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Diffstat (limited to 'doc/ref/api-procedures.texi')
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1 files changed, 55 insertions, 6 deletions
diff --git a/doc/ref/api-procedures.texi b/doc/ref/api-procedures.texi index e749fdc98..bef3386e8 100644 --- a/doc/ref/api-procedures.texi +++ b/doc/ref/api-procedures.texi @@ -599,12 +599,61 @@ A @code{case-lambda*} clause matches if the arguments fill the required arguments, but are not too many for the optional and/or rest arguments. -Keyword arguments are possible with @code{case-lambda*}, but they do -not contribute to the ``matching'' behavior. That is to say, -@code{case-lambda*} matches only on required, optional, and rest -arguments, and on the predicate; keyword arguments may be present but -do not contribute to the ``success'' of a match. In fact a bad keyword -argument list may cause an error to be raised. +Keyword arguments are possible with @code{case-lambda*} as well, but +they do not contribute to the ``matching'' behavior, and their +interactions with required, optional, and rest arguments can be +surprising. + +For the purposes of @code{case-lambda*} (and of @code{case-lambda}, as a +special case), a clause @dfn{matches} if it has enough required +arguments, and not too many positional arguments. The required +arguments are any arguments before the @code{#:optional}, @code{#:key}, +and @code{#:rest} arguments. @dfn{Positional} arguments are the +required arguments, together with the optional arguments. + +In the absence of @code{#:key} or @code{#:rest} arguments, it's easy to +see how there could be too many positional arguments: you pass 5 +arguments to a function that only takes 4 arguments, including optional +arguments. If there is a @code{#:rest} argument, there can never be too +many positional arguments: any application with enough required +arguments for a clause will match that clause, even if there are also +@code{#:key} arguments. + +Otherwise, for applications to a clause with @code{#:key} arguments (and +without a @code{#:rest} argument), a clause will match there only if +there are enough required arguments and if the next argument after +binding required and optional arguments, if any, is a keyword. For +efficiency reasons, Guile is currently unable to include keyword +arguments in the matching algorithm. Clauses match on positional +arguments only, not by comparing a given keyword to the available set of +keyword arguments that a function has. + +Some examples follow. + +@example +(define f + (case-lambda* + ((a #:optional b) 'clause-1) + ((a #:optional b #:key c) 'clause-2) + ((a #:key d) 'clause-3) + ((#:key e #:rest f) 'clause-4))) + +(f) @result{} clause-4 +(f 1) @result{} clause-1 +(f) @result{} clause-4 +(f #:e 10) clause-1 +(f 1 #:foo) clause-1 +(f 1 #:c 2) clause-2 +(f #:a #:b #:c #:d #:e) clause-4 + +;; clause-2 will match anything that clause-3 would match. +(f 1 #:d 2) @result{} error: bad keyword args in clause 2 +@end example + +Don't forget that the clauses are matched in order, and the first +matching clause will be taken. This can result in a keyword being bound +to a required argument, as in the case of @code{f #:e 10}. + @node Higher-Order Functions @subsection Higher-Order Functions |