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Add back SPDX license expressions to version 3 branch
Includes the changes from PR #839 adding AdditionRef and also PR #857 correcting a typo Signed-off-by: Gary O'Neall <[email protected]>
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| # Annex D SPDX license expressions (Normative) | ||
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| ## D.1 Overview <a name="D.1"></a> | ||
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| Often a single license can be used to represent the licensing terms of a source code or binary file, but there are situations where a single license identifier is not sufficient. A common example is when software is offered under a choice of one or more licenses (e.g., GPL-2.0-only OR BSD-3-Clause). Another example is when a set of licenses is needed to represent a binary program constructed by compiling and linking two (or more) different source files each governed by different licenses (e.g., LGPL-2.1-only AND BSD-3-Clause). | ||
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| SPDX License Expressions provide a way for one to construct expressions that more accurately represent the licensing terms typically found in open source software source code. A license expression could be a single license identifier found on the SPDX License List; a user defined license reference denoted by the LicenseRef-`[idString]`; a license identifier combined with an SPDX exception; or some combination of license identifiers, license references and exceptions constructed using a small set of defined operators (e.g., `AND`, `OR`, `WITH` and `+`). We provide the definition of what constitutes a valid SPDX License Expression in this section. | ||
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| The exact syntax of license expressions is described below in [ABNF](http://tools.ietf.org/html/rfc5234). | ||
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| ```text | ||
| idstring = 1*(ALPHA / DIGIT / "-" / "." ) | ||
| license-id = <short form license identifier in Annex A.1> | ||
| license-exception-id = <short form license exception identifier in Annex A.2> | ||
| license-ref = ["DocumentRef-"(idstring)":"]"LicenseRef-"(idstring) | ||
| addition-ref = ["DocumentRef-"(idstring)":"]"AdditionRef-"(idstring) | ||
| simple-expression = license-id / license-id"+" / license-ref | ||
| addition-expression = license-exception-id / addition-ref | ||
| compound-expression = (simple-expression / | ||
| simple-expression "WITH" addition-expression / | ||
| compound-expression "AND" compound-expression / | ||
| compound-expression "OR" compound-expression / | ||
| "(" compound-expression ")" ) | ||
| license-expression = (simple-expression / compound-expression) | ||
| ``` | ||
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| In the following sections we describe in more detail `<license-expression>` construct, a licensing expression string that enables a more accurate representation of the licensing terms of modern-day software. | ||
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| A valid `<license-expression>` string consists of either: | ||
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| (i) a simple license expression, such as a single license identifier; or | ||
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| (ii) a more complex expression constructed by combining smaller valid expressions using Boolean license operators. | ||
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| There MUST NOT be white space between a license-id and any following `+`. This supports easy parsing and backwards compatibility. There MUST be white space on either side of the operator "WITH". There MUST be white space and/or parentheses on either side of the operators `AND` and `OR`. | ||
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| In the `tag:value` format, a license expression MUST be on a single line, and MUST NOT include a line break in the middle of the expression. | ||
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| ## D.2 Case sensitivity <a name="D.2"></a> | ||
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| License expression operators (`AND`, `OR` and `WITH`) should be matched in a *case-sensitive* manner. | ||
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| License identifiers (including license exception identifiers) used in SPDX documents or source code files should be matched in a *case-insensitive* manner. In other words, `MIT`, `Mit` and `mIt` should all be treated as the same identifier and referring to the same license. | ||
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| However, please be aware that it is often important to match with the case of the canonical identifier on the [SPDX License List](https://spdx.org/licenses). This is because the canonical identifier's case is used in the URL of the license's or exception's entry on the List, and because the canonical identifier is translated to a URI in RDF documents. | ||
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| ## D.3 Simple license expressions <a name="D.3"></a> | ||
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| A simple `<license-expression>` is composed one of the following: | ||
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| * An SPDX License List Short Form Identifier. For example: CDDL-1.0 | ||
| * An SPDX License List Short Form Identifier with a unary "+" operator suffix to represent the current version of the license or any later version. For example: CDDL-1.0+ | ||
| * An SPDX user defined license reference: ["DocumentRef-"1\*(idstring)":"]"LicenseRef-"1*(idstring) | ||
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| Some examples: | ||
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| ```text | ||
| LicenseRef-23 | ||
| LicenseRef-MIT-Style-1 | ||
| DocumentRef-spdx-tool-1.2:LicenseRef-MIT-Style-2 | ||
| ``` | ||
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| The current set of valid license identifiers can be found in Annex [A.1](SPDX-license-list.md#A.1). For the most up to date set of exceptions please see [spdx.org/licenses](https:/ | ||
| /spdx.org/licenses). | ||
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| ## D.4 Composite license expressions <a name="D.4"></a> | ||
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| ### D.4.1 Introduction <a name="D.4.1"></a> | ||
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| More expressive composite license expressions can be constructed using "OR", "AND", and "WITH" operators similar to constructing mathematical expressions using arithmetic operators. | ||
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| For the `tag:value` format, any license expression that consists of more than one license identifier and/or LicenseRef, may optionally be encapsulated by parentheses: "( )". | ||
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| Nested parentheses can also be used to specify an order of precedence which is discussed in more detail in [D.4.5](#D.4.5). | ||
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| ### D.4.2 Disjunctive "OR" operator <a name="D.4.2"></a> | ||
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| If presented with a choice between two or more licenses, use the disjunctive binary "OR" operator to construct a new license expression, where both the left and right operands are valid license expression values. | ||
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| For example, when given a choice between the LGPL-2.1-only or MIT licenses, a valid expression would be: | ||
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| ```text | ||
| LGPL-2.1-only OR MIT | ||
| ``` | ||
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| The "OR" operator is commutative, meaning that the above expression should be considered equivalent to: | ||
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| ```text | ||
| MIT OR LGPL-2.1-only | ||
| ``` | ||
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| An example representing a choice between three different licenses would be: | ||
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| ```text | ||
| LGPL-2.1-only OR MIT OR BSD-3-Clause | ||
| ``` | ||
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| ### D.4.3 Conjunctive "AND" operator <a name="D.4.3"></a> | ||
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| If required to simultaneously comply with two or more licenses, use the conjunctive binary "AND" operator to construct a new license expression, where both the left and right operands are a valid license expression values. | ||
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| For example, when one is required to comply with both the LGPL-2.1-only or MIT licenses, a valid expression would be: | ||
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| ```text | ||
| LGPL-2.1-only AND MIT | ||
| ``` | ||
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| The "AND" operator is commutative, meaning that the above expression should be considered equivalent to: | ||
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| ```text | ||
| MIT AND LGPL-2.1-only | ||
| ``` | ||
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| An example where all three different licenses apply would be: | ||
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| ```text | ||
| LGPL-2.1-only AND MIT AND BSD-2-Clause | ||
| ``` | ||
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| ### D.4.4 Additive "WITH" operator <a name="D.4.4"></a> | ||
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| Sometimes license texts are found with additional text, which might or might not modify the original license terms. | ||
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| In this case, use the binary "WITH" operator to construct a new license expression to represent the special situation. A valid `<license-expression>` is where the left operand is a `<simple-expression>` value and the right operand is a `<addition-expression>` that represents the additional text. | ||
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| The `<addition-expression>` can be either a `<license-exception-id>` from the SPDX License List, or a user defined addition reference in the form ["DocumentRef-"(idstring)":"]"AdditonRef-"(idstring) | ||
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| For example, when the Bison exception is to be applied to GPL-2.0-or-later, the expression would be: | ||
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| ```text | ||
| GPL-2.0-or-later WITH Bison-exception-2.2 | ||
| ``` | ||
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| The current set of valid license exceptions identifiers can be found in Annex [A.2](SPDX-license-list.md#A.2). For the most up to date set of exceptions please see [spdx.org/licenses](https://spdx.org/licenses). | ||
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| ### D.4.5 Order of precedence and parentheses <a name="D.4.5"></a> | ||
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| The order of application of the operators in an expression matters (similar to mathematical operators). The default operator order of precedence of a `<license-expression>` a is: | ||
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| ```text | ||
| + | ||
| WITH | ||
| AND | ||
| OR | ||
| ``` | ||
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| where a lower order operator is applied before a higher order operator. | ||
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| For example, the following expression: | ||
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| ```text | ||
| LGPL-2.1-only OR BSD-3-Clause AND MIT | ||
| ``` | ||
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| represents a license choice between either LGPL-2.1-only and the expression BSD-3-Clause AND MIT because the AND operator takes precedence over (is applied before) the OR operator. | ||
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| When required to express an order of precedence that is different from the default order a `<license-expression>` can be encapsulated in pairs of parentheses: ( ), to indicate that the operators found inside the parentheses takes precedence over operators outside. This is also similar to the use of parentheses in an algebraic expression e.g., (5+7)/2. | ||
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| For instance, the following expression: | ||
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| ```text | ||
| MIT AND (LGPL-2.1-or-later OR BSD-3-Clause) | ||
| ``` | ||
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| states the OR operator should be applied before the AND operator. That is, one should first select between the LGPL-2.1-or-later or the BSD-3-Clause license before applying the MIT license. | ||
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| ### D.4.6 License expressions in RDF <a name="D.4.6"></a> | ||
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| A conjunctive license can be expressed in RDF via a `<spdx:ConjunctiveLicenseSet>` element, with an spdx:member property for each element in the conjunctive license. Two or more members are required. | ||
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| ```text | ||
| <spdx:ConjunctiveLicenseSet> | ||
| <spdx:member rdf:resource="http://spdx.org/licenses/GPL-2.0-only"/> | ||
| <spdx:ExtractedLicensingInfo rdf:about | ||
| ="http://example.org#LicenseRef-EternalSurrender"> | ||
| <spdx:extractedText> | ||
| In exchange for using this software, you agree to give | ||
| its author all your worldly possessions. You will not | ||
| hold the author liable for all the damage this software | ||
| will inevitably cause not only to your person and | ||
| property, but to the entire fabric of the cosmos. | ||
| </spdx:extractedText> | ||
| <spdx:licenseId>LicenseRef-EternalSurrender</spdx:licenseId> | ||
| </spdx:ExtractedLicensingInfo> | ||
| </spdx:ConjunctiveLicenseSet> | ||
| ``` | ||
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| A disjunctive license can be expressed in RDF via a `<spdx:DisjunctiveLicenseSet>` element, with an spdx:member property for each element in the disjunctive license. Two or more members are required. | ||
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| ```text | ||
| <spdx:DisjunctiveLicenseSet> | ||
| <spdx:member rdf:resource="http://spdx.org/licenses/GPL-2.0-only"/> | ||
| <spdx:member> | ||
| <spdx:ExtractedLicensingInfo rdf:about | ||
| ="http://example.org#LicenseRef-EternalSurrender"> | ||
| <spdx:extractedText> | ||
| In exchange for using this software, you agree to | ||
| give its author all your worldly possessions. You | ||
| will not hold the author liable for all the damage | ||
| this software will inevitably cause not only to | ||
| your person and property, but to the entire fabric | ||
| of the cosmos. | ||
| </spdx:extractedText> | ||
| <spdx:licenseId>LicenseRef-EternalSurrender</spdx:licenseId> | ||
| </spdx:ExtractedLicensingInfo> | ||
| </spdx:member> | ||
| </spdx:DisjunctiveLicenseSet> | ||
| ``` | ||
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| A License Exception can be expressed in RDF via a `<spdx:LicenseException>` element. This element has the following unique mandatory (unless specified otherwise) attributes: | ||
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| * `comment` - An `rdfs:comment` element describing the nature of the exception. | ||
| * `seeAlso` (optional, one or more)- An `rdfs:seeAlso` element referencing external sources of information on the exception. | ||
| * `example` (optional) - Text describing examples of this exception. | ||
| * `name` - The full human readable name of the item. | ||
| * `licenseExceptionId` - The identifier of an exception in the SPDX License List to which the exception applies. | ||
| * `licenseExceptionText` - Full text of the license exception. | ||
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| ```text | ||
| <rdf:Description rdf:about | ||
| ="http://example.org#SPDXRef-ButIdDontWantToException"> | ||
| <rdfs:comment>This exception may be invalid in some | ||
| jurisdictions.</rdfs:comment> | ||
| <rdfs:seeAlso>http://dilbert.com/strip/1997-01-15</rdfs:seeAlso> | ||
| <spdx:example>So this one time, I had a license exception | ||
| …</spdx:example> | ||
| <spdx:licenseExceptionText> | ||
| A user of this software may decline to follow any subset of | ||
| the terms of this license upon finding any or all such terms | ||
| unfavorable. | ||
| </spdx:licenseExceptionText> | ||
| <spdx:name>"But I Don't Want To" Exception</spdx:name> | ||
| <spdx:licenseExceptionId>SPDXRef-ButIdDontWantToException</spdx:licenseExceptionId> | ||
| <rdf:type rdf:resource | ||
| ="http://spdx.org/rdf/terms#LicenseException"/> | ||
| </rdf:Description> | ||
| ``` |