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Improve documentation of 'alist-get' (Bug#34708)

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* lisp/subr.el (alist-get): Enhance part of docstring explaining usage
in place expressions.
This commit is contained in:
Michael Heerdegen 2019-03-12 15:13:55 +01:00
parent 92acab73e0
commit 6dfd28c320
1 changed files with 24 additions and 2 deletions
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26
lisp/subr.el
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@ -756,9 +756,31 @@ Elements of ALIST that are not conses are ignored."
If KEY is not found in ALIST, return DEFAULT.
Use TESTFN to lookup in the alist if non-nil. Otherwise, use `assq'.
This is a generalized variable suitable for use with `setf'.
You can use `alist-get' in PLACE expressions. This will modify
an existing association (more precisely, the first one if
multiple exist), or add a new element to the beginning of ALIST,
destructively modifying the list stored in ALIST.
Example:
(setq foo '((a . 0)))
(setf (alist-get 'a foo) 1
(alist-get 'b foo) 2)
foo => ((b . 2) (a . 1))
When using it to set a value, optional argument REMOVE non-nil
means to remove KEY from ALIST if the new value is `eql' to DEFAULT."
means to remove KEY from ALIST if the new value is `eql' to
DEFAULT (more precisely the first found association will be
deleted from the alist).
Example:
(setq foo '((a . 1) (b . 2)))
(setf (alist-get 'b foo nil 'remove) nil)
foo => ((a . 1))"
(ignore remove) ;;Silence byte-compiler.
(let ((x (if (not testfn)
(assq key alist)