The only key that's automatically defined in every meta table is @racket['#,pollen-here-path-key], which holds the absolute path to the source file. For instance, you could retrieve this value with @racket[(select-from-metas '#,pollen-here-path-key metas)].
The splicing tag is useful when you want to return a list of X-expressions in a situation where you can only return one. For instance, @secref["Tag_functions"] can only return one X-expression. But if we wrap the list of X-expressions in a splicing tag, they behave like a single X-expression. Later, Pollen will merge the list elements into the surrounding expression (as shown above).
@examples[#:eval my-eval
(require pollen/tag)
(code:comment @#,t{wrong: function returns a list of X-expressions})
(define-tag-function (multi attrs elems)
'("foo" "bar"))
(code:comment @#,t{right: function returns a list of X-expressions})
(code:comment @#,t{as elements inside a splicing tag})
(define-tag-function (multi2 attrs elems)
'(\@ "foo" "bar"))
]
Though the splicing tag is cosmetically identical to the abbreviated notation of @litchar{@"@"} for @racket[unquote-splicing], and has a similar purpose, it's not the same thing. The splicing tag isn't a variable —it's just a symbol that Pollen treats specially when generating output.
If @racket[_condition] is true, put the @racket[_pollen-args] into the document. Within a template file, usually invoked like so:
@verbatim{◊when/splice[@racketvarfont{condition}]{The text to insert.}}
The inserted text can contain its own nested Pollen commands.
@racket[when/splice] can be more convenient than @racket[when], because @racket[when] will only use the last argument between the curly braces. @racket[when/splice], by contrast, treats everything between the curly braces as a block.
Functions for retrieving data out of Pollen source files. These are not the only options –you can, of course, use any of the usual Racket functions.
@defproc[
(get-doc
[doc-source (or/c pagenode? pathish?)])
(or/c txexpr? string?)]
Retrieve the @racket[doc] export from @racket[_doc-source], which can be either a path, path string, or pagenode that can be resolved into a source path. If @racket[_doc-source] cannot be resolved, raise an error.
If @racket[_doc-source] is a relative path or pagenode, it is treated as being relative to @racket[current-project-root]. If that's not what you want, you'll need to convert it explicitly to a complete-path (e.g., with @racket[path->complete-path] or @racket[->complete-path]).
Retrieve the @racket[metas] export from @racket[_meta-source], which can be either a path, path string, or pagenode that can be resolved into a source path. If @racket[_meta-source] cannot be resolved, raise an error.
If @racket[_meta-source] is a relative path or pagenode, it is treated as being relative to @racket[current-project-root]. If that's not what you want, you'll need to convert it explicitly to a complete-path (e.g., with @racket[path->complete-path] or @racket[->complete-path]).
Find matches for @racket[_key] in @racket[_value-source]. The @racket[_value-source] can be 1)a hashtable of @racket[metas], 2)a tagged X-expression representing a @racket[doc], or 3)a pagenode or path that identifies a source file that provides @racket[metas] and @racket[doc]. In that case, first look for @racket[_key] in @code{metas} (using @racket[select-from-metas]) and then in @code{doc} (using @racket[select-from-doc]).
With @racket[select], you get the first result; with @racket[select*], you get them all.
In both cases, you get @racket[#f] if there are no matches.
Note that if @racket[_value-source] is a relative path or pagenode, it is treated as being relative to @racket[current-project-root]. If that's not what you want, you'll need to convert it explicitly to a complete-path (e.g., with @racket[path->complete-path] or @racket[->complete-path]).
Look up the value of @racket[_key] in @racket[_doc-source]. The @racket[_doc-source] argument can be either 1)a tagged X-expression representing a @racket[doc] or 2)a pagenode or source path that identifies a source file that provides @racket[doc]. If no value exists for @racket[_key], you get @racket[#f].
Note that if @racket[_doc-source] is a relative path or pagenode, it is treated as being relative to @racket[current-project-root]. If that's not what you want, you'll need to convert it explicitly to a complete-path (e.g., with @racket[path->complete-path] or @racket[->complete-path]).
Look up the value of @racket[_key] in @racket[_meta-source]. The @racket[_meta-source] argument can be either 1)a hashtable representing @racket[metas] or 2)a pagenode or source path that identifies a source file that provides @racket[metas]. If no value exists for @racket[_key], you get @racket[#f].
Note that if @racket[_meta-source] is a relative path or pagenode, it is treated as being relative to @racket[current-project-root]. If that's not what you want, you'll need to convert it explicitly to a complete-path (e.g., with @racket[path->complete-path] or @racket[->complete-path]).
Holds the @racket[metas] of the current Pollen source. In tag functions, rather than pass @racket[metas] as an argument, you can refer to @racket[(current-metas)] within the body of the function. Likewise, if your tag function calls other tag functions, they can all invoke @racket[(current-metas)] instead of passing the value around.
