hy/core/macros.hy: Add an (if-not) macro
Sometimes it is better to start with the false condition, sometimes that makes the code clearer. For that, the (if-not) macro, which simply reverses the order of the condition blocks, can be of great use. Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>
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@ -679,13 +679,16 @@ would thrown a `NameError`.
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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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if / if-not
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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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the evaluation of the condition is `False`. The `if-not` form (*new in
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0.9.13*) is similar, but the first block after the test will be
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executed when the test fails, while the other, conditional one, when
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the test succeeds - opposite of the order of the `if` form.
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Example usage:
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@ -695,6 +698,10 @@ Example usage:
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(print "lets go shopping")
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(print "lets go and work"))
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(if-not (money-left? account)
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(print "lets go and work")
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(print "lets go shopping"))
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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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@ -138,6 +138,13 @@
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ret)
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(defmacro if-not [test not-branch &optional [yes-branch nil]]
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"Like `if`, but execute the first branch when the test fails"
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(if (nil? yes-branch)
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`(if (not ~test) ~not-branch)
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`(if (not ~test) ~not-branch ~yes-branch)))
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(defmacro when [test &rest body]
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"Execute `body` when `test` is true"
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`(if ~test (do ~@body)))
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@ -176,3 +176,12 @@
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(assert (in ":res_" s1))
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(assert (in ":res_" s2))
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(assert (not (= s1 s2))))
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(defn test-if-not []
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(assert (= (if-not True :yes :no)
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:no))
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(assert (= (if-not False :yes :no)
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:yes))
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(assert (nil? (if-not True :yes)))
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(assert (= (if-not False :yes)
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:yes)))
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