76 lines
3.5 KiB
Racket
76 lines
3.5 KiB
Racket
#lang scribble/manual
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@require[scribble/example
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"utils.rkt"
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@for-label[phc-toolkit/untyped
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extensible-parser-specifications
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generic-syntax-expanders
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racket/base
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syntax/parse
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(only-in racket/base [... …])]]
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@title{Pre operations}
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@defform[#:kind "eh-mixin expander"
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(~named-seq #,tribute-name #,ntax-pattern ...)]{
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Equivalent to @racket[{~seq #,ntax-pattern ...}], but also binds the
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@racket[#,tribute-name] to the whole sequence. If the sequence appears inside
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an @racket[~optional] or @racket[~or] clause that fails, the
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@racket[_attribute-name] is still bound to the empty sequence.
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Known issues: this may not behave as expected if @racket[~named-seq] appears
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under ellipses.
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This probably should bind the sequence attribute @emph{before} the "global"
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operations, instead of being a "post" operation, and may be changed in that way
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the future.}
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@defform[#:kind "eh-mixin expander"
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(~maybe/empty #,ntax-pattern ...)]{
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Optionally matches @racket[{~seq #,ntax-pattern ...}]. If the match fails, it
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matches these same sequence of patterns against the empty syntax list
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@racket[#'()]. This form can be used in an ellipsis-head position. This is
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implemented in both cases as a "pre" action.}
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@defform[#:kind "eh-mixin expander"
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(~optional/else #,ntax-pattern
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maybe-defaults
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else-post-fail ...
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maybe-name)
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#:grammar
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[(maybe-defaults (code:line)
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(code:line #:defaults (default-binding ...)))
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(else-post-fail
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(code:line #:else-post-fail message #:when condition)
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(code:line #:else-post-fail #:when condition message)
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(code:line #:else-post-fail message #:unless unless-condition)
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(code:line #:else-post-fail #:unless unless-condition message))
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(maybe-name (code:line)
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(code:line #:name #,tribute-name))]]{
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Like @racket[~optional], but with conditional post-failures when the pattern is
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not matched. An @racket[~optional/else] pattern can be matched zero or one time
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as part of the @racket[~seq-no-order] or @racket[~no-order]. When it is not
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matched (i.e. matched zero times):
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@itemlist[
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@item{it uses the default values for the attributes as specified with
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@racket[#:defaults].}
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@item{for each @racket[#:else-post-fail] clause, it checks whether the
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@racket[condition] or @racket[unless-condition] is true or false,
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respectively. If this is the case the whole @racket[~seq-no-order] or
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@racket[~no-order] is rejected with the given @racket[_message]. The
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behaviour of @racket[#:else-post-fail] is the same as the behaviour of
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@racket[~post-fail], except that the "post" conditional failure can only be
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executed if the optional @racket[_syntax-pattern] was not matched.
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Note that there is an implicit cut (@racket[~!]) between the no-order
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patterns and the "post" checks, so after a @racket[~post-fail] fails,
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@racket[syntax-parse] does not backtrack and attempt different combinations
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of patterns to match the sequence, nor does it backtrack and attempt to match
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a shorter sequence. This is by design, as it allows for better error messages
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(syntax-parse would otherwise attempt and possibly succeed in matching a
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shorter sequence, then just treat the remaining terms as
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"unexpected terms").}]
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The meaning of @racket[#:name #,tribute-name] option is the same as for
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@racket[~optional].}
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