One document matched: draft-gregorio-uritemplate-04.xml
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<rfc category="std" ipr="pre5378Trust200902" docName="draft-gregorio-uritemplate-04">
<front>
<title>URI Template</title>
<author initials='J.C.' surname="Gregorio" fullname='Joe Gregorio' role="editor">
<organization>Google</organization>
<address>
<email>joe@bitworking.org</email>
<uri>http://bitworking.org/</uri>
</address>
</author>
<author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
<organization abbrev="Day Software">Day Software</organization>
<address>
<email>fielding@day.com</email>
<uri>http://www.day.com/</uri>
</address>
</author>
<author initials='M.H.' surname="Hadley" fullname='Marc Hadley' role="editor">
<organization>Oracle</organization>
<address>
<email>Marc.Hadley@oracle.com</email>
<uri>http://oracle.com/</uri>
</address>
</author>
<author initials='M.N.' surname="Nottingham" fullname='Mark Nottingham' role="editor">
<organization></organization>
<address>
<email>mnot@pobox.com</email>
<uri>http://mnot.net/</uri>
</address>
</author>
<author initials='D.O.' surname="Orchard" fullname='David Orchard'>
<organization></organization>
<address>
<uri>http://www.pacificspirit.com/</uri>
</address>
</author>
<date day="08" month="Mar" year="2010"/>
<abstract>
<t>
A URI Template is a compact sequence of characters
for describing a range of Uniform Resource Identifiers
through variable expansion.
This specification defines the URI Template
syntax and the process for expanding a URI Template into a
URI, along with guidelines for the
use of URI Templates on the Internet.
</t>
</abstract>
<note title="Editorial Note (to be removed by RFC Editor)">
<t>
To provide feedback on this Internet-Draft, join the
<eref target="http://lists.w3.org/Archives/Public/uri/">W3C URI
mailing list (http://lists.w3.org/Archives/Public/uri/)</eref>.
</t>
</note>
</front>
<middle>
<section title="Introduction" anchor="intro">
<section title="Overview" anchor="overview">
<t>
A Uniform Resource Identifier (URI) <xref target="RFC3986"/>
is often used to identify a specific resource within a common
space of similar resources. For example, personal web spaces
are often delegated using a common pattern, such as
</t>
<figure><artwork>
http://example.com/~fred/
http://example.com/~mark/
</artwork></figure>
<t>
or a set of dictionary entries might be grouped in a hierarchy
by the first letter of the term, as in
</t>
<figure><artwork>
http://example.com/dictionary/c/cat
http://example.com/dictionary/d/dog
</artwork></figure>
<t>
or a service interface might be invoked with various user input
in a common pattern, as in
</t>
<figure><artwork>
http://example.com/search?q=cat&lang=en
http://example.com/search?q=dog&lang=fr
</artwork></figure>
<t>
URI Templates provide a mechanism for abstracting a space of
resource identifiers such that the variable parts can be easily
identified and described. URI templates can have many uses,
including discovery of available services, configuring resource
mappings, defining computed links, specifying interfaces, and
other forms of programmatic interaction with resources.
For example, the above resources could be described by the
following URI templates:
</t>
<figure><artwork>
http://example.com/~{username}/
http://example.com/dictionary/{term:1}/{term}
http://example.com/search{?q,lang}
</artwork></figure>
<t>
We define the following terms:
<list style="symbols">
<t>expression -
The text between '{' and '}', including the enclosing braces,
as defined in <xref target="syntax"/>.</t>
<t>expansion -
The string result obtained from a template expression after
processing it according to its expression type, list of variable
names, and value modifiers, as defined in <xref target="expansion"/>.</t>
<t>template processor -
A program or library that, given a URI Template and a set of
variables with values, transforms the template string into a
URI-reference by parsing the template for expressions and
substituting each one with its corresponding expansion.</t>
</list>
</t>
<t>
A URI Template provides both a structural description of a URI
space and, when variable values are provided, a simple instruction
on how to construct a URI corresponding to those values.
A URI Template is transformed into a URI-reference by replacing each
delimited expression with its value as defined by the
expression type and the values of variables named within the
expression. The expression types range from simple value
expansion to multiple key=value lists. The expansions
are based on the URI generic syntax, allowing an implementation
to process any URI Template without knowing the scheme-specific
requirements of every possible resulting URI.
</t>
<t>
A URI Template may be provided in absolute form, as in the examples
above, or in relative form if a suitable base URI is defined.
</t>
<t>
Although the URI syntax is used for the result, the template
string is allowed to contain the broader set of characters
that can be found in IRI references <xref target="RFC3987"/>.
A URI Template is therefore also an IRI template, and the result
of template processing can be rendered as an IRI by transforming
the pct-encoded sequences to their corresponding Unicode
character if the character is not in the reserved set.
</t>
</section>
<section title="Expression Types" anchor="types">
<t>
URI Templates are similar to a macro language with a fixed set of
macro definitions: the expression type determines the expansion
process. For example, the following URI Template includes a
form-style parameter expression, as indicated by the "?" operator
appearing before the variable names.
</t>
<figure><artwork>
http://www.example.com/foo{?query,number}
</artwork></figure>
<t>
Each template expression describes, in a machine-readable manner,
how a URI is to be constructed. In this example,
the expansion process for templates beginning with the
question-mark ("?") operator follows the same pattern
as form-style interfaces on the World Wide Web.
</t>
<figure><artwork>
http://www.example.com/foo{?query,number}
\_____________/
|
|
For each defined variable in [ 'query', 'number' ],
substitute "?" if it is the first substitution or "&"
thereafter, followed by the variable name, '=', and the
variable's value.
</artwork></figure>
<t>
If the variables have the values
</t>
<figure><artwork>
query := "mycelium"
number := 100
</artwork></figure>
<t>
then the expansion of the above URI Template is
</t>
<figure><artwork>
http://www.example.com/foo?query=mycelium&number=100
</artwork></figure>
<t>
Alternatively, if 'query' is undefined, then the expansion
would be
</t>
<figure><artwork>
http://www.example.com/foo?number=100
</artwork></figure>
<t>
or if both variables are undefined, then it would be
</t>
<figure><artwork>
http://www.example.com/foo
</artwork></figure>
<t>
The following table summarizes each type of template
expression by its associated operator and cross-references
the section of this document that defines the operator and
its specific expansion process. The example expansions are
based on the following variables and values:
</t>
<figure><artwork>
var := "value";
hello := "Hello World!";
undef := null;
empty := "";
list := [ "val1", "val2", "val3" ];
keys := [("key1", "val1"), ("key2", "val2")];
path := "/foo/bar"
x := "1024";
y := "768";
.-----------------------------------------------------------------.
| Sec | Op | Description |
| | | Expression | Expansion |
|-----+-----+-----------------------------------------------------|
| 3.3 | | Simple expansion with comma-separated values |
| | | |
| | | {var} value |
| | | {hello} Hello%20World%21 |
| | | {path}/here %2Ffoo%2Fbar/here |
| | | {x,y} 1024,768 |
| | | {var=default} value |
| | | {undef=default} default |
| | | {list} val1,val2,val3 |
| | | {list*} val1,val2,val3 |
| | | {list+} list.val1,list.val2,list.val3 |
| | | {keys} key1,val1,key2,val2 |
| | | {keys*} key1,val1,key2,val2 |
| | | {keys+} keys.key1,val1,keys.key2,val2 |
| | | |
|-----+-----+-----------------------------------------------------|
| 3.4 | + | Reserved expansion with comma-separated values |
| | | |
| | | {+var} value |
| | | {+hello} Hello%20World! |
| | | {+path}/here /foo/bar/here |
| | | {+path,x}/here /foo/bar,1024/here |
| | | {+path}{x}/here /foo/bar1024/here |
| | | {+empty}/here /here |
| | | {+undef}/here /here |
| | | {+list} val1,val2,val3 |
| | | {+list*} val1,val2,val3 |
| | | {+list+} list.val1,list.val2,list.val3 |
| | | {+keys} key1,val1,key2,val2 |
| | | {+keys*} key1,val1,key2,val2 |
| | | {+keys+} keys.key1,val1,keys.key2,val2 |
| | | |
|-----+-----+-----------------------------------------------------|
| 3.5 | ; | Path-style parameters, semicolon-prefixed |
| | | |
| | | {;x,y} ;x=1024;y=768 |
| | | {;x,y,empty} ;x=1024;y=768;empty |
| | | {;x,y,undef} ;x=1024;y=768 |
| | | {;list} ;val1,val2,val3 |
| | | {;list*} ;val1;val2;val3 |
| | | {;list+} ;list=val1;list=val2;list=val3 |
| | | {;keys} ;key1,val1,key2,val2 |
| | | {;keys*} ;key1=val1;key2=val2 |
| | | {;keys+} ;keys.key1=val1;keys.key2=val2 |
| | | |
|-----+-----+-----------------------------------------------------|
| 3.6 | ? | Form-style parameters, ampersand-separated |
| | | |
| | | {?x,y} ?x=1024&y=768 |
| | | {?x,y,empty} ?x=1024&y=768&empty= |
| | | {?x,y,undef} ?x=1024&y=768 |
| | | {?list} ?list=val1,val2,val3 |
| | | {?list*} ?val1&val2&val3 |
| | | {?list+} ?list=val1&list=val2&list=val3 |
| | | {?keys} ?keys=key1,val1,key2,val2 |
| | | {?keys*} ?key1=val1&key2=val2 |
| | | {?keys+} ?keys.key1=val1&keys.key2=val2 |
| | | |
|-----+-----+-----------------------------------------------------|
| 3.7 | / | Hierarchical path segments, slash-separated |
| | | |
| | | {/var} /value |
| | | {/var,empty} /value/ |
| | | {/var,undef} /value |
| | | {/list} /val1,val2,val3 |
| | | {/list*} /val1/val2/val3 |
| | | {/list*,x} /val1/val2/val3/1024 |
| | | {/list+} /list.val1/list.val2/list.val3 |
| | | {/keys} /key1,val1,key2,val2 |
| | | {/keys*} /key1/val1/key2/val2 |
| | | {/keys+} /keys.key1/val1/keys.key2/val2 |
| | | |
|-----+-----+-----------------------------------------------------|
| 3.8 | . | Label expansion, dot-prefixed |
| | | |
| | | X{.var} X.value |
| | | X{.empty} X. |
| | | X{.undef} X |
| | | X{.list} X.val1,val2,val3 |
| | | X{.list*} X.val1.val2.val3 |
| | | X{.list*,x} X.val1.val2.val3.1024 |
| | | X{.list+} X.list.val1.list.val2.list.val3 |
| | | X{.keys} X.key1,val1,key2,val2 |
| | | X{.keys*} X.key1.val1.key2.val2 |
| | | X{.keys+} X.keys.key1.val1.keys.key2.val2 |
| | | |
`-----+-----+-----------------------------------------------------'
</artwork></figure>
</section>
<section title="Design Considerations" anchor="design">
<t>
The URI Template syntax has been designed to carefully balance
the need for a powerful expansion mechanism with the need for
ease of implementation. The syntax is designed to be trivial
to parse while at the same time providing enough flexibility to
express many common template scenarios. Implementations are able
to parse the template and perform the expansions in a single pass.
</t>
<t>
Templates are simple and readable when used with common
examples because the single-character operators match the URI
generic syntax delimiters. The operator's associated delimiter
(";", "?", "/", and ".") is omitted when none of the listed variables
are defined. Likewise, the expansion process for ";"
(path-style parameters) will omit the "=" when the variable value
is empty, whereas the process for "?" (form-style parameters)
will not omit the "=" when the value is empty. Multiple variables
and list values have their values joined with "," if there is no
predefined joining mechanism for the operator. Only one operator,
plus ("+"), will substitute unencoded reserved characters found
inside the variable values; the other operators will pct-encode
reserved characters found in the variable values prior to expansion.
</t>
<t>
The most common cases for URI spaces can be described with
simple URI Template expressions. If we were only concerned with
URI generation, then the template syntax could be limited to
just simple variable expansion, since more complex forms could
be generated by changing the variable values. However, URI
Templates have the additional goal of describing the layout of
identifiers in terms of preexisting data values. The template
syntax therefore includes operators that reflect how
resource identifiers are commonly allocated. Likewise, since
prefix and suffix substrings are often used to partition large
spaces of resources, modifiers on variable values provide a way
to specify those substrings.
</t>
<t>
Mechanisms similar to URI Templates have been defined within
several specifications, including WSDL, WADL and OpenSearch.
This specification extends and formally defines the syntax so
that URI Templates can be used consistently across multiple
Internet applications and within Internet message fields.
</t>
</section>
<section title="Limitations" anchor="limitations">
<t>
Since a URI Template describes a superset of the identifiers,
there is no implication that every possible expansion for
each delimited variable expression corresponds to a URI of an
existing resource. Our expectation is that an application
constructing URIs according to the template will be provided
with an appropriate set of values for the variables being
substituted and will be able to cope with any errors that might
occur when the resulting URI is used for name resolution or access.
</t>
<t>
URI Template expressions are not URIs: they do not identify
an abstract or physical resource, they are not parsed as URIs,
and should not be used in places where a URI would be expected
unless the template expressions will be expanded by a template
processor prior to use. Distinct field, element, or attribute
names should be used to differentiate protocol elements that
carry a URI Template from those that expect a URI-reference.
</t>
</section>
<section title="Notational Conventions" anchor="notation">
<t>
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL",
"SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED",
"MAY", and "OPTIONAL" in this document are to be
interpreted as described in <xref target="RFC2119"/>.
</t>
<t>
This specification uses the Augmented Backus-Naur Form (ABNF)
notation of <xref target="RFC5234"/>. The following ABNF rules
are imported from the normative references <xref target="RFC5234"/>,
<xref target="RFC3986"/>, and <xref target="RFC3987"/>.
</t>
<figure><artwork type="abnf">
ALPHA = %x41-5A / %x61-7A ; A-Z / a-z
DIGIT = %x30-39 ; 0-9
HEXDIG = DIGIT / "A" / "B" / "C" / "D" / "E" / "F"
pct-encoded = "%" HEXDIG HEXDIG
unreserved = ALPHA / DIGIT / "-" / "." / "_" / "~"
reserved = gen-delims / sub-delims
gen-delims = ":" / "/" / "?" / "#" / "[" / "]" / "@"
sub-delims = "!" / "$" / "&" / "'" / "(" / ")"
/ "*" / "+" / "," / ";" / "="
ucschar = %xA0-D7FF / %xF900-FDCF / %xFDF0-FFEF
/ %x10000-1FFFD / %x20000-2FFFD / %x30000-3FFFD
/ %x40000-4FFFD / %x50000-5FFFD / %x60000-6FFFD
/ %x70000-7FFFD / %x80000-8FFFD / %x90000-9FFFD
/ %xA0000-AFFFD / %xB0000-BFFFD / %xC0000-CFFFD
/ %xD0000-DFFFD / %xE1000-EFFFD
iprivate = %xE000-F8FF / %xF0000-FFFFD / %x100000-10FFFD
</artwork></figure>
<t>
This specification uses the terms "character" and "coded
character set" in accordance with the definitions provided
in <xref target="RFC2978"/>, and "character encoding" in
place of what <xref target="RFC2978"/> refers to as a "charset".
</t>
<t>
A URI Template is defined as a sequence of characters and therefore
has the same issues as URIs with regard to codepoints and character sets.
That is, URI Template characters are frequently encoded as octets
for transport or presentation. This specification does not mandate
any particular character encoding for mapping between URI Template
characters and the octets used to store or transmit those characters.
When a URI Template appears in a protocol element, the character
encoding is defined by that protocol; without such a definition,
a URI Template is assumed to be in the same character encoding as
the surrounding text.
</t>
<t>
A URI Template and its associated variable values are converted
to a normal form of UTF-8 <xref target="RFC3629"/> prior to
template processing, as defined in <xref target="unicode-normal"/>.
</t>
<t>
The ABNF notation defines its terminal values to be
non-negative integers (codepoints) that are a superset of the
US-ASCII coded character set <xref target="ASCII"/>. This
specification defines terminal values as codepoints within the
Unicode coded character set <xref target="UNIV4"/>. Thus, a string
of characters in a URI Template is assumed to be transformed into
its corresponding sequence of Unicode codepoints prior to testing
for a match with the URI Template grammar.
</t>
</section>
</section>
<section title="URI Template Syntax" anchor="syntax">
<t>
A URI Template is a string of printable Unicode characters
that contains zero or more embedded variable expressions, each
expression being delimited by a matching pair of braces ('{', '}').
</t>
<figure><artwork type="abnf">
URI-Template = *( literals / expression )
</artwork></figure>
<section title="Literals" anchor="literals">
<t>
The characters outside of expressions in a URI Template string
are intended to be translated literally to the URI-reference.
</t>
<figure><artwork type="abnf">
literals = %x21 / %x23-24 / %x26 / %x28-3B / %x3D / %x3F-5B
/ %x5D-5F / %x61-7A / %x7E / ucschar / iprivate
/ pct-encoded
; any Unicode character except: CTL, SP,
; DQUOTE, "'", "%" (aside from pct-encoded),
; "<", ">", "\", "^", "`", "{", "|", "}"
</artwork></figure>
</section>
<section title="Expressions" anchor="expressions">
<t>
Template expressions are the parameterized parts of a URI Template.
Each expression contains an optional operator, which defines the
expression type and its corresponding expansion process, followed by a
comma-separated list of variable specifiers (variable names and
optional value modifiers). If no operator is provided, the expression
defaults to simple variable expansion of unreserved values.
</t>
<figure><artwork type="abnf">
expression = "{" [ operator ] variable-list "}"
operator = "+" / "." / "/" / ";" / "?" / op-reserve
op-reserve = "|" / "!" / "@"
; reserved for local use: "$" / "(" / ")"
</artwork></figure>
<t>
</t>
<t>
The operator characters have been chosen to reflect each of their roles
as reserved characters in the URI generic syntax. The operators defined
by this specification include:
plus ("+") for substituting values that may contain reserved characters;
dot (".") for substituting values as a sequence of name labels prefixed
by ".";
slash ("/") for substituting values as a sequence of path segments
separated by "/";
semicolon (";") for substituting key=value pairs as path parameters
prefixed by ";"; and,
question-mark ("?") for substituting a query component beginning
with "?" and consisting of key=value pairs separated by "&".
These operators will be described in detail in <xref target="expansion"/>.
</t>
<t>
The operator characters pipe ("|"), exclamation ("!"), and at-sign ("@")
are reserved for future extensions. A processor that unexpectedly
encounters such an extension operator SHOULD pass the expression
through unexpanded and MAY also indicate a warning to the invoking
application.
</t>
<t>
The expression syntax specifically excludes use of the dollar ("$")
and parentheses ["(" and ")"] characters so that they remain
available for local language extensions outside the scope of this
specification.
</t>
</section>
<section title="Variables" anchor="variables">
<t>
After the operator (if any), each expression contains a list of
one or more comma-separated variable specifiers (varspec).
The variable names serve multiple purposes: documentation for
what kinds of values are expected, identifiers for associating
values within a URI Template processor, and the string to use
for each key on key=value expansions.
</t>
<figure><artwork type="abnf">
variable-list = varspec *( "," varspec )
varspec = varname [ modifier ] [ "=" default ]
varname = varchar *( varchar / "." )
varchar = ALPHA / DIGIT / "_" / ucschar / iprivate
/ pct-encoded
</artwork></figure>
<t>
An expression MAY reference variables that are unknown
to the template processor or whose value is set to a special
"undefined" value, such as undef or null. Such undefined
variables are given special treatment by the expansion process.
</t>
<t>
A variable value that is a string of length zero is not considered
undefined; it has the defined value of an empty string.
</t>
<t>
A variable may have a composite or structured value, such as a
list of values, an associative array of (key, value) pairs, or
a structure of components defined by some separate schema.
Such value types are not directly indicated by the template
syntax, but do have an impact on the expansion process. A
composite or structured value with zero member values is
considered undefined.
</t>
<t>
If a variable appears more than once in an expression or
within multiple expressions of a URI Template, the value of
that variable MUST remain static throughout the expansion
process (i.e., the variable must have the same value for the
purpose of calculating each expansion).
</t>
</section>
<section title="Value Modifiers" anchor="modifiers">
<t>
Any of the variables can have a modifier indicating that its
value is exploded into components or is limited to a prefix,
suffix, or the remainder of a prefix or suffix of the variable value.
</t>
<figure><artwork type="abnf">
modifier = explode / partial
</artwork></figure>
<section title="Component Values" anchor="component-values">
<t>
The explode modifiers ("*" and "+") indicate that the
variable represents a composite value that may be substituted
in full or partial forms, depending on the variable's type or
schema. Since URI Templates do not contain an indication of
type or schema, this is assumed to be determined by context.
An example context is a mark-up element or header field that
contains one attribute that is a template and one or more other
attributes that define the schema applicable to variables found
in the template. Likewise, a typed programming language might
differentiate variables as strings, lists, associative arrays,
or structures.
</t>
<figure><artwork type="abnf">
explode = ( "*" / "+" )
</artwork></figure>
<t>
The primary difference between the two explode modifiers is
that an asterisk ("*") indicates that just the component names
and values are included in the expansion, whereas the plus
("+") indicates that each component name is prefixed with the
given variable name and a period ("."), thereby enabling multiple
variables with the same component names to be disambiguated.
</t>
<t>
Component modifiers improve brevity in the URI Template syntax.
For example, a resource that provides a geographic map for a
given street address might accept a hundred permutations on
fields for address input, including partial addresses (e.g.,
just the city or postal code). Such a resource could be
described as a template with each and every address component
listed in order, or with a far more simple template that makes
use of an explode modifier, as in
</t>
<figure><artwork>
/mapper{?address*}
</artwork></figure>
<t>
or
</t>
<figure><artwork>
/directions{?from+,to+}
</artwork></figure>
<t>
along with some context that defines each variable (address, from,
and to) as adhering to a given addressing standard (e.g., UPU S42
or AS/NZS 4819:2003). A recipient aware of the schema can then
provide appropriate expansions, such as:
</t>
<figure><artwork>
/mapper?city=Newport%20Beach&state=CA
/directions?from.zipcode=92660&to.zipcode=90210
</artwork></figure>
<t>
The expansion process for variables, as defined in
<xref target="expansion"/>, is dependent on both the
operator being used and, if one of the explode modifiers is
used, the type and schema of the variable being substituted.
</t>
</section>
<section title="Prefix and Suffix Values" anchor="partial-values">
<t>
Prefix and suffix modifiers are often used to partition an identifier
space hierarchically, as is common in reference indices and hash-based
storage, or to limit the substituted value to a maximum number of
characters.
</t>
<figure><artwork type="abnf">
partial = ( substring / remainder ) offset
substring = ":"
remainder = "^"
offset = [ from-end ] 1*DIGIT
from-end = "-"
</artwork></figure>
<t>
The offset refers to a maximum number of characters from either
the beginning (prefix) or end (suffix) of the variable's value as
a Unicode string. Note that this numbering is in characters, not
octets, in order to avoid substituting improperly encoded values
due to splitting a multi-octet UTF-8 encoded character or a
pct-encoded triplet.
</t>
<t>
A substring modifier requires that only the indicated prefix or
suffix be used in the expansion. A remainder modifier requires
that only the remainder of the value, excluding the indicated prefix
or suffix, be used in the expansion. If the offset is greater than
the length of the variable's value, then the entire string is used
for a substring and the empty string is used for a remainder.
</t>
<t>
The following examples illustrate how modifiers work with the
different variable types. More complex examples are provided
in <xref target="examples"/>.
</t>
<figure><artwork>
Given the variable assignments:
var := "value";
name := [ "Fred", "Wilma", "Pebbles" ];
Example Template Expansion
{var} value
{var:20} value
{var:3} val
{var^3} ue
{var:-3} lue
{var^-3} va
{?name} ?name=Fred,Wilma,Pebbles
{?name:1} ?name=F
</artwork></figure>
</section>
</section>
<section title="Value Defaults" anchor="defaults">
<t>
Any of the variables may also be supplied with a default value
to be used when a template processor determines the variable to be
undefined. The default value is limited to the unreserved and
pct-encoded characters of a URI-reference, since our intention
is for the default to be presented in the exact form that it would
appear in the resulting URI. The default is not affected by the
variable modifiers; it is assumed that the default string
provided in the expression already reflects any necessary
substring or remainder processing.
</t>
<figure><artwork type="abnf">
default = *( unreserved / pct-encoded )
</artwork></figure>
<t>
The following examples illustrate how default values work with
different variable types. More complex examples are provided
in <xref target="examples"/>.
</t>
<figure><artwork>
Given the variable assignments:
var := "value";
empty := "";
undef := null;
name := [ "Fred", "Wilma", "Pebbles" ];
favs := [("color","red"), ("volume","high")];
empty_list := [];
empty_keys := [];
Example Template Expansion
{var=default} value
{undef=default} default
x{empty}y xy
x{empty=_}y xy
x{undef}y xy
x{undef=_}y x_y
x{empty_list}y xy
x{empty_list=_}y xy
x{empty_list*}y xy
x{empty_list*=_}y x_y
x{empty_list+}y xy
x{empty_list+=_}y xempty_list._y
x{empty_keys}y xy
x{empty_keys=_}y xy
x{empty_keys*}y xy
x{empty_keys*=_}y x_y
x{empty_keys+}y xy
x{empty_keys+=_}y xempty_keys._y
x{?name=none} x?name=Fred,Wilma,Pebbles
x{?favs=none} x?favs=color,red,volume,high
x{?favs*=none} x?color=red&volume=high
x{?favs+=none} x?favs.color=red&favs.volume=high
x{?undef} x
x{?undef=none} x?undef=none
x{?empty} x?empty=
x{?empty=none} x?empty=
x{?empty_list} x
x{?empty_list=none} x?empty_list=none
x{?empty_list*} x
x{?empty_list*=none} x?none
x{?empty_list+} x
x{?empty_list+=none} x?empty_list.none
x{?empty_keys} x
x{?empty_keys=none} x?empty_keys=none
x{?empty_keys*} x
x{?empty_keys*=none} x?none
x{?empty_keys+} x
x{?empty_keys+=none} x?empty_keys.none
x{;name=none} x;name=Fred,Wilma,Pebbles
x{;favs=none} x;favs=color,red,volume,high
x{;favs*=none} x;color=red;volume=high
x{;favs+=none} x;favs.color=red;favs.volume=high
x{;undef} x
x{;undef=none} x;undef=none
x{;empty} x;empty
x{;empty=none} x;empty
x{;empty_list} x
x{;empty_list=none} x;empty_list=none
x{;empty_list*} x
x{;empty_list*=none} x;none
x{;empty_list+} x
x{;empty_list+=none} x;empty_list.none
x{;empty_keys} x
x{;empty_keys=none} x;empty_keys=none
x{;empty_keys*} x
x{;empty_keys*=none} x;none
x{;empty_keys+} x
x{;empty_keys+=none} x;empty_keys.none
x{/name=none} x/Fred,Wilma,Pebbles
x{/name*=none} x/Fred/Wilma/Pebbles
x{/name+=none} x/name.Fred/name.Wilma/name.Pebbles
x{/favs=none} x/color,red,volume,high
x{/favs*=none} x/color/red/volume/high
x{/favs+=none} x/favs.color/red/favs.volume/high
x{/undef} x
x{/undef=none} x/none
x{/empty} x/
x{/empty=none} x/
x{/empty_list} x
x{/empty_list=none} x/none
x{/empty_list*} x
x{/empty_list*=none} x/none
x{/empty_list+} x
x{/empty_list+=none} x/empty_list.none
x{/empty_keys} x
x{/empty_keys=none} x/none
x{/empty_keys*} x
x{/empty_keys*=none} x/none
x{/empty_keys+} x
x{/empty_keys+=none} x/empty_keys.none
</artwork></figure>
</section>
</section>
<section title="URI Template Expansion" anchor="expansion">
<t>
The process of URI Template expansion is to scan the template
string from beginning to end, copying literal characters as-is
and replacing each expression with the result of applying the
expression's operator to the value of each variable named in
the expression.
</t>
<t>
If a template processor encounters an error, such as an operator
that it does not understand or a character sequence that does not
match the <URI-Template> grammar, then processing of the
template SHOULD cease, the URI-reference result SHOULD be undefined,
and the location and type of error SHOULD be indicated to the
invoking application.
</t>
<t>
If a template processor encounters a warning, such as the use of
an operator character reserved for future extension, then the
processing of the template SHOULD NOT cease, and the location
and type of warning SHOULD be indicated to the invoking application.
</t>
<section title='Unicode normalization' anchor="unicode-normal">
<t>
The Unicode Standard <xref target="UNIV4"/> defines various
equivalences between sequences of characters for various
purposes. Unicode Standard Annex #15 <xref target="UTR15"/> defines
various Normalization Forms for these equivalences, in
particular Normalization Form KC (NFKC: Compatibility
Decomposition followed by Canonical Composition). Since
different Normalized Form unicode strings will have
different UTF-8 representations, the only way to guarantee
that template processors will produce the same URI is to
require a common Normalized Form.
</t>
<t>
The string values for the URI Template and template variables
MUST be in NFKC and encoded as UTF-8 <xref target="RFC3629"/>
prior to use in the template expansion process (US-ASCII is a
proper subset of NFKC UTF-8). The remaining sections defining
the expansion process assume strings are in NFKC UTF-8.
</t>
</section>
<section title='Literal expansion' anchor="literal-expansion">
<t>
If the literal character is allowed anywhere in the URI
syntax (unreserved / reserved), then it is copied directly
to the result string. Otherwise, the pct-encoded equivalent
of the literal character is copied to the result string by
encoding the character in UTF-8 (a sequence of octets) and
then encoding each octet as a pct-encoded triplet.
</t>
</section>
<section title='Expression expansion' anchor="expression-expansion">
<t>
Each expression is indicated by an opening brace ("{") character
and continues until the next closing brace ("}"). The expression
is expanded by determining the expression type and then following
that type's expansion process for each comma-separated varspec
in the expression.
</t>
<t>
The expression type is determined by looking at the first
character after the opening brace. If the character is an
operator, then remember the expression type associated with
that operator for later expansion decisions and skip to the
next character for the varspec list. If the first character
is not an operator, then the expression type is simple
expansion and the first character is the beginning of the
varspec list.
</t>
<t>
If the expression does not contain any varspec, as in
"{}" or "{,}", then a template processor SHOULD copy that
invalid expression to the result string, continue processing
the remainder of the template, and indicate that an error
occurred to the caller.
</t>
<t>
If the template contains an opening brace without a corresponding
closing brace (the template ends in mid-expression), then a
processor SHOULD attempt to process the template as if it ended
in a closing brace and indicate that an error occurred to the caller.
</t>
</section>
<section title='Variable and modifier expansion' anchor="variable-expansion">
<t>
A variable that is undefined has no value and thus is
excluded from the expansion. A variable defined as composite
or component values is undefined if it contains zero members or
all of its components are undefined. If all of the variables are
undefined, then the expression's expansion is the empty string.
</t>
<t>
A variable defined as a single value is expanded by converting its
value to a NFKC UTF-8 string, replacing any character within
the string that is not in the unreserved set with its
corresponding sequence of pct-encoded octets, applying any prefix
or suffix modifier (<xref target="partial-values"/>), and then
appending the result to the URI-reference.
</t>
<t>
A variable defined as a list of values is substituted as a
string of comma-separated single values when no explode modifier
is given. If the "*" modifier is used, then each value is
separated by the default delimiter for the expression type.
If the "+" operator is used, then the variable name is
prepended to the expansion list as if it were the initial
value in the list. If a partial modifier is indicated,
the modifier is applied to the combined string of values.
The list expansion is then appended to the result string.
</t>
<t>
A variable defined as an associative array is expanded as a list
of alternating key and value pairs, excluding any keys for which
the corresponding value is undefined. If no explode modifier is
used, then the list is substituted as comma-separated single
values. If the "*" modifier is given, then the list is delimited
as key=value pairs according to the default delimiters defined
by the expression type. If the "+" modifier is used, the values
are substituted as in the "*" case, except that each key name is
prefixed by the variable name and a ".", as in "name.key=value".
</t>
<t>
When a variable containing component values is given without an
explode modifier, the value of each defined component is
substituted, separated by a comma (",") character, in the order
indicated by the variable's schema or, if the schema is unknown,
in the order provided by the variable's value. A structure of
component values is expanded as a list of the component values
in the order implied by a preorder (depth-first) traversal of that
structure, excluding any components that are undefined.
</t>
<t>
When an explode modifier is used with an operator that substitutes
variables as key=value pairs, the key is determined as follows.
If the modifier is an asterisk ("*"), then each "key" is the name
of the component. If the modifier is a plus ("+"), then each key
is the variable name followed by a period (".") and the component
name. In both cases, if the component names have a hierarchical
structure, then the component subnames are also appended to the
key, each separated by a period.
</t>
<t>
When an explode modifier is used with the hierarchical ("/")
operator, the slash delimiter is substituted before each
defined component's value if the modifier is "*", or before each
conjunction of component name and value (e.g., "name.value") if
the modifier is "+".
</t>
</section>
<section title='Simple expansion: {var}' anchor="op-default">
<t>
The default expression type when no operator is given is
simple expansion: the value of each defined variable is
substituted in the order given, modified as indicated by the
optional modifiers, with each value separated by a comma
character (","). A variable that is undefined has no value
and thus is excluded from the expansion. If all of the
variables are undefined, then the expansion is the empty string.
</t>
<figure><preamble>For example,</preamble><artwork>
foo := "fred"
"{foo}" -> "fred"
"{foo,foo}" -> "fred,fred"
"{bar,foo}" -> "fred"
"{bar=wilma}" -> "wilma"
</artwork></figure>
</section>
<section title='Reserved expansion: {+var}' anchor="op-plus">
<t>
Reserved expansion is identical to simple expansion except that the
substituted values may contain characters in the reserved set.
</t>
<figure><preamble>For example,</preamble><artwork>
foo := "That's right!"
"{foo}" -> "That%27s%20right%21"
"{+foo}" -> "That%27s%20right!"
base := "http://example.com/home/"
"{base}index" -> "http%3A%2F%2Fexample.com%2Fhome%2Findex"
"{+base}index" -> "http://example.com/home/index"
</artwork></figure>
<t>
The same expansion process is followed as in <xref target="op-default"/>
except that, instead of replacing any character within each value
string that is not in the unreserved set with its corresponding
sequence of pct-encoded octets, replace any character within each
value string that is not in the set of unreserved or reserved
characters with its corresponding sequence of pct-encoded octets.
</t>
</section>
<section title='Path-style parameter expansion: {;var}' anchor="op-semicolon">
<t>
TBD.
</t>
</section>
<section title='Form-style parameter expansion: {?var}' anchor="op-question">
<t>
TBD.
</t>
</section>
<section title='Hierarchical path expansion: {/var}' anchor="op-slash">
<t>
TBD.
</t>
</section>
<section title="Label expansion with dot-prefix: {.var}" anchor="op-dot">
<t>
TBD.
</t>
</section>
</section>
<section title="Examples" anchor="examples">
<t>
TBD.
</t>
</section>
<section title="Security Considerations">
<t>
A URI Template does not contain active or executable content. Other
security considerations are the same as those for URIs, see section 7
of <xref target="RFC3986"/>.
</t>
</section>
<section title="IANA Considerations" anchor="iana">
<t>
No IANA actions are required by this document.
</t>
</section>
<section title="Acknowledgments" anchor="ack">
<t>
The following people made significant contributions to this
specification: Mike Burrows, Michaeljohn Clement, DeWitt Clinton,
John Cowan, James H. Manger, and James Snell.
</t>
</section>
</middle>
<back>
<references title='Normative References'>
&ASCII;
&UNIV4;
&UTR15;
&rfc2119;
&rfc2978;
&rfc3986;
&rfc3987;
&rfc3629;
&rfc5234;
</references>
<section title="Example URI Template Parser" anchor="parser">
<t>
Parsing a valid URI Template expression does not require
building a parser from the given ABNF. Instead, the set of allowed
characters in each part of URI Template expression has been
chosen to avoid complex parsing, and breaking an expression into
its component parts can be achieved by a series of splits
of the character string.
</t>
<t>
Here is example Python code that parses a URI Template
expression and returns the operator, argument, and
variables as a tuple. The variables are returned
as a dictionary of variable names mapped to their
default values. If no default is given then the
name maps to None.
</t>
<t>
TBD.
</t>
</section>
<section title="Revision History (to be removed by RFC Editor)" anchor="revisions">
<t>04 - Changed the operator syntax to a single character that is
analogous to its reserved role within the URI generic syntax,
resulting in templates that are far more readable for the common
cases. Added value modifiers for prefix and suffix expansion.
Added explode modifiers to allow expansion of complex variables
and lists according to (external) variable types or schema.
Replaced use of "expansion" with "expression", since expansion
is traditionally used to refer to the result after expanding
a macro (not the macro itself). Made applicable to any hypertext
reference string, such that the process for template expansion
also includes transforming the surrounding string into a proper
URI-reference rather than assuming it is already in absolute URI form.
Rewrote the text accordingly.</t>
<t>03 - Added more examples. Introduced error conditions and defined
their handling. Changed listjoin to list. Changed -append to -suffix,
and allowed -prefix and -suffix to accept list variables. Clarified
the handling of unicode.</t>
<t>02 - Added operators and came up with coherent
percent-encoding and reserved character story.
Added large examples section which is extracted
and tested against the implementation.
</t>
<t>01
</t>
<t>00 - Initial Revision.
</t>
</section>
</back>
</rfc>
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