I also agree with the point that .lock extension better suits here and adheres 
to a convention already established in other languages. I personally would 
prefer the file to be named Package.lock, not Package.pins and also think that 
it would help newcomers who already used other package managers, especially 

With best regards, Max.

> On 14 Oct 2016, at 17:43, Alexis via swift-evolution 
> <swift-evolution@swift.org> wrote:
>> On Oct 14, 2016, at 2:01 AM, Ankit Aggarwal via swift-evolution 
>> <swift-evolution@swift.org <mailto:swift-evolution@swift.org>> wrote:
>> Hi,
>> We're proposing version pinning feature in Swift Package Manager. The 
>> proposal is available here 
>> <https://github.com/aciidb0mb3r/swift-evolution/blob/version-pinning/proposals/NNNN-Version-Pinning.md>
>>  and also in this email:
>> Feedback welcomed!
>> Thanks,
>> Ankit
>> --------
>> Package Manager Version Pinning
>> Proposal: SE-XXXX
>> Author: Daniel Dunbar <https://github.com/ddunbar>, Ankit Aggarwal 
>> <https://github.com/aciidb0mb3r>
>> Review Manager: TBD
>> Status: Discussion
>> Introduction
>> This is a proposal for adding package manager features to "pin" or "lock" 
>> package dependencies to particular versions.
>> Motivation
>> As used in this proposal, version pinning refers to the practice of 
>> controlling exactly which specific version of a dependency is selected by 
>> the dependency resolution algorithm, independent from the semantic 
>> versioning specification. Thus, it is a way of instructing the package 
>> manager to select a particular version from among all of the versions of a 
>> package which could be chosen while honoring the dependency constraints.
>> Terminology
>> We have chosen to use "pinning" to refer to this feature, over "lockfiles", 
>> since the term "lock" is already overloaded between POSIX file locks and 
>> locks in concurrent programming.
> I’ve never seen this cause any actual confusion, nor has anyone I know who 
> teaches/develops these sorts of tools. As far as I can tell, the broader 
> programming community is rapidly converging on this as standard terminology:
> * Gemfile.lock (Ruby)
> * Cargo.lock (Rust)
> * Composer.lock (PHP)
> * yarn.lock (JS)
> * pubspec.lock (Dart)
> * Podfile.lock (Swift/Objc!)
> Diverging from this seems counter-productive.
>> Philosophy
>> Our philosophy with regard to pinning is that we actively want to encourage 
>> packages to develop against the latest semantically appropriate versions of 
>> their dependencies, in order to foster rapid development amongst the 
>> ecosystem and strong reliance on the semantic versioning concept. Our design 
>> for version pinning is thus intended to be a feature for package authors and 
>> users to use in crafting specific workflows, not be a mechanism by which 
>> most of the packages in the ecosystem pin themselves to specific versions of 
>> each other.
>> Use Cases
>> Our proposal is designed to satisfy several different use cases for such a 
>> behavior:
>> Standardizing team workflows
>> When collaborating on a package, it can be valuable for team members (and 
>> continuous integration) to all know they are using the same exact version of 
>> dependencies, to avoid "works for me" situations.
>> This can be particularly important for certain kinds of open source projects 
>> which are actively being cloned by new users, and which want to have some 
>> measure of control around exactly which available version of a dependency is 
>> selected.
>> Difficult to test packages or dependencies
>> Complex packages which have dependencies which may be hard to test, or hard 
>> to analyze when they break, may choose to maintain careful control over what 
>> versions of their upstream dependencies they recommend -- even if 
>> conceptually they regularly update those recommendations following the true 
>> semantic version specification of the dependency.
>> Dependency locking w.r.t. deployment
>> When stabilizing a release for deployment, or building a version of a 
>> package for deployment, it is important to be able to lock down the exact 
>> versions of dependencies in use, so that the resulting product can be 
>> exactly recreated later if necessary.
>> Proposed solution
>> We will introduce support for an optional new file Package.pins adjacent to 
>> the Package.swift manifest, called the "pins file". We will also introduce a 
>> number of new commands (see below) for maintaining the pins file.
>> This file will record the active version pin information for the package, 
>> including data such as the package identifier, the pinned version, and 
>> explicit information on the pinned version (e.g., the commit hash/SHA for 
>> the resolved tag).
>> The exact file format is unspecified/implementation defined, however, in 
>> practice it will be a JSON data file.
>> This file may be checked into SCM by the user, so that its effects apply to 
>> all users of the package. However, it may also be maintained only locally 
>> (e.g., placed in the .gitignore file). We intend to leave it to package 
>> authors to decide which use case is best for their project.
>> In the presence of a Package.pins file, the package manager will respect the 
>> pinned dependencies recorded in the file whenever it needs to do dependency 
>> resolution (e.g., on the initial checkout or when updating).
>> The pins file will not override Manifest specified version requirements and 
>> it will be an error (with proper diagnostics) if there is a conflict between 
>> the pins and the manifest specification.
>> Detailed Design
>> We will add a new command pin to swift package tool with following semantics:
>> $ swift package pin ( [--all] | [<package-name>] [<version>] ) [--message 
>> <message>]
>> The package-name refers to the name of the package as specified in its 
>> manifest.
>> This command pins one or all dependencies. The command which pins a single 
>> version can optionally take a specific version to pin to, if unspecified (or 
>> with --all) the behaviour is to pin to the current package version in use. 
>> Examples: 
>> $ swift package pin --all - pins all the dependencies.
>> $ swift package pin Foo - pins Foo at current resolved version.
>> $ swift package pin Foo 1.2.3 - pins Foo at 1.2.3. The specified version 
>> should be valid and resolvable.
>> The --reason option is an optional argument to document the reason for 
>> pinning a dependency. This could be helpful for user to later remember why a 
>> dependency was pinned. Example: 
>> $ swift package pin Foo --reason "The patch updates for Foo are really 
>> unstable and need screening."
>> Dependencies are never automatically pinned, pinning is only ever taken as a 
>> result of an explicit user action.
>> We will add a new command unpin:
>> $ swift package unpin ( [--all] | [<package-name>] )
>> This is the counterpart to the pin command, and unpins one or all packages.
>> We will fetch and resolve the dependencies when running the pin commands, in 
>> case we don't have the complete dependency graph yet.
>> We will extend the workflow for update to honour version pinning, that is, 
>> it will only update packages which are unpinned, and it will only update to 
>> versions which can satisfy the existing pins. The update command will, 
>> however, also take an optional argument --repin:
>> $ swift package update [--repin]
>> Update command errors if there are no unpinned packages which can be updated.
>> Otherwise, the behaviour is to update all unpinned packages to the latest 
>> possible versions which can be resolved while respecting the existing pins.
>> The [--repin] argument can be used to lift the version pinning restrictions. 
>> In this case, the behaviour is that all packages are updated, and packages 
>> which were previously pinned are then repinned to the latest resolved 
>> versions.
>> The update and checkout will both emit logs, notifying the user that pinning 
>> is in effect.
>> The swift package show-dependencies subcommand will be updated to indicate 
>> if a dependency is pinned.
>> As a future extension, we anticipate using the SHA information recorded in a 
>> pins file as a security feature, to prevent man-in-the-middle attacks on 
>> parts of the package graph.
>> Impact on existing code
>> There will be change in the behaviours of swift build and swift package 
>> update in presence of the pins file, as noted in the proposal however the 
>> existing package will continue to build without any modifications.
>> Alternative considered
>> We considered making the pinning behavior default on running swift build, 
>> however we think that pinning by default is likely to make the package graph 
>> more constrained than it should be. It drives the user away from taking full 
>> advantage of semantic versioning. We think it will be good for the package 
>> ecosystem if such a restriction is not the default behavior and that this 
>> design will lead to faster discovery of bugs and fixes in the upstream.
> I agree with the others that this is the better solution.
> With regards to the constraining problem, the key insight adopted by 
> Cargo/Yarn/Bundler is to distinguish libraries from applications. A library 
> shouldn’t pin its dependencies, while an application should. This ensures 
> that the ecosystem itself is maximally unconstrained, while ensuring actual 
> applications continue to reliably build, regardless of ecosystem changes and 
> the computer that it was built on. If a version of a library has trouble 
> building with different versions, it should ideally specify that with its 
> dependency constraints, not a lockfile.
> This also ensures that there’s diverse testing of versions: CI for 
> applications will verify particular configurations, while CI for libraries 
> will verify the latest-and-greatest works.
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