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draft-gregorio-uritemplate-04.txt
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Network Working Group J. Gregorio, Ed.
Internet-Draft Google
Intended status: Standards Track R. Fielding, Ed.
Expires: September 9, 2010 Day Software
M. Hadley, Ed.
Oracle
M. Nottingham, Ed.
D. Orchard
Mar 08, 2010
URI Template
draft-gregorio-uritemplate-04
Abstract
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.
Editorial Note (to be removed by RFC Editor)
To provide feedback on this Internet-Draft, join the W3C URI mailing
list (http://lists.w3.org/Archives/Public/uri/) [1].
Status of this Memo
This Internet-Draft is submitted to IETF in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet-
Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt.
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
Gregorio, et al. Expires September 9, 2010 [Page 1]
Internet-Draft URI Template Mar 2010
This Internet-Draft will expire on September 9, 2010.
Copyright Notice
Copyright (c) 2010 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the BSD License.
This document may contain material from IETF Documents or IETF
Contributions published or made publicly available before November
10, 2008. The person(s) controlling the copyright in some of this
material may not have granted the IETF Trust the right to allow
modifications of such material outside the IETF Standards Process.
Without obtaining an adequate license from the person(s) controlling
the copyright in such materials, this document may not be modified
outside the IETF Standards Process, and derivative works of it may
not be created outside the IETF Standards Process, except to format
it for publication as an RFC or to translate it into languages other
than English.
Gregorio, et al. Expires September 9, 2010 [Page 2]
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.2. Expression Types . . . . . . . . . . . . . . . . . . . . . 5
1.3. Design Considerations . . . . . . . . . . . . . . . . . . 8
1.4. Limitations . . . . . . . . . . . . . . . . . . . . . . . 9
1.5. Notational Conventions . . . . . . . . . . . . . . . . . . 9
2. URI Template Syntax . . . . . . . . . . . . . . . . . . . . . 11
2.1. Literals . . . . . . . . . . . . . . . . . . . . . . . . . 11
2.2. Expressions . . . . . . . . . . . . . . . . . . . . . . . 11
2.3. Variables . . . . . . . . . . . . . . . . . . . . . . . . 12
2.4. Value Modifiers . . . . . . . . . . . . . . . . . . . . . 13
2.4.1. Component Values . . . . . . . . . . . . . . . . . . . 13
2.4.2. Prefix and Suffix Values . . . . . . . . . . . . . . . 14
2.5. Value Defaults . . . . . . . . . . . . . . . . . . . . . . 15
3. URI Template Expansion . . . . . . . . . . . . . . . . . . . . 18
3.1. Unicode normalization . . . . . . . . . . . . . . . . . . 18
3.2. Literal expansion . . . . . . . . . . . . . . . . . . . . 18
3.3. Expression expansion . . . . . . . . . . . . . . . . . . . 19
3.4. Variable and modifier expansion . . . . . . . . . . . . . 19
3.5. Simple expansion: {var} . . . . . . . . . . . . . . . . . 20
3.6. Reserved expansion: {+var} . . . . . . . . . . . . . . . . 21
3.7. Path-style parameter expansion: {;var} . . . . . . . . . . 21
3.8. Form-style parameter expansion: {?var} . . . . . . . . . . 21
3.9. Hierarchical path expansion: {/var} . . . . . . . . . . . 21
3.10. Label expansion with dot-prefix: {.var} . . . . . . . . . 22
4. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
5. Security Considerations . . . . . . . . . . . . . . . . . . . 22
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 22
7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 22
8. Normative References . . . . . . . . . . . . . . . . . . . . . 22
Appendix A. Example URI Template Parser . . . . . . . . . . . . . 23
Appendix B. Revision History (to be removed by RFC Editor) . . . 23
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 24
Gregorio, et al. Expires September 9, 2010 [Page 3]
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1. Introduction
1.1. Overview
A Uniform Resource Identifier (URI) [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
http://example.com/~fred/
http://example.com/~mark/
or a set of dictionary entries might be grouped in a hierarchy by the
first letter of the term, as in
http://example.com/dictionary/c/cat
http://example.com/dictionary/d/dog
or a service interface might be invoked with various user input in a
common pattern, as in
http://example.com/search?q=cat&lang=en
http://example.com/search?q=dog&lang=fr
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:
http://example.com/~{username}/
http://example.com/dictionary/{term:1}/{term}
http://example.com/search{?q,lang}
We define the following terms:
o expression - The text between '{' and '}', including the enclosing
braces, as defined in Section 2.
o 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 Section 3.
o 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.
Gregorio, et al. Expires September 9, 2010 [Page 4]
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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.
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.
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 [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.
1.2. Expression Types
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.
http://www.example.com/foo{?query,number}
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.
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.
If the variables have the values
query := "mycelium"
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number := 100
then the expansion of the above URI Template is
http://www.example.com/foo?query=mycelium&number=100
Alternatively, if 'query' is undefined, then the expansion would be
http://www.example.com/foo?number=100
or if both variables are undefined, then it would be
http://www.example.com/foo
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:
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 |
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| | | |
|-----+-----+-----------------------------------------------------|
| 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 |
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| | | {/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 |
| | | |
`-----+-----+-----------------------------------------------------'
1.3. Design Considerations
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.
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.
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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.
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.
1.4. Limitations
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.
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.
1.5. Notational Conventions
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 [RFC2119].
This specification uses the Augmented Backus-Naur Form (ABNF)
notation of [RFC5234]. The following ABNF rules are imported from
the normative references [RFC5234], [RFC3986], and [RFC3987].
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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
This specification uses the terms "character" and "coded character
set" in accordance with the definitions provided in [RFC2978], and
"character encoding" in place of what [RFC2978] refers to as a
"charset".
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.
A URI Template and its associated variable values are converted to a
normal form of UTF-8 [RFC3629] prior to template processing, as
defined in Section 3.1.
The ABNF notation defines its terminal values to be non-negative
integers (codepoints) that are a superset of the US-ASCII coded
character set [ASCII]. This specification defines terminal values as
codepoints within the Unicode coded character set [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.
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2. URI Template Syntax
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 ('{', '}').
URI-Template = *( literals / expression )
2.1. Literals
The characters outside of expressions in a URI Template string are
intended to be translated literally to the URI-reference.
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),
; "<", ">", "\", "^", "`", "{", "|", "}"
2.2. Expressions
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.
expression = "{" [ operator ] variable-list "}"
operator = "+" / "." / "/" / ";" / "?" / op-reserve
op-reserve = "|" / "!" / "@"
; reserved for local use: "$" / "(" / ")"
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 Section 3.
The operator characters pipe ("|"), exclamation ("!"), and at-sign
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("@") 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.
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.
2.3. Variables
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.
variable-list = varspec *( "," varspec )
varspec = varname [ modifier ] [ "=" default ]
varname = varchar *( varchar / "." )
varchar = ALPHA / DIGIT / "_" / ucschar / iprivate
/ pct-encoded
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.
A variable value that is a string of length zero is not considered
undefined; it has the defined value of an empty string.
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.
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).
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2.4. Value Modifiers
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.
modifier = explode / partial
2.4.1. Component Values
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.
explode = ( "*" / "+" )
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.
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
/mapper{?address*}
or
/directions{?from+,to+}
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:
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/mapper?city=Newport%20Beach&state=CA
/directions?from.zipcode=92660&to.zipcode=90210
The expansion process for variables, as defined in Section 3, 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.
2.4.2. Prefix and Suffix Values
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.
partial = ( substring / remainder ) offset
substring = ":"
remainder = "^"
offset = [ from-end ] 1*DIGIT
from-end = "-"
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.
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.
The following examples illustrate how modifiers work with the
different variable types. More complex examples are provided in
Section 4.
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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
2.5. Value Defaults
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.
default = *( unreserved / pct-encoded )
The following examples illustrate how default values work with
different variable types. More complex examples are provided in
Section 4.
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
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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
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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
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3. URI Template Expansion
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.
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.
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.
3.1. Unicode normalization
The Unicode Standard [UNIV4] defines various equivalences between
sequences of characters for various purposes. Unicode Standard Annex
#15 [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.
The string values for the URI Template and template variables MUST be
in NFKC and encoded as UTF-8 [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.
3.2. Literal expansion
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.