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pollen/metas.rkt

59 lines
2.7 KiB
Racket

#lang racket/base
(require racket/list pollen/top txexpr) ; pollen/top needed for metaroot
(provide split-metas-to-hash)
(define (meta-element? x)
(or (trivial-meta-element? x) (nontrivial-meta-element? x)))
(define (trivial-meta-element? x) ; trivial meta has no attributes.
(and (possible-meta-element? x) (empty? (get-attrs x))))
(define (nontrivial-meta-element? x) ; nontrivial meta has attributes that are valid.
(and (possible-meta-element? x)
(let ([attrs (get-attrs x)])
(and (not (empty? attrs)) (andmap valid-meta-attr? attrs)))))
(define (possible-meta-element? x)
(and (txexpr? x) (equal? 'meta (get-tag x))))
(define (valid-meta-attr? x)
(or (and (list? x) (symbol? (first x)) (string? (second x)))
(error 'is-meta-element? "error: meta must be a symbol / string pair, instead got: ~v" x)))
(define (explode-meta-element me)
;; convert a meta with multiple key/value pairs into multiple metas with a single key/value pair
;; only gets nontrivial metas to start.
(let loop ([me me][acc empty])
(cond
[(not (trivial-meta-element? me)) ; meta might become trivial during loop (no attrs)
(define attrs (get-attrs me))
(loop `(meta ,(cdr attrs)) (cons `(meta ,(list (car attrs))) acc))]
[else (reverse acc)])))
(define (split-meta-elements x) ; pull metas out of doc and put them into meta-elements accumulator
(define-values (thing-without-meta-elements meta-elements) (splitf-txexpr x meta-element?))
;; trivial metas are discarded
(define exploded-meta-elements (append-map explode-meta-element (filter nontrivial-meta-element? meta-elements)))
(values thing-without-meta-elements exploded-meta-elements))
(define (split-metas-to-hash x)
(define-values (doc-without-metas meta-elements) (split-meta-elements x))
;; 'metaroot is the hook for the meta decoder function.
;; If it's not a defined identifier, it just hits #%top and becomes `(metaroot ,@metas ...)
;; because of `explode-meta-element`, meta-elements will be a list of metas with a single key/value pair
;; metaroot can rely on this
(define metas-xexpr (apply metaroot meta-elements))
(define (first-attribute x) (car (get-attrs x)))
(define (meta-key x) (first (first-attribute x)))
(define (meta-value x) (second (first-attribute x)))
(define (meta-element->assoc me) (cons (meta-key me) (meta-value me)))
(define metas (make-hash (map meta-element->assoc (cdr metas-xexpr))))
(values doc-without-metas metas))
(module+ test
(require rackunit)
(define x '(root (meta ((foo "bar"))) "hello" (p (meta ((foo "zam"))) (meta) "there")))
(define-values (doc-without-metas metahash) (split-metas-to-hash x))
(check-equal? doc-without-metas '(root "hello" (p "there")))
(check-equal? (hash-ref metahash 'foo) "zam"))