Propose semver support in ytt's standard library

proposals/ytt/005-semver/README.md

@@ -0,0 +1,217 @@
+---
+title: "Semver support in the standard library"
+authors: [ "Max Brauer <[email protected]>" ]
+status: "draft"
+approvers: [ ? ]
+---
+
+# Semver
+
+## Problem Statement
+
+Configuration authors frequently encounter strings which denote the version of a
+software. [Semantic versioning](https://semver.org/) is a widely adopted scheme for versioning
+software. In this proposal the term "semver" is used both for _the_ scheme itself as well _an_ instance of a version.
+
+A semantic version is a string, e.g. `3.2.12`, `0.23.0-beta`, `1.99.2-next+build.5`, which contains structured
+information about the version. Configuration authors may want
+to read a version's constituents, mutate them and compare different versions. For example, when authoring `ytt`
+templates:
+
+* I want to assert that a given string is a well-formed semver.
+* I want to change a semver's representation from string another to data structure, e.g. object or array.
+* I want to compare semvers, e.g. expect a given version to be larger than a known version.
+* I want to (de,in)crement the major, minor and/or patch version of a semver.
+* I want to add, change or remove a semver's prerelease identifier or metadata.
+
+Starlark's standard library provides the means to achieve this. However, it sets a high bar and burdens configuration
+authors to write boilerplate code for common, industry standard operations.
+
+## Terminology / Concepts
+
+A semantic version encodes different properties about the version in a string of the
+form `<major>.<minor>.<patch>(-<pre-release>)(+<metadata>)`.
+
+Semantic versions are strictly sortable.
+
+For its full specification refer to [Semantic versioning](https://semver.org/).
+
+## Proposal
+
+The proposal is to include a built-in `@ytt:semver` library. Configuration authors can then load it and programmatically
+work with strings representing semvers.
+
+The semver library contains means for turning a semver string into a structured representation. This can result in an
+error in the case of a malformed semver. If the operation succeeds, it returns a `struct` representing the semver.
+
+Now, it is possible to read the _major_, _minor_, _patch_ versions as integers, as well as the _pre-release_
+version and _build metadata_ as a strings. This enables authors to change its representation at will.
+
+It is also possible to programmatically construct a semver `struct` by providing _major_, _minor_, _patch_ versions as
+required integers, as well as the _pre-release_ version and _build metadata_ as optional strings. This enables authors
+to construct semvers either from values known in advance or which are received in a representation other than a semver.
+
+Given two semver `structs`, they can be compared. This enables authors to test if a semver is large or small enough,
+or whether it is within a range.
+
+For its implementation, the suggestion is to use [blang/semver](https://github.com/blang/semver). It is a
+well-established module, contains an API to address all conceivable case and is already used by Carvel's `vendir` for
+its `tools sort-semver` subcommand.
+
+### Goals and Non-goals
+
+* Enable configuration authors to programmatically work with semvers out-of-the-box.
+
+* Education around and proliferation of the semver scheme is not considered to be `ytt`'s responsibility.
+
+### Specification / Use Cases
+
+#### semver·version
+
+`semver.version([major=int, minor=int, patch=int, prerelease=string, metadata=string])` returns a struct representing a
+semantic version. The fields `major`, `minor` and `patch` are strings and default to `0`. The fields `prerelease` and
+`metadata` are strings and default to `None`. If any of the arguments are of the wrong type or contain forbidden
+characters, it is a dynamic error.
+
+```yaml
+#@ load("@ytt:semver", "semver")
+
+#@ semver.version()                                        #! 0.0.0
+#@ semver.version(1, 2, 3)                                 #! 1.2.3
+#@ semver.version(1, 2, 3, "beta")                         #! 1.2.3-beta
+#@ semver.version(1, 2, 3, metadata="build.5")             #! 1.2.3+build.5
+#@ semver.version(1, 2, 3, "next", "nightly")              #! 1.2.3-next+nightly
+#@ semver.version(minor=1, prerelease="alpha")             #! 0.1.0-alpha
+#@ semver.version(1, 2, -5)                                #! ⚡️ error
+#@ semver.version(prerelease="$send-ytt-coins$")           #! ⚡️ error
+#@ semver.version(meta="__dunder-score")                   #! ⚡️ error
+```
+
+Use cases:
+
+```yaml
+#@ load("@ytt:semver", "semver")
+#@ load("@ytt:yaml", "yaml")
+#@ load("@ytt:json", "json")
+#@ load("@ytt:assert", "assert")
+
+#@ v = semver.version(1, 2, 3)
+#@ d = dict(major=v.major, minor=v.minor, patch=v.patch, prerelease=v.prerelease, metadata=v.metadata) 
+
+--- #@ d
+
+---
+version.yaml: #@ yaml.encode(d)
+version.json: #@ json.encode(d)
+
+#@ d.update(major=d["major"]+1)
+#@ next_major = semver.version(**d)
+
+#@ if v.prerelease:
+#@   assert.fail("Sorry, we don't do prereleases") 
+#@ end
+
+#@ forbidden = ["nightly", "dev"]
+#@ if v.metadata in forbidden:
+#@   assert.fail("Sorry, that version is forbidden") 
+#@ end
+```
+
+#### semver·from_str
+
+`semver.from_str(s)` parses `s` as a semantic version and returns its `semver.version()` struct representation. If `s`
+is malformed, it is a dynamic error.
+
+```yaml
+#@ load("@ytt:semver", "semver")
+
+#@ semver.from_str("1.2.3")                                #! semver.version(1, 2, 3)
+#@ semver.from_str("1.2.3-pre")                            #! semver.version(1, 2, 3, "pre")
+#@ semver.from_str("1.2.3+meta")                           #! semver.version(1, 2, 3, metadata="meta")
+#@ semver.from_str("1.2.3-pre+meta")                       #! semver.version(1, 2, 3, "pre", "meta")
+#@ semver.from_str("1.2.-5")                               #! ⚡️ error
+#@ semver.from_str("Homer Simpson")                        #! ⚡️ error
+```
+
+#### version·cmp
+
+`version.cmp(other)` compares two semantic versions and returns `0` if they are equal, `-1` if `other` is greater
+and `1` if other is smaller. If `other` is not a `version` struct, it is a dynamic error.
+
+Usage of `version.(eq, lt, lte, gt, gte)` is recommended instead. However, there may be cases where a numeric return
+value is preferred.
+
+```yaml
+#@ load("@ytt:semver", "semver")
+
+#@ semver.version().cmp(semver.version())                  #! 0
+#@ semver.version(1).cmp(semver.version(2))                #! -1
+#@ semver.version(1, 2, 3).cmp(semver.version(2, 0, 3))    #! 1
+#@ semver.version(1, 2, 3).cmp(dict())                     #! ⚡️ error
+```
+
+#### version·(eq, lt, lte, gt, gte)
+
+All of `version.eq(version)`, `version.lt(version)`, `version.lte(version)`, `version.gt(version)`
+and `version.gte(version)` compare `version` with `other` and return `True` if the operator holds and `False` otherwise.
+If `other` is not a `version` struct, it is a dynamic error.
+
+```yaml
+#@ load("@ytt:data", "data")
+#@ load("@ytt:semver", "semver")
+#@ load("@ytt:assert", "assert")
+
+#@ v1_2_3 = semver.version(1, 2, 3)
+#@ given_version = semver.from_str(data.values.version)
+
+---
+cool_config: nice
+#@ if/end given_version.lt(v1_2_3)
+ye_olde_confyg: they don't make 'em like this any more
+
+#@ if given_version.gte(semver.version(10, 20, 30))
+#@   assert.fail("Sorry, but that's too hot for us")
+#@ end
+```
+
+#### version·to_str
+
+`version.to_str()` returns the string representation of the `version` struct.
+
+```yaml
+#@ load("@ytt:semver", "semver")
+
+#@ semver.version().to_str()                               #! "0.0.0"
+#@ semver.version(1, 2, 3).to_str()                        #! "1.2.3"
+#@ semver.version(1, 2, 3, "beta").to_str()                #! "1.2.3-beta"
+#@ semver.version(1, 2, 3, metadata="build.5").to_str()    #! "1.2.3+build.5"
+#@ semver.version(1, 2, 3, "next", "nightly").to_str()     #! "1.2.3-next+nightly"
+#@ semver.version(minor=1, prerelease="alpha").to_str()    #! "0.1.0-alpha"
+```
+
+Use cases:
+
+```yaml
+#@ load("@ytt:semver", "semver")
+
+#@ all_the_minor_versions = [semver.version(minor=i).to_str() for i in range(10)]
+---
+allowed_versions: #@ all_the_minor_versions
+```
+
+### Other Approaches Considered
+
+* A Github-hosted Starlark library like `awesome-ytt/semver` containing a similar API could be fetched with `vendir` and
+  then loaded. This wouldn't in reach for the widest possible spectrum of configuration authors. It would require to
+  reimplement what existing, established Go modules already have done.
+
+## Open Questions
+
+* There's a host of conceivable additional operations available for semver, e.g. test against range, sort list of
+  versions, bumping, etc. It is debatable if that should be part of the standard library. Testing against a range and
+  sorting are likelier candidates for a standard library. Bumping is possible with the primitives presented.
+* Maybe configuration authors would benefit from a `version.to_dict()`(or even `version.to_list()`
+  and `version.to_tuple()`) to ease the transformation into a serializable data structure. On the other hand, at least
+  in the case of `version.to_dict()` the author may disagree with the keys.
+
+## Answered Questions

proposals/ytt/README.md

@@ -5,3 +5,4 @@ Feature proposals for `ytt`. See each proposal for their status.
 - [Schemas](001-schemas/)
 - [Raw data values](002-raw-data-values/)
 - [OpenAPI Schemas](003-openapi-schemas/)
+- [Semver](005-semver/)