
|
||
modules can be imported with a single `require`.
|
||
|
||
The following example will import macros from `module-1` and `module-2`:
|
||
|
||
.. code-block:: clj
|
||
|
||
(require module-1 module-2)
|
||
|
||
|
||
rest / cdr
|
||
----------
|
||
|
||
`rest` and `cdr` return the collection passed as an argument without the first
|
||
element:
|
||
|
||
.. code-block:: clj
|
||
|
||
=> (rest (range 10))
|
||
[1, 2, 3, 4, 5, 6, 7, 8, 9]
|
||
|
||
|
||
slice
|
||
-----
|
||
|
||
`slice` 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 slice. Two
|
||
optional parameters can be used to give the start and end position of the
|
||
subset. If they are not supplied, default value of None will be used instead.
|
||
Third optional parameter is used to control step between the elements.
|
||
|
||
`slice` follows the same rules as the Python counterpart. Negative indecies are
|
||
counted starting from the end of the list.
|
||
Some examples of
|
||
usage:
|
||
|
||
.. code-block:: clj
|
||
|
||
=> (def collection (range 10))
|
||
|
||
=> (slice collection)
|
||
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
|
||
|
||
=> (slice collection 5)
|
||
[5, 6, 7, 8, 9]
|
||
|
||
=> (slice collection 2 8)
|
||
[2, 3, 4, 5, 6, 7]
|
||
|
||
=> (slice collection 2 8 2)
|
||
[2, 4, 6]
|
||
|
||
=> (slice collection -4 -2)
|
||
[6, 7]
|
||
|
||
|
||
throw / raise
|
||
-------------
|
||
|
||
the `throw` or `raise` forms can be used to raise an Exception at runtime.
|
||
|
||
|
||
Example usage
|
||
|
||
.. code-block:: clj
|
||
|
||
(throw)
|
||
; re-rase the last exception
|
||
|
||
(throw IOError)
|
||
; Throw an IOError
|
||
|
||
(throw (IOError "foobar"))
|
||
; Throw an IOError("foobar")
|
||
|
||
|
||
`throw` can acccept a single argument (an `Exception` class or instance), or
|
||
no arguments to re-raise the last Exception.
|
||
|
||
|
||
try
|
||
---
|
||
|
||
the `try` form is used to start a `try` / `catch` block. The form is used
|
||
as follows
|
||
|
||
.. code-block:: clj
|
||
|
||
(try
|
||
(error-prone-function)
|
||
(catch [e ZeroDivisionError] (print "Division by zero"))
|
||
(else (print "no errors"))
|
||
(finally (print "all done")))
|
||
|
||
`try` must contain at least one `catch` block, and may optionally have an
|
||
`else` or `finally` block. If an error is raised with a matching catch
|
||
block during execution of `error-prone-function` then that catch block will
|
||
be executed. If no errors are raised the `else` block is executed. Regardless
|
||
if an error was raised or not, the `finally` block is executed as last.
|
||
|
||
|
||
unless
|
||
------
|
||
|
||
`unless` macro is a shorthand for writing a if-statement that checks if the
|
||
given conditional is False. The following shows how the macro expands into code.
|
||
|
||
.. code-block:: clj
|
||
|
||
(unless conditional statement)
|
||
|
||
(if conditional
|
||
None
|
||
(do statement))
|
||
|
||
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 `when` macro. The following
|
||
shows how the macro is expanded into code.
|
||
|
||
.. code-block:: clj
|
||
|
||
(when conditional statement)
|
||
|
||
(if conditional (do statement))
|
||
|
||
while
|
||
-----
|
||
|
||
`while` form is used to execute a single or more blocks as long as a condition
|
||
is being met.
|
||
|
||
The following example will output "hello world!" on screen indefinetely:
|
||
|
||
.. code-block:: clj
|
||
|
||
(while True (print "hello world!"))
|
||
|
||
|
||
with
|
||
----
|
||
|
||
`with` is used to wrap execution of a block with a context manager. The context
|
||
manager can then set up the local system and tear it down in a controlled
|
||
manner. Typical example of using `with` is 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)
|
||
|
||
The following example will open file `NEWS` and print its content on screen. The
|
||
file is automatically closed after it has been processed.
|
||
|
||
.. code-block:: clj
|
||
|
||
(with [f (open "NEWS")] (print (.read f)))
|
||
|
||
|
||
with-decorator
|
||
--------------
|
||
|
||
`with-decorator` is used to wrap a function with another. The function performing
|
||
decoration should accept a single value, the function being decorated and return
|
||
a new function. `with-decorator` takes two parameters, the function performing
|
||
decoration and the function being decorated.
|
||
|
||
In the following example, `inc-decorator` is used to decorate function `addition`
|
||
with a function that takes two parameters and calls the decorated function with
|
||
values that are incremented by 1. When 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))))
|
||
=> (with-decorator inc-decorator (defn addition [a b] (+ a b)))
|
||
=> (addition 1 1)
|
||
4
|
||
|
||
|
||
yield
|
||
-----
|
||
|
||
`yield` is used to create a generator object, that returns 1 or more values.
|
||
The generator is iterable and therefore can be used in loops, list
|
||
comprehensions and other similar constructs.
|
||
|
||
Especially the second example shows how generators can be used to generate
|
||
infinite series without consuming infinite amount of memory.
|
||
|
||
.. code-block:: clj
|
||
|
||
=> (defn multiply [bases coefficients]
|
||
... (foreach [(, base coefficient) (zip bases coefficients)]
|
||
... (yield (* base coefficient))))
|
||
|
||
=> (multiply (range 5) (range 5))
|
||
<generator object multiply at 0x978d8ec>
|
||
|
||
=> (list-comp value [value (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-comp x [x (take 15 (random-numbers 1 50))])])
|
||
[7, 41, 6, 22, 32, 17, 5, 38, 18, 38, 17, 14, 23, 23, 19]
|