◊(Local Yarn Code "Check-in [6b60dccb]")

I use a couple of macros to define tag functions that automatically branch into other functions
depending on the current output target format. This allows me to put the format-specific tag
functions in separate files that have separate places in the dependency chain. So if only the HTML
tag functions have changed and not those for PDF, the makefile can ensure only the HTML files are
rebuilt.

@defproc[#:kind "syntax"
 (poly-branch-tag (id symbol?))
 (poly-branch-tag (tag symbol?))
 (-> txexpr?)]
Define a new tag function (using @racket[define-tag-function]) for @racket[_id], which will
automatically pass all of its attributes and elements to a tag function whose name is the value
returned by @racket[current-poly-target], followed by a hyphen, followed by @racket[_id]. So
whenever the current output format is @racket['html], the function defined by
@racket[(poly-branch-tag _p)] will branch to a function named @racket[html-p]; when the current
format is @racket['pdf], it will branch to @racket[pdf-p], and so forth.

Defines a new function @racket[_tag] which will automatically pass all of its arguments to a
function whose name is the value returned by @racket[current-poly-target], followed by a hyphen,
followed by @racket[_tag]. So whenever the current output format is @racket['html], the function
defined by @racket[(poly-branch-tag _p)] will branch to a function named @racket[html-p]; when the
current format is @racket['pdf], it will branch to @racket[pdf-p], and so forth.

You @emph{must} define these branch functions separately, and you must define one for @emph{every}
output format included in the definition of @racket[poly-targets] in this file’s @racket[setup]
submodule. If you do not,  you will get “unbound identifier” errors at expansion time.
submodule. If you do not, you will get “unbound identifier” errors at expansion time.

The convention in this project is to define and provide these branch functions in separate files:
see, e.g., @filepath{tags-html.rkt}.

Functions defined with this macro @emph{do not} accept keyword arguments. If you need keyword
arguments, see @racket[poly-branch-kwargs-tag].

@defproc[#:kind "syntax"
         (poly-branch-func (id symbol?))
         (poly-branch-kwargs-tag (id symbol?))
         (-> txexpr?)]

@margin-note{In writing this documentation it dawned on me that I only need one of these macros. So
at some point the current @code{poly-branch-tag} will be eliminated, and @code{poly-branch-func}
then renamed to @code{poly-branch-tag}. See @ticket{fbbb5ed9}.}
Works just like @racket[poly-branch-tag], but uses Pollen’s @racket[define-tag-function] so that
keyword arguments will automatically be parsed as X-expression attributes.

@margin-note{The thought behind having two macros so similar is that, by cutting out handling for keyword
Like @racket[poly-branch-tag], but uses @racket[define] instead of @racket[define-tag-function].
arguments, @racket[poly-branch-tag] could produce simpler and faster code. I have not verified if
this intuition is meaningful or correct.}

@section{Markup reference}

These are the tags that can be used in any of @italic{The Local Yarn}’s Pollen documents (articles,
etc).

@defproc[(p [element xexpr?] ...) txexpr?]

    (map resolve-module-path '("tags-html.rkt"
                               "snippets-html.rkt"
                               "dust.rkt"
                               "crystalize.rkt"))))

;; Macro for defining tag functions that automatically branch based on the 
;; current output format and the list of poly-targets in the setup module.
;; Use this macro when you know you will need keyword arguments.
;; 
(define-syntax (poly-branch-tag stx)
;;
(define-syntax (poly-branch-kwargs-tag stx)
  (syntax-parse stx
    [(_ TAG:id)
     (with-syntax ([((POLY-TARGET POLY-FUNC) ...) 
                    (for/list ([target (in-list (setup:poly-targets))])
                              (list target (format-id stx "~a-~a" target #'TAG)))]
                   [DEFAULT-FUNC (format-id stx "html-~a" #'TAG)])
       #'(define-tag-function (TAG attributes elems)
           (define args (cons attributes elems))
           (case (current-poly-target)
             [(POLY-TARGET) (apply POLY-FUNC args)] ... 
             [else (apply DEFAULT-FUNC args)])))]))

;; Like above, but uses define instead of define-tag-function, so arguments
;; are given ‘straight’ rather than being parsed out as attributes.
;; Like above, but uses `define` instead of `define-tag-function`.
;; Use this when you know you will not need keyword arguments.
;;
(define-syntax (poly-branch-func stx)
(define-syntax (poly-branch-tag stx)
  (syntax-parse stx
    [(_ TAG:id)
     (with-syntax ([((POLY-TARGET POLY-FUNC) ...) 
                    (for/list ([target (in-list (setup:poly-targets))])
                              (list target (format-id stx "~a-~a" target #'TAG)))]
                   [DEFAULT-FUNC (format-id stx "html-~a" #'TAG)])
       #'(define (TAG . args)

(poly-branch-tag newthought)
(poly-branch-tag smallcaps)
(poly-branch-tag center)
(poly-branch-tag section)
(poly-branch-tag subsection)
(poly-branch-tag code)
(poly-branch-tag blockcode)
(poly-branch-tag verse)          ; [#:title ""] [#:italic "no"]
(poly-branch-kwargs-tag verse)          ; [#:title ""] [#:italic "no"]

(poly-branch-func link)
(poly-branch-func url)
(poly-branch-func fn)
(poly-branch-func fndef)
(poly-branch-tag link)
(poly-branch-tag url)
(poly-branch-tag fn)
(poly-branch-tag fndef)

(poly-branch-tag note)
(poly-branch-kwargs-tag note)

;; Not yet implemented
; (poly-branch-tag table)         ; #:columns ""
; (poly-branch-tag inline-math)
; (poly-branch-tag margin-note)
; (poly-branch-tag noun)
; (poly-branch-func index-entry entry)