METAL Reference

The Macro Expansion Template Attribute Language (METAL) standard is a facility for HTML/XML macro preprocessing. It can be used in conjunction with or independently of TAL and TALES.

Introduction

Macros provide a way to define a chunk of presentation in one template, and share it in others, so that changes to the macro are immediately reflected in all of the places that share it. Additionally, macros are always fully expanded, even in a template’s source text, so that the template appears very similar to its final rendering

METAL Namespace

The METAL namespace URI and recommended alias are currently defined as:

xmlns:metal="http://xml.zope.org/namespaces/metal"

Just like the TAL namespace URI, this URI is not attached to a web page; it’s just a unique identifier.

Zope does not require an XML namespace declaration when creating templates with a content-type of text/html. However, it does require an XML namespace declaration for all other content-types.

METAL Statements

METAL defines a number of statements:

metal:define-macro
Define a macro.
metal:use-macro
Use a macro.
metal:define-slot
Define a macro customization point.
metal:fill-slot
Customize a macro.

Although METAL does not define the syntax of expression non-terminals, leaving that up to the implementation, a canonical expression syntax for use in METAL arguments is described in TALES Specification.

define-macro: Define a macro

Syntax

metal:define-macro syntax:

argument ::= Name

Description

The metal:define-macro statement defines a macro. The macro is named by the statement expression, and is defined as the element and its sub-tree.

In Zope, a macro definition is available as a sub-object of a template’s macros object. For example, to access a macro named header in a template named master.html, you could use the path expression:

master.html/macros/header

Examples

Simple macro definition:

<p metal:define-macro="copyright">
  Copyright 2009, <em>Foobar</em> Inc.
</p>

define-slot: Define a macro customization point

Syntax

metal:define-slot syntax:

argument ::= Name

Description

The metal:define-slot statement defines a macro customization point or slot. When a macro is used, its slots can be replaced, in order to customize the macro. Slot definitions provide default content for the slot. You will get the default slot contents if you decide not to customize the macro when using it.

The metal:define-slot statement must be used inside a metal:define-macro statement.

Slot names must be unique within a macro.

Examples

Simple macro with slot:

<p metal:define-macro="hello">
  Hello <b metal:define-slot="name">World</b>
</p>

This example defines a macro with one slot named name. When you use this macro you can customize the b element by filling the name slot.

fill-slot: Customize a macro

Syntax

metal:fill-slot syntax:

argument ::= Name

Description

The metal:fill-slot statement customizes a macro by replacing a slot in the macro with the statement element (and its content).

The metal:fill-slot statement must be used inside a metal:use-macro statement. Slot names must be unique within a macro.

If the named slot does not exist within the macro, the slot contents will be silently dropped.

Examples

Given this macro:

<p metal:define-macro="hello">
  Hello <b metal:define-slot="name">World</b>
</p>

You can fill the name slot like so:

<p metal:use-macro="container/master.html/macros/hello">
  Hello <b metal:fill-slot="name">Kevin Bacon</b>
</p>

use-macro: Use a macro

Syntax

metal:use-macro syntax:

argument ::= expression

Description

The metal:use-macro statement replaces the statement element with a macro. The statement expression describes a macro definition.

In Zope the expression will generally be a path expression referring to a macro defined in another template. See “metal:define-macro” for more information.

The effect of expanding a macro is to graft a subtree from another document (or from elsewhere in the current document) in place of the statement element, replacing the existing sub-tree. Parts of the original subtree may remain, grafted onto the new subtree, if the macro has slots. See metal:define-slot for more information. If the macro body uses any macros, they are expanded first.

When a macro is expanded, its metal:define-macro attribute is replaced with the metal:use-macro attribute from the statement element. This makes the root of the expanded macro a valid use-macro statement element.

Examples

Basic macro usage:

<p metal:use-macro="container/other.html/macros/header">
  header macro from defined in other.html template
</p>

This example refers to the header macro defined in the other.html template which is in the same folder as the current template. When the macro is expanded, the p element and its contents will be replaced by the macro. Note: there will still be a metal:use-macro attribute on the replacement element.