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Document return value of pcase (Bug#30425)

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* doc/lispref/control.texi (Pattern matching case statement):
* lisp/emacs-lisp/pcase.el (pcase): State that pcase returns nil if no
patterns match.
(pcase-exhaustive): State that an error is signaled if no patterns
match.
This commit is contained in:
Noam Postavsky 2018-03-31 00:40:43 -04:00
parent 8a2466ff72
commit e50196e8a9
2 changed files with 7 additions and 3 deletions
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3
doc/lispref/control.texi
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@ -317,7 +317,8 @@ list of the form @code{(@var{pattern} @var{body-forms}@dots{})}.
@var{pattern} of each clause, in textual order. If the value matches,
the clause succeeds; @code{pcase} then evaluates its @var{body-forms},
and returns the value of the last of @var{body-forms}. Any remaining
@var{clauses} are ignored.
@var{clauses} are ignored. If no clauses match, then the @code{pcase}
form evaluates to @code{nil}.
The @var{pattern} part of a clause can be of one of two types:
@dfn{QPattern}, a pattern quoted with a backquote; or a

7
lisp/emacs-lisp/pcase.el
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@ -118,7 +118,9 @@ two element list, binding its elements to symbols named `foo' and
A significant difference from `cl-destructuring-bind' is that, if
a pattern match fails, the next case is tried until either a
successful match is found or there are no more cases.
successful match is found or there are no more cases. The CODE
expression corresponding to the matching pattern determines the
return value. If there is no match the returned value is nil.
Another difference is that pattern elements may be quoted,
meaning they must match exactly: The pattern \\='(foo bar)
@ -211,7 +213,8 @@ Emacs Lisp manual for more information and examples."
;;;###autoload
(defmacro pcase-exhaustive (exp &rest cases)
"The exhaustive version of `pcase' (which see)."
"The exhaustive version of `pcase' (which see).
If EXP fails to match any of the patterns in CASES, an error is signaled."
(declare (indent 1) (debug pcase))
(let* ((x (gensym "x"))
(pcase--dontwarn-upats (cons x pcase--dontwarn-upats)))