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Merge pull request #893 from pyrmont/docs.nested-loops

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Clarify nested loop behaviour in loop macro
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Calvin Rose 2021-12-09 14:09:59 -06:00 committed by GitHub
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@ -532,16 +532,19 @@
(defmacro loop
```
A general purpose loop macro. This macro is similar to the Common Lisp
loop macro, although intentionally much smaller in scope.
The head of the loop should be a tuple that contains a sequence of
either bindings or conditionals. A binding is a sequence of three values
that define something to loop over. They are formatted like:
A general purpose loop macro. This macro is similar to the Common Lisp loop
macro, although intentionally much smaller in scope. The head of the loop
should be a tuple that contains a sequence of either bindings or
conditionals. A binding is a sequence of three values that define something
to loop over. Bindings are written in the format:
binding :verb object/expression
Where `binding` is a binding as passed to def, `:verb` is one of a set of
keywords, and `object` is any expression. The available verbs are:
where `binding` is a binding as passed to def, `:verb` is one of a set of
keywords, and `object` is any expression. Each subsequent binding creates a
nested loop within the loop created by the previous binding.
The available verbs are:
* `:iterate` - repeatedly evaluate and bind to the expression while it is
truthy.
@ -565,14 +568,19 @@
where `:modifier` is one of a set of keywords, and `argument` is keyword-dependent.
`:modifier` can be one of:
* `:while expression` - breaks from the loop if `expression` is falsey.
* `:until expression` - breaks from the loop if `expression` is truthy.
* `:let bindings` - defines bindings inside the loop as passed to the `let` macro.
* `:before form` - evaluates a form for a side effect before the next inner loop.
* `:after form` - same as `:before`, but the side effect happens after the next inner loop.
* `:while expression` - breaks from the current loop if `expression` is
falsey.
* `:until expression` - breaks from the current loop if `expression` is
truthy.
* `:let bindings` - defines bindings inside the current loop as passed to the
`let` macro.
* `:before form` - evaluates a form for a side effect before the next inner
loop.
* `:after form` - same as `:before`, but the side effect happens after the
next inner loop.
* `:repeat n` - repeats the next inner loop `n` times.
lets try putting a loop item on multiple lines.
* `:when condition` - only evaluates the loop body when condition is true.
* `:when condition` - only evaluates the current loop body when `condition`
is true.
The `loop` macro always evaluates to nil.
```