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vers: a mostly universal version range specifier

This specification is a new syntax for dependency and vulnerable version ranges.

Context

Software package version ranges and version constraints are essential:

  • When resolving the dependencies of a package to express which subset of the versions are supported. For instance a dependency or requirement statement such as "I require package foo, version 2.0 or later versions" defines a range of acceptable foo versions.
  • When stating that a known vulnerability or bug affects a range of package versions. For instance a security advisory such as "vulnerability 123 affects package bar, version 3.1 and version 4.2 but not version 5" defines a range of vulnerable "bar" package versions.

Version ranges can be replaced by a list enumerating all the versions of interest. But in practice, all the versions may not yet exist when defining an open version range such as "v2.0 or later".

Therefore, a version range is a necessary, compact and practical way to reference multiple versions rather than listing all the versions.

Problem

Several version range notations exist and have evolved separately to serve the specific needs of each package ecosystem, vulnerability databases and tools.

There is no (mostly) universal notation for version ranges and there is no universal way to compare two versions, even though the concepts that exist in most version range notations are similar.

Each package type or ecosystem may define their own ranges notation and version comparison semantics for dependencies. And for security advisories, the lack of a portable and compact notation for vulnerable package version ranges means that these ranges may be either ambiguous or hard to compute and may be best replaced by complete enumerations of all impacted versions, such as in the NVD CPE Match feed.

Because of this, expressing and resolving a version range is often a complex, or error prone task.

In particular the need for common notation for version has emerged based on the usage of Package URLs referencing vulnerable package version ranges such as in vulnerability databases like VulnerableCode.

To better understand the problem, here are some of the many notations and conventions in use:

The way two versions are compared as equal, lesser or greater is a closely related topic:

  • Each package ecosystem may have evolved its own peculiar version string conventions, semantics and comparison procedure.
  • For instance, semver is a prominent specification in this domain but this is just one of the many ways to structure a version string.
  • Debian, RPM, PyPI, RubyGems, and Composer have their own subtly different approach on how to determine how two versions are compared as equal, greater or lesser.

Solution

A solution to the many version range syntaxes is to design a new simplified notation to unify them all with:

  • a mostly universal and minimalist, compact notation to express version ranges from many different package types and ecosystems.
  • the package type-specific definitions to normalize existing range expressions in this common notation.
  • the designation of which algorithm or procedure to use when comparing two versions such that it is possible to resolve if a version is within or outside of a version range.

We call this solution "version range specifier" or "vers" and it is described in this document.

Version range specifier

A version range specifier (aka. "vers") is a URI string using the vers URI-scheme with this syntax:

vers:<versioning-scheme>/<version-constraint>|<version-constraint>|...

For example, to define a set of versions that contains either version 1.2.3, or any versions greater than or equal to 2.0.0 but less than 5.0.0 using the node-semver versioning scheme used with the npm Package URL type, the version range specifier will be:

vers:npm/1.2.3|>=2.0.0|<5.0.0

vers is the URI-scheme and is an acronym for "VErsion Range Specifier". It has been selected because it is short, obviously about version and available for a future formal URI-scheme registration at IANA.

The pipe "|" is used as a simple separator between <version-constraint>. Each <version-constraint> in this pipe-separated list contains a comparator and a version:

<comparator:version>

This list of <version-constraint> are signposts in the version timeline of a package that specify version intervals.

A <version> satisfies a version range specifier if it is contained within any of the intervals defined by these <version-constraint>.

Using version range specifiers

vers primary usage is to test if a version is within a range.

An version is within a version range if falls in any of the intervals defined by a range. Otherwise, the version is outside of the version range.

Some important usages derived from this include:

  • Resolving a version range specifier to a list of concrete versions. In this case, the input is one or more known versions of a package. Each version is then tested to check if it lies within or outside the range. For example, given a vulnerability and the vers describing the vulnerable versions of a package, this process is used to determine if an existing package version is vulnerable.
  • Selecting one of several versions that are within a range. In this case, given several versions that are within a range and several packages that express package dependencies qualified by a version range, a package management tools will determine and select the set of package versions that satisfy all the version ranges constraints of all dependencies. This usually requires deploying heuristics and algorithms (possibly complex such as sat solvers) that are ecosystem- and tool-specific and outside of the scope for this specification; yet vers could be used in tandem with purl to provide an input to this dependencies resolution process.

Examples

A single version in an npm package dependency:

  • originally seen as a dependency on version "1.2.3" in a package.json manifest
  • the version range spec is: vers:npm/1.2.3

A list of versions, enumerated:

  • vers:pypi/0.0.0|0.0.1|0.0.2|0.0.3|1.0|2.0pre1

A complex statement about a vulnerability in a "maven" package that affects multiple branches each with their own fixed versions at https://repo1.maven.org/maven2/org/apache/tomee/apache-tomee/ Note how the constraints are sorted:

  • "affects Apache TomEE 8.0.0-M1 - 8.0.1, Apache TomEE 7.1.0 - 7.1.2, Apache TomEE 7.0.0-M1 - 7.0.7, Apache TomEE 1.0.0-beta1 - 1.7.5."
  • a normalized version range spec is: vers:maven/>=1.0.0-beta1|<=1.7.5|>=7.0.0-M1|<=7.0.7|>=7.1.0|<=7.1.2|>=8.0.0-M1|<=8.0.1
  • alternatively, four vers express the same range, using one vers for each vulnerable "branches": - vers:tomee/>=1.0.0-beta1|<=1.7.5 - vers:tomee/>=7.0.0-M1|<=7.0.7 - vers:tomee/>=7.1.0|<=7.1.2 - vers:tomee/>=8.0.0-M1|<=8.0.1

Conversing RubyGems custom syntax for dependency on gem. Note how the pessimistic version constraint is expanded:

  • 'library', '~> 2.2.0', '!= 2.2.1'
  • the version range spec is: vers:gem/>=2.2.0|!= 2.2.1|<2.3.0

URI scheme

The vers URI scheme is an acronym for "VErsion Range Specifier". It has been selected because it is short, obviously about version and available for a future formal registration for this URI-scheme at the IANA registry.

The URI scheme is followed by a colon ":".

<versioning-scheme>

The <versioning-scheme> (such as npm, deb, etc.) determines:

  • the specific notation and conventions used for a version string encoded in this scheme. Versioning schemes often specify a version segments separator and the meaning of each version segments, such as [major.minor.patch] in semver.
  • how two versions are compared as greater or lesser to determine if a version is within or outside a range.
  • how a versioning scheme-specific range notation can be transformed in the vers simplified notation defined here.

By convention the versioning scheme should be the same as the Package URL package type for a given package ecosystem. It is OK to have other schemes beyond the purl type. A scheme could be specific to a single package name.

The <versioning-scheme> is followed by a slash "/".

<version-constraint>

After the <versioning-scheme> and "/" there are one or more <version-constraint> separated by a pipe "|". The pipe "|" has no special meaning beside being a separator.

Each <version-constraint> of this list is either a single <version> as in 1.2.3 for example or the combination of a <comparator> and a <version> as in >=2.0.0 using this syntax:

<comparator><version>

A single version that means that a version equal to this version satisfies the range spec. Equality is based on the equality of two normalized version strings according to their versioning scheme. For most schemes, this is a simple string equality. But schemes can specify normalization and rules for equality such as pypi with PEP440.

The special star "*" comparator matches any version. It must be used alone exclusive of any other constraint and must not be followed by a version. For example "vers:deb/*" represent all the versions of a Debian package. This includes past, current and possible future versions.

Otherwise, the <comparator> is one of these comparison operators:

  • "!=": Version exclusion or inequality comparator. This means a version must not be equal to the provided version that must be excluded from the range. For example: "!=1.2.3" means that version "1.2.3" is excluded.
  • "<", "<=": Lesser than or lesser-or-equal version comparators point to all versions less than or equal to the provided version. For example "<=1.2.3" means less than or equal to "1.2.3".
  • ">", ">=": Greater than or greater-or-equal version comparators point to all versions greater than or equal to the provided version. For example ">=1.2.3" means greater than or equal to "1.2.3".

The <versioning-scheme> defines:

  • how to compare two version strings using these comparators, and
  • the structure of a version string such as "1.2.3" if any. For instance, the semver specification for version numbers defines a version as composed primarily of three dot-separated numeric segments named major, minor and patch.

Normalized, canonical representation and validation

The construction and validation rules are designed such that a vers is easier to read and understand by human and straight forward to process by tools, attempting to avoid the creation of empty or impossible version ranges.

  • Spaces are not significant and removed in a canonical form. For example "<1.2.3|>=2.0" and " < 1.2. 3 | > = 2 . 0" are equivalent.
  • A version range specifier contains only printable ASCII letters, digits and punctuation.
  • The URI scheme and versioning scheme are always lowercase as in vers:npm.
  • The versions are case-sensitive, and a versioning scheme may specify its own case sensitivity.
  • If a version in a <version-constraint> contains separator or comparator characters (i.e. ><=!*|), it must be quoted using the URL quoting rules. This should be rare in practice.

The list of <version-constraint>s of a range are signposts in the version timeline of a package. With these few and simple validation rules, we can avoid the creation of most empty or impossible version ranges:

  • Constraints are sorted by version. The canonical ordering is the versions order. The ordering of <version-constraint> is not significant otherwise but this sort order is needed when check if a version is contained in a range.
  • Versions are unique. Each version must be unique in a range and can occur only once in any <version-constraint> of a range specifier, irrespective of its comparators. Tools must report an error for duplicated versions.
  • There is only one star: "*" must only occur once and alone in a range, without any other constraint or version.

Starting from a de-duplicated and sorted list of constraints, these extra rules apply to the comparators of any two contiguous constraints to be valid:

  • "!=" constraint can be followed by a constraint using any comparator, i.e., any of "=", "!=", ">", ">=", "<", "<=" as comparator (or no constraint).

Ignoring all constraints with "!=" comparators:

  • A "=" constraint must be followed only by a constraint with one of "=", ">", ">=" as comparator (or no constraint).

And ignoring all constraints with "=" or "!=" comparators, the sequence of constraint comparators must be an alternation of greater and lesser comparators:

  • "<" and "<=" must be followed by one of ">", ">=" (or no constraint).
  • ">" and ">=" must be followed by one of "<", "<=" (or no constraint).

Tools must report an error for such invalid ranges.

Parsing and validating version range specifiers

To parse a version range specifier string:

  • Remove all spaces and tabs.

  • Start from left, and split once on colon ":".

  • The left hand side is the URI-scheme that must be lowercase. - Tools must validate that the URI-scheme value is vers.

  • The right hand side is the specifier.

  • Split the specifier from left once on a slash "/".

  • The left hand side is the <versioning-scheme> that must be lowercase. Tools should validate that the <versioning-scheme> is a known scheme.

  • The right hand side is a list of one or more constraints. Tools must validate that this constraints string is not empty ignoring spaces.

  • If the constraints string is equal to "*", the <version-constraint> is "*". Parsing is done and no further processing is needed for this vers. A tool should report an error if there are extra characters beyond "*".

  • Strip leading and trailing pipes "|" from the constraints string.

  • Split the constraints on pipe "|". The result is a list of <version-constraint>. Consecutive pipes must be treated as one and leading and trailing pipes ignored.

  • For each <version-constraint>: - Determine if the <version-constraint> starts with one of the two comparators:

    • If it starts with ">=", then the comparator is ">=".
    • If it starts with "<=", then the comparator is "<=".
    • If it starts with "!=", then the comparator is "!=".
    • If it starts with "<", then the comparator is "<".
    • If it starts with ">", then the comparator is ">".
    • Remove the comparator from <version-constraint> string start. The remaining string is the version.
    • Otherwise the version is the full <version-constraint> string (which implies an equality comparator of "=")
    • Tools should validate and report an error if the version is empty.
    • If the version contains a percent "%" character, apply URL quoting rules to unquote this string.
    • Append the parsed (comparator, version) to the constraints list.

Finally:

  • The results are the <versioning-scheme> and the list of <comparator, version> constraints.

Tools should optionally validate and simplify the list of <comparator, version> constraints once parsing is complete:

  • Sort and validate the list of constraints.
  • Simplify the list of constraints.

Version constraints simplification

Tools can simplify a list of <version-constraint> using this approach:

These pairs of contiguous constraints with these comparators are valid:

  • != followed by anything
  • =, <, or <= followed by =, !=, >, or >=
  • >, or >= followed by !=, <, or <=

These pairs of contiguous constraints with these comparators are redundant and invalid (ignoring any != since they can show up anywhere):

  • =, < or <= followed by < or <=: this is the same as < or <=
  • > or >= followed by =, > or >=: this is the same as > or >=

A procedure to remove redundant constraints can be:

  • Start from a list of constraints of comparator and version, sorted by version and where each version occurs only once in any constraint.

  • If the constraints list contains a single constraint (star, equal or anything) return this list and simplification is finished.

  • Split the constraints list in two sub lists:

    • a list of "unequal constraints" where the comparator is "!="
    • a remainder list of "constraints" where the comparator is not "!="

  • If the remainder list of "constraints" is empty, return the "unequal constraints" list and simplification is finished.

  • Iterate over the constraints list, considering the current and next contiguous constraints, and the previous constraint (e.g., before current) if it exists:

    • If current comparator is ">" or ">=" and next comparator is "=", ">" or ">=", discard next constraint

    • If current comparator is "=", "<" or "<=" and next comparator is <" or <=", discard current constraint. Previous constraint becomes current if it exists.

    • If there is a previous constraint:

      • If previous comparator is ">" or ">=" and current comparator is "=", ">" or ">=", discard current constraint
      • If previous comparator is "=", "<" or "<=" and current comparator is <" or <=", discard previous constraint

  • Concatenate the "unequal constraints" list and the filtered "constraints" list

  • Sort by version and return.

Checking if a version is contained within a range

To check if a "tested version" is contained within a version range:

  • Start from a parsed a version range specifier with:

    • a versioning scheme
    • a list of constraints of comparator and version, sorted by version and where each version occurs only once in any constraint.

  • If the constraint list contains only one item and the comparator is "*", then the "tested version" is IN the range. Check is finished.

  • Select the version equality and comparison procedures suitable for this versioning scheme and use these for all version comparisons performed below.

  • If the "tested version" is equal to the any of the constraint version where the constraint comparator is for equality (any of "=", "<=", or ">=") then the "tested version" is in the range. Check is finished.

  • If the "tested version" is equal to the any of the constraint version where the constraint comparator is "=!" then the "tested version" is NOT in the range. Check is finished.

  • Split the constraint list in two sub lists:

    • a first list where the comparator is "=" or "!="
    • a second list where the comparator is neither "=" nor "!="

  • Iterate over the current and next contiguous constraints pairs (aka. pairwise) in the second list.

  • For each current and next constraint:

    • If this is the first iteration and current comparator is "<" or <=" and the "tested version" is less than the current version then the "tested version" is IN the range. Check is finished.
    • If this is the last iteration and next comparator is ">" or >=" and the "tested version" is greater than the next version then the "tested version" is IN the range. Check is finished.
    • If current comparator is ">" or >=" and next comparator is "<" or <=" and the "tested version" is greater than the current version and the "tested version" is less than the next version then the "tested version" is IN the range. Check is finished.
    • If current comparator is "<" or <=" and next comparator is ">" or >=" then these versions are out the range. Continue to the next iteration.

  • Reaching here without having finished the check before means that the "tested version" is NOT in the range.

Notes and caveats

  • Comparing versions from two different versioning schemes is an error. Even though there may be some similarities between the semver version of an npm and the deb version of its Debian packaging, the way versions are compared specific to each versioning scheme and may be different. Tools should report an error in this case.
  • All references to sorting or ordering of version constraints means sorting by version. And sorting by versions always implies using the versioning scheme-specified version comparison and ordering.

Some of the known versioning schemes

These are a few known versioning schemes for some common Package URL types (aka. ecosystem).

TODO: add Rust, composer and archlinux, nginx, tomcat, apache.

A separate document will provide details for each versioning scheme and:

  • how to convert its native range notation to the vers notation and back.
  • how to compare and sort two versions in a range.

This versioning schemes document will also explain how to convert CVE and OSV ranges to vers.

Implementations

Related efforts and alternative

  • CUDF defines a generic range notation similar to Debian and integer version numbers from the sequence of versions for universal dependencies resolution https://www.mancoosi.org/cudf/primer/
  • OSV is an "Open source vulnerability DB and triage service." It defines vulnerable version range semantics using a minimal set of comparators for use with package "ecosystem" and version range "type". https://github.com/google/osv
  • libversion is a library for general purpose version comparison using a unified procedure designed to work with many package types. https://github.com/repology/libversion
  • unified-range is a library for uniform version ranges based on the Maven version range spec. It support Apache Maven and npm ranges https://github.com/snyk/unified-range
  • dephell specifier is a library to parse and evaluate version ranges and "work with version specifiers (can parse PEP-440, SemVer, Ruby, NPM, Maven)" https://github.com/dephell/dephell_specifier

Why not reuse existing version range notations?

Most existing version range notations are tied to a specific version string syntax and are therefore not readily applicable to other contexts. For example, the use of elements such as tilde and caret ranges in RubyGems, npm or Dart notations implies that a certain structure exists in the version string (semver or semver- like). The inclusion of these additional comparators is a result of the history and evolution in a given package ecosystem to address specific needs.

In practice, the unified and reduced set of comparators and syntax defined for vers has been designed such that all these notations can be converted to a vers and back from a vers to the original notation.

In contrast, this would not be possible with existing notations. For instance, the Python notation may not work with npm semver versions and reciprocally.

There are likely to be a few rare cases where round tripping from and to vers may not be possible, and in any case round tripping to and from vers should produce equivalent results and even if not strictly the same original strings.

Another issue with existing version range notations is that they are primarily designed for dependencies and not for vulnerable ranges. In particular, a vulnerability may exist for multiple "version branches" of a given package such as with Django 2.x and 3.x. Several version range notations have difficulties to communicate these as typically all the version constraints must be satisfied. In contrast, a vulnerability can affect multiple disjoint version ranges of a package and any version satisfying these constraints would be vulnerable: it may not be possible to express this with a notation designed exclusively for dependent versions resolution.

Finally, one of the goals of this spec is to be a compact yet obvious Package URL companion for version ranges. Several existing and closely related notations designed for vulnerable ranges are verbose specifications designed for use in API with larger JSON documents.

Why not use the OSV Ranges?

See:

vers and the OSSF OSV schema vulnerable ranges are equivalent and vers provides a compact range notation while OSV provides more verbose JSON notation.

vers borrows the design from and was informed by the OSV schema spec and its authors.

OSV uses a minimalist set of only three comparators:

  • "=" to enumerate versions,
  • ">=" for the version that introduced a vulnerability, and
  • "<" for the version that fixed a vulnerability.

OSV Ranges support neither ">" nor "!=" comparators making it difficult to express some ranges that must exclude a version. This may not be an issue for most vulnerable ranges yet:

  • this makes it difficult or impossible to precisely express certain dependency and vulnerable ranges when a version must be excluded and the set of existing versions is not yet known,
  • this make some ranges more verbose such as with the CVE v5 API ranges notation that can include their upper limit and would need two constraints.

Another high level difference between the two specifications are the codes used to qualify a range package "ecosystem" value that resembles closely the Package URL package "type" used in vers. This spec will provide a strict mapping between the OSV ecosystem and the vers versioning schemes values.

Why not use the CVE v5 API Ranges?

See:

The version 5 of the CVE JSON data format defines version ranges with a starting version, a versionType, and an upper limit for the version range as lessThan or lessThanOrEqual or as an enumeration of versions. The versionType and the package collectionURL possible values are only indicative and left out of this specification and both seem strictly equivalent to the Package URL "type" on the one hand and the vers versioning scheme on the other hand.

The semantics and expressiveness of each range are similar and vers provides a compact notation rather than a more verbose JSON notation. vers supports strictly the conversion of any CVE v5 range to its notation and further provides a concrete list of well known versioning schemes. vers design was informed by the CVE v5 API schema spec and its authors.

When CVE v5 becomes active, this spec will provide a strict mapping between the CVE versionType and the vers versioning schemes values. Furthermore, this spec and the Package URL "types" should be updated accordingly to provide a mapping with the upcoming CVE collectionURL that will be effectively used.

There is one issue with CVE v5: it introduces a new trailing "*" notation that does not exists in most version ranges notations and may not be computable easily in many cases. The description of the "lessThan" property is:

The non-inclusive upper limit of the range. This is the least version NOT in the range. The usual version syntax is expanded to allow a pattern to end in an asterisk (*), indicating an arbitrarily large number in the version ordering. For example, {version: 1.0 lessThan: 1.*} would describe the entire 1.X branch for most range kinds, and {version: 2.0, lessThan: *} describes all versions starting at 2.0, including 3.0, 5.1, and so on.

The conversion to vers range should be:

  • with a version 1.0 and "lessThan": "*", the vers equivalent is: >=1.0.

  • with a version 1.0 and "lessThan": "2.*", the vers equivalent can be computed for semver versions as >=1.0|<2 but is not accurate unless as versioning schemes have different rules. For instance, pre-release may be treated in some case as part of the v1. branch and in some other cases as part of the v2. branch. It is not clear if with "2.*" the CVE v5 spec means:

    • <2
    • or something that excludes any version string that starts with 2.

And in this case, with the expression "lessThan": "2.*" using a semver version, it is not clear if 2.0.0-alpha is "lessThan"; semver sorts it before 2.0 and after 1.0, e.g., in semver 2.0.0-alpha is "less than" 2.

Why not use the NVD CPE Ranges?

See:

The version ranges notation defined in the JSON schema of the CVE API payload uses these four fields: versionStartIncluding, versionStartExcluding, versionEndIncluding and versionEndExcluding. For example:

"versionStartIncluding": "7.3.0",
"versionEndExcluding": "7.3.31",
"versionStartExcluding" : "9.0.0",
"versionEndIncluding" : "9.0.46",

In addition to these ranges, the NVD publishes a list of concrete CPE with versions resolved for a range with daily updates at https://nvd.nist.gov/vuln/data-feeds#cpeMatch

Note that the NVD CVE configuration is a complex specification that goes well beyond version ranges and is used to match comprehensive configurations across multiple products and version ranges. vers focus is exclusively versions.

In contrast with vers compact notation, the NVD JSON notation is more verbose, yet vers supports strictly the conversion of any CPE range.

Why not use node-semver ranges?

See:

The node-semver spec is similar but much more complex than this spec. This is an AND of ORs constraints with a few practical issues:

  • A space means "AND", therefore white spaces are significant. Having significant white spaces in a string makes normalization more complicated and may be a source of confusion if you remove the spaces from the string. vers avoids the ambiguity of spaces by ignoring them.
  • The advanced range syntax has grown to be rather complex using hyphen ranges, stars ranges, carets and tilde constructs that are all tied to the JavaScript and npm ways of handling versions in their ecosystem and are bound furthermore to the semver semantics and its npm implementation. These are not readily reusable elsewhere. The multiple comparators and modifiers make the notation grammar more complex to parse and process for a machine and harder to read for human.

Notations that are directly derived from node-semver as used in Rust and PHP Composer have the same issues.

Why not use Python PEP-0440 ranges?

See:

The Python pep-0440 "Version Identification and Dependency Specification" provides a comprehensive specification for Python package versioning and a notation for "version specifiers" to express the version constraints of dependencies.

This specification is similar to this vers spec, with more operators and aspects specific to the versions used only in the Python ecosystem.

  • In particular pep-0440 uses tilde, triple equal and wildcard star operators that are specific to how two Python versions are compared.
  • The comma separator between constraints is a logical "AND" rather than an "OR". The "OR" does not exist in the syntax making some version ranges harder to express, in particular for vulnerabilities that may affect several exact versions or ranges for multiple parallel release branches. Ranges such as "Django 1.2 or later, or Django 2.2 or later or Django 3.2 or later" are difficult to express without an "OR" logic.

Why not use RubyGems requirements notation?

See:

The RubyGems specification suggests but does not enforce using semver. It uses operators similar to the node-semver spec with the different of the "~>" aka. pessimistic operator vs. a plain "~" tilde used in node-semver. This operator implies some semver-like versioning, yet gem version are not strictly semver. This makes the notation complex to implement and impractical to reuse in places that do not use the same Ruby-specific semver-like semantics.

Why not use fewer comparators with only =, >= and <?

For instance, the OSV schema adopts a reduced set of only three comparators:

  • "=" is implied when used to enumerate vulnerable versions
  • ">=" (greater or equal) is for the version that introduces a vulnerability
  • "<" (lesser) is for the version that fixes a vulnerability

This approach is simpler and works well for most vulnerable ranges but it faces limitations when converting from other notations:

  • ">" cannot be converted reliably to ">=" unless you know all the versions and these will never change.
  • "<=" cannot be converted reliably to "<" unless you know all the versions and these will never change.
  • "!=" cannot be converted reliably: there is no ">" comparator to create an unequal equivalent of "><"; and a combo of ">=" and "<" is not equivalent to inequality unless you know all the versions and these will never change.

Why not use richer comparators such as tilde, caret and star?

Some existing notations such as used with npm, gem, python, or composer provide syntactic shorthand such as:

  • a "pessimistic operator" using tilde, ~> or =~ as in "~1.3" or "~>1.2.3"
  • a caret ^ prefix as in "^ 1.2"
  • using a star in a version segment as in "1.2.*"
  • dash-separated ranges as in "1.2 - 1.4"
  • arbitrary string equality such as "===1.2"

Most of these notations can be converted without loss to the vers notation. Furthermore these notations typically assume a well defined version string structure specific to their package ecosystem and are not reusable in another ecosystem that would not use the exact same version conventions.

For instance, the tilde and caret notations demand that you can reliably infer the next version (aka. "bump") from a given version; this is possible only if the versioning scheme supports this operation reliably for all its accepted versions.

Why not use mathematical interval notation for ranges?

Apache Maven and NuGet use a mathematical interval notation with comma-separated "[", "]", "(" and ")" to declare version ranges.

All other known range notations use the more common ">", "<", and "=" as comparators. vers adopts this familiar approach.

References

Here are some of the discussions that led to the creation of this specification:

License

This document is licensed under the MIT license