A set of small but handy functions for improving the readability and reliability of programs that operate on tagged X-expressions (for short, @italic{txexpr}s).
A txexpr is a list with a symbol in the first position— the @italic{tag} — followed by a series of @italic{elements}, which are other X-expressions. Optionally, a txexpr can have a list of @italic{attributes} in the second position.
The last one is a common mistake. Because the key–value pair is not enclosed in a @racket[list], it's interpreted as a nested txexpr within the first txexpr, as you may not find out until you try to read its attributes:
@margin-note{There's no way of eliminating this ambiguity, short of always requiring an attribute list — empty if necessary —in your txexpr. See also @racket[xexpr-drop-empty-attributes].}
After converting to and from HTML, we get back the original X-expression. Well, almost. The brackets turned into parentheses — no big deal, since they mean the same thing in Racket. Also, per its usual practice, @racket[string->xexpr] added an empty attribute list after @racket[em]. This is also benign.
@section{Why not just use @exec{match}, @exec{quasiquote}, and so on?}
If you prefer those, please do. But I've found two benefits to using module functions:
@bold{Readability.} In code that already has a lot of matching and quasiquoting going on, these functions make it easy to see where & how txexprs are being used.
@bold{Reliability.} Because txexprs come in two close but not quite equal forms, careful coders will always have to take both casesinto account.
The programming is trivial, but the annoyance is real.
@section{Interface}
@deftogether[(
@defproc[
(txexpr?
[v any/c])
boolean?]
@defproc[
(txexpr-tag?
[v any/c])
boolean?]
@defproc[
(txexpr-attr?
[v any/c])
boolean?]
@defproc[
(txexpr-attr-key?
[v any/c])
boolean?]
@defproc[
(txexpr-attr-value?
[v any/c])
boolean?]
@defproc[
(txexpr-element?
[v any/c])
boolean?]
)]
Predicates for @racket[_txexpr]s that implement this grammar:
Convert @racket[_x] to an HTML string. Better than @racket[xexpr->string] because consistent with the HTML spec, it will not escape text that appears within @code{script} or @code{style} blocks. For convenience, this function will take any X-expression, not just tagged X-expressions.
Assemble a @racket[_txexpr] from its parts. If you don't have attributes, but you do have elements, you'll need to pass @racket[empty] as the second argument. Note that unlike @racket[xml->xexpr], if the attribute list is empty, it's not included in the resulting expression.
Predicate for functions that handle @racket[_txexpr-attrs]. Covers values that are easily converted into pairs of @racket[_attr-key] and @racket[_attr-value]. Namely: single @racket[_xexpr-attr]s, lists of @racket[_xexpr-attr]s (i.e., what you get from @racket[get-attrs]), or interleaved symbols and strings (each pair will be concatenated into a single @racket[_xexpr-attr]).
@deftogether[(
@defproc[
(attrs->hash [x can-be-txexpr-attrs?] ...)
hash?]
@defproc[
(hash->attrs
[h hash?])
txexpr-attrs?]
)]
Convert @racket[_attrs] to an immutable hash, and back again.
Returns @racket[#t] if @racket[_attrs] and @racket[_other-attrs] contain the same keys and values, @racket[#f] otherwise. The order of attributes is irrelevant.
Given a @racket[_key], look up the corresponding @racket[_value] in the attributes of a @racket[_txexpr]. Asking for a nonexistent key produces an error.
Like @racket[attr-ref], but returns a recursively gathered list of all the @racket[_value]s for that key within @racket[_tx]. Asking for a nonexistent key produces @racket[null].
@examples[#:eval my-eval
(define tx '(div [[class "red"]] "Hello" (em ([class "blue"]) "world")))
Combine a series of attributes into a single @racket[_txexpr-attrs] item. This function addresses three annoyances that surface in working with txexpr attributes.
@itemlist[#:style 'ordered
@item{You can pass the attributes in multiple forms. See @racket[can-be-txexpr-attrs?] for further details.}
@item{Attributes with the same name are merged, with the later value taking precedence (i.e., @racket[hash] behavior). }
@item{Attributes are sorted in alphabetical order.}]
Recursively apply @racket[_proc] to all elements, leaving tags and attributes alone. Using plain @racket[map] will only process elements at the top level of the current @racket[_txexpr]. Usually that's not what you want.
In practice, most @racket[_xexpr-element]s are strings. But woe befalls those who pass string procedures to @racket[map-elements], because an @racket[_xexpr-element] can be any kind of @racket[xexpr?], and an @racket[xexpr?] is not necessarily a string.
Like @racket[map-elements], but skips any @racket[_txexprs] that evaluate to @racket[#t] under @racket[_exclude-test]. The @racket[_exclude-test] gets a whole txexpr as input, so it can test any of its parts.
Be careful with the wider consequences of exclusion tests. When @racket[_exclude-test] is true, the @racket[_txexpr] is excluded, but so is everything underneath that @racket[_txexpr]. In other words, there is no way to re-include (un-exclude?) elements nested under an excluded element.
Recursively descend through @racket[_txexpr] and extract all elements that match @racket[_pred]. Returns two values: a @racket[_txexpr] with the matching elements removed, and the list of matching elements. Sort of esoteric, but I've needed it more than once, so here it is.
Ordinarily, the result of the split operation is to remove the elements that match @racket[_pred]. But you can change this behavior with the optional @racket[_replace-proc] argument.
Designed to be used with @racketmodname[rackunit]. Check whether @racket[_tx1] and @racket[_tx2] are @racket[equal?] except for ordering of attributes (which ordinarily has no semantic significance). Return @racket[void] if so, otherwise raise a check failure.
