processing-instructions.md

Processing instructions

A processing instruction is markup gsx never renders as an element: a bare placeholder, or a region wrapping content meant to be replaced later. Both forms are useful targets for client-side patching, such as Chrome's declarative partial updates, which patch by name via <template for="…">. <template> itself is an ordinary element in gsx and needs no special syntax.

Marker

<?marker name=VALUE> is a void placeholder:

component Row(id string) {
	<li>
		<?marker name={id}>
	</li>
}

<li><?marker name="row-1"></li>

Region

<?start name=VALUE> … <?end> wraps temporary content. Regions nest, and each <?end> closes its nearest open <?start>:

component Feed() {
	<?start name="feed">
		<li>Loading…</li>
	<?end>
}

<?start name="feed"><li>Loading…</li><?end>

name

VALUE is a string literal or {expr}, optionally piped through |> stages — the same grammar as any attribute value. name is required on marker and start; <?end> takes no attributes.

An {expr} must be a string, []byte, or fmt.Stringer — or have a registered renderer, which is applied before the name is written. Any other type is a compile error.

A name can't contain > or ": neither is escapable in processing-instruction data, since it is never entity-decoded. A literal containing one is a compile error; an {expr} producing one is a render error.

Restrictions

marker, start, and end are the only valid targets — any other target, such as <?foo>, is a compile error. Source must terminate with >; the XML-style <?marker name="a"?> is a compile error naming the ?>.

Only <?end> closes a region. Any close tag reached inside one — including a fragment's </> — is a compile error naming <?end> as the expected terminator.

Processing instructions are markup only. They are not valid as a Go-expression value:

x := <?marker name="a"> // error: not supported as a Go expression value

Patch by name

Pair a marker or region with <template for="…"> so client code can replace it:

component Row(id string) {
	<li>
		<?marker name={id}>
	</li>
	<template for={id}>
		<li>Updated</li>
	</template>
}