spinneret
2021-10-21
Common Lisp HTML5 generator.
Spinneret
In the crowded space of Common Lisp HTML generators, Spinneret occupies the following coordinates:
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Modern. Targets HTML5. Does not treat XML and HTML as the same problem. Assumes you will be serving your documents as UTF-8.
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Composable. Makes it easy to refactor HTML generation into separate functions and macros.
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Pretty. Treats HTML as a document format, not a serialization. Output is idiomatic and readable, following the coding style of the HTML5 specification.
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Aggressive. If something can be interpreted as HTML, then it will be, meaning that some Lisp forms can't be mixed with HTML syntax. In the trade-off between 90% convenience and 10% correctness Spinneret is on the side of convenience.
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Bilingual. Spinneret (after loading
spinneret/ps) has the same semantics in Lisp and Parenscript.
HTML generation with Spinneret looks like this:
(in-package #:spinneret)
(defparameter *shopping-list*
'("Atmospheric ponds"
"Electric gumption socks"
"Mrs. Leland's embyronic television combustion"
"Savage gymnatic aggressors"
"Pharmaceutical pianos"
"Intravenous retribution champions"))
(defparameter *user-name* "John Q. Lisper")
(defparameter *last-login* "12th Never")
(defmacro with-page ((&key title) &body body)
`(with-html
(:doctype)
(:html
(:head
(:title ,title))
(:body ,@body))))
(defun shopping-list ()
(with-page (:title "Home page")
(:header
(:h1 "Home page"))
(:section
("~A, here is *your* shopping list: " *user-name*)
(:ol (dolist (item *shopping-list*)
(:li (1+ (random 10)) item))))
(:footer ("Last login: ~A" *last-login*))))
Which produces:
<!DOCTYPE html>
<html lang=en>
<head>
<meta charset=UTF-8>
<title>Home page</title>
</head>
<body>
<header>
<h1>Home page</h1>
</header>
<section>
John Q. Lisper, here is <em>your</em> shopping list:
<ol>
<li>10 Atmospheric ponds
<li>6 Electric gumption socks
<li>4 Mrs. Leland's embyronic television combustion
<li>9 Savage gymnatic aggressors
<li>6 Pharmaceutical pianos
<li>9 Intravenous retribution champions
</ol>
</section>
<footer>
Last login: 12th Never
</footer>
</body>
</html>
(Pretty-printing is pretty fast, but Spinneret obeys *print-pretty*
should you want to turn it off.)
Printing style
Spinneret tries hard to produce human-writable output – output that looks like a human being wrote it. Sometimes, however, you may have markup to render that there is no human-writable way to render, because no human being would ever write it.
In these cases you can set or bind the *html-style* variable to
control Spinneret’s print style. The default is :human, which means
to attempt to produce human-writable output. It can also be set to
:tree, which simply prints every element as if it were a block
element, and every run of text on a new line.
(let ((*html-style* :human))
(with-html
(:div
(:p "Text " (:a "link text") " more text"))))
=> <div>
<p>Text <a>link text</a> more text
</div>"
(let ((*html-style* :tree))
(with-html-string
(:div
(:p "Text " (:a "link text") " more text"))))
=> <div>
<p>
Text
<a>
link text
</a>
more text
</p>
</div>
With *html-style* bounds to :tree, and *print-pretty* bounds to
nil, output is verbose but predictable:
(let ((*html-style* :tree)
(*print-pretty* nil))
(with-html-string
(:div
(:p "Text " (:a "link text") " more text"))))
=> "<div><p>Text <a>link text</a> more text</p></div>"
Inserted spaces
By default, when objects are output to HTML, spaces are inserted betweeen them. This is nearly always the right thing to do, but in some special cases, the spaces may be a problem. They can be turned off by setting the flag *suppress-inserted-spaces* to t.
Line wrapping
When pretty-printing, Spinneret makes the best decisions about line
wrapping that it can, given the information it has about how to get
the print length of various types. But, in the case of user-defined
types, it has no way to tell in advance how long they will be when
printed. If you find Spinneret is making bad line-breaking decisions
with your types, you can help it out by specializing html-length.
For example, if you use PURI, you could help Spinneret pretty-print
PURI URIs by teaching it how to get their length:
(defmethod html-length ((uri puri:uri))
;; Doesn't cons.
(length (puri:render-uri uri nil)))
Syntax
The rules for WITH-HTML are these:
-
All generated forms write to
*html*. -
A keyword in function position is interpreted as a tag name. If the name is not valid as a tag, it is ignored.
Certain keywords are recognized as pseudo-tags and given special treatment:
:RAW :DOCTYPE :!DOCTYPE :CDATA :!-- :COMMENT :HTML :HEAD :H* :TAG
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The pseudotag :RAW can be used to bypass Spinneret’s implicit escaping for raw output. This allows inserting HTML literals, and bypasses pretty printing.
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The pseudotags :!– and :COMMENT insert comments into the output.
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The pseudotag :H* renders as one of :H1 through :H6 depending on how many :SECTION elements it is dynamically nested inside. At the top level, :H* is equivalent to :H1. Inside the dynamic extent of one :SECTION tag, it is equivalent to :H2; inside two section tags, it is equivalent to :H3; and so forth up to :H6.
-
The pseudotag :TAG allows dynamic selection of a tag.
The value of the LANG attribute of HTML is controlled by
*html-lang*; the value of the meta charset attribute is controlled by*html-charset*.Constant classes and ids can be specified with a selector-like syntax. E.g.:
(:div#wrapper (:div.section ...)) ≡ (:div :id "wrapper" (:div :class "section" ...)) -
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Keyword-value pairs following a tag are interpreted as attributes. HTML syntax may not be used in attribute values. Attributes with nil values are omitted from the output. Boolean attributes with non-nil values are minimized.
Duplicate attributes are handled like duplicate keyword arguments: all values are evaluated, but only the leftmost value is used. The exception is the handling of tokenized attributes, such as :CLASS or :REL. The class of a tag is the union of all its :CLASS arguments.
The argument :DATASET introduces a list of :DATA-FOO arguments:
(:p :dataset (:duck (dolomphious) :fish 'fizzgigious :spoon "runcible")) ≡ (:p :data-duck (dolomphious) :data-fish 'fizzgigious :data-spoon "runcible")For flexibility, even at the cost of efficiency, the argument :ATTRS introduces a form to evaluate at run time for a plist of extra attributes and values.
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Forms after the attributes are treated as arguments. Each non-nil (primary) value returned by an argument to a tag is written to the stream by HTML, a generic function on which you can define your own methods. By default only literal arguments are printed. Literal arguments are strings, characters, numbers and symbols beside NIL.
WITH-HTML-STRING is like WITH-HTML, but intercepts the generated HTML at run time and returns a string.
Dynamic output
For flexibility, even at the cost of efficiency, the pseudo-attribute :ATTRS introduces a form to evaluate at run time for a plist of extra attributes and values.
(:p :attrs (list :id "dynamic!"))
=> <p id="dynamic!">
Similarly, the pseudo-tag :TAG allows you to select a tag at run time.
(:tag :name "div"
(:tag :name "p"
(:tag :name "span"
"Hello.")))
≡ (:div (:p (:span "Hello")))
Note that :TAG only allows you to select a tag, not create one. The tag must still be one that is known to Spinneret to be valid. (That is, either defined as part of HTML or matching the requirements for a custom element.)
For maximum dynamicity, you can combine :TAG and :ATTRS:
(:tag :name "div" :attrs (list :id "dynamic!"))
=> <div id=dynamic!></div>
Interpreting trees
For the ne plus ultra of flexibility, you can interpret trees at runtime using a subset of Spinneret syntax:
(interpret-html-tree `(:div :id "dynamic!"))
=> <div id=dynamic!></div>
Markdown
If the additional system spinneret/cl-markdown is loaded, then a
string in function position is first compiled as Markdown (using
CL-MARKDOWN), then passed to format as a control string and
applied to its arguments.
This is useful for inline formatting, like links, where sexps would be clumsy:
(with-html
("Here is some copy, with [a link](~a)" link))
(with-html
(:span "Here is some copy, with "
(:a :href link "a link.")))
get-html-path
Sometimes it is useful for a piece of HTML-generating code to know
where in the document it appears. You might, for example, want to
define a tabulate function that prints list-of-lists as rows of
cells, but only prints the surrounding <table></table> if it is not
already within a table. The function get-html-path returns a list of
open tags, from latest to earliest. Usually it will look something
like
(get-html-path) ;-> '(:table :section :body :html)
Thus `tabulate' could be written
(defun tabulate (&rest rows)
(with-html
(flet ((tabulate ()
(loop for row in rows do
(:tr (loop for cell in row do
(:td cell))))))
(if (find :table (get-html-path))
(tabulate)
(:table (:tbody (tabulate)))))))
Note that get-html-path returns a freshly-consed list each time it
is called.
deftag
The stumbling block for all sexp-based HTML generators is order of evaluation. It's tempting to write something like this:
;; Doesn't work
(defun field (control)
(with-html (:p control)))
(defun input (default &key name label (type "text"))
(with-html
(:label :for name label)
(:input :name name :id name :type type :value default)))
But it won't work: in (field (input "Default" :name "why" :label "Reason")), (input) gets evaluated before (field), and the HTML
is printed inside-out.
Macros do work:
(defmacro field (control)
`(with-html (:p ,control)))
(defmacro input (name label &key (type "text"))
`(with-html
(:label :for ,name ,label)
(:input :name ,name :id ,name :type ,type)))
But we can do better than this. Spinneret provides a macro-writing
macro, deftag, which lets you refactor HTML without hiding it.
(deftag field (control attrs)
`(:p ,@attrs ,@control))
(deftag input (default attrs &key name label (type "text"))
(once-only (name)
`(progn
(:label :for ,name ,label)
(:input :name ,name :id ,name :type ,type
,@attrs
:value (progn ,@default)))))
A macro defined using deftag takes its arguments just like an HTML
element. Instead of
(input "Default" :name "why" :label "Reason") ; defmacro
You write
(input :name "why" :label "Reason" "Default") ; deftag
The macro re-arranges the arguments so they can be bound to an
ordinary lambda list, like the one above: the body of the tag is bound
to the first argument, and matching attributes are bound to keywords.
Multiple :class arguments, :dataset, and other shorthands are
handled exactly as in the usual HTML syntax.
But the great advantage of deftag is how it handles attributes which
are not bound to keywords. In the definition of input using
deftag, you see that the attrs catch-all argument is spliced into
the call to :input. This means that any unhandled attributes pass
through to the actual input element.
(input :name "why" :label "Reason" :required t :class "special" "Default")
=> <label for=why>Reason</label>
<input class=special name=why id=why type=text required value=Default>
In effect, input extends the :input tag, almost like a subclass.
This is a very idiomatic and expressive way of building abstractions
over HTML.
(Spinneret used to provide a more elaborate way of building HTML
abstractions, deftemplate, but deftag is simpler and more useful.)
Parenscript
To use Spinneret with Parenscript, load the system spinneret/ps.
The semantics of Spinneret in Parenscript are almost the same. There
is no with-html-string, and with-html returns a
DocumentFragment.
If Markdown support is enabled, strings in function position are still
parsed as Markdown, but supplying arguments triggers an error (since
Parenscript does not have format).
get-html-path is not implemented for Parenscript.
Neither :ATTRS nor :TAG is available in Parenscript.
Validation
Spinneret does not do document validation, but it does warn about invalid tags and attributes.
Although HTML5 does include a mechanism for application-specific
attributes (the data- prefix), some client-side frameworks choose to
employ their own prefixes instead. You can disable validation for a
given prefix by adding it to *unvalidated-attribute-prefixes*.
(pushnew "ng-" *unvalidated-attribute-prefixes* :test #’equal)
You can disable attribute validation altogether by adding the empty string to the list:
;; Disable attribute validation.
(setf *unvalidated-attribute-prefixes* '(""))
Tags are considered valid if they are defined as part of the HTML standard, or if they match the rules for the name of a custom element – basically, start with an ASCII alphabetic character and include a hyphen. For custom elements, attributes are not validated.