Proposal: Allow installing the latest package of a major version

[jfmengels]

Quick Summary: I want a way to install a package knowing only its major version like elm install author/package@2.

This is a proposal that is an addition to the proposals to install specific versions of a package (that I'm sure exists somewhere).

This is a feature request for when the package installer becomes the next focus. I imagine that when that is released elm-json would become obsolete and that elm becomes the only tool that users will have to use. This proposal aims to indicate some use-cases that elm-json currently fills for tooling purposes, so that they also be included and that batch of work.

Use-case

While this could potentially be useful for Elm developers when they want to use the latest version of an outdated major version of a package, this would mostly be for tooling.

This behavior currently exists in elm-json, and elm-review depends on it. I'll explain the process for the latter.

elm-review takes the user's configuration, which is an Elm application with configuration as Elm code, and a list of elm-review packages as dependencies (and maybe other Elm packages if they wrote custom rules).

It then

• creates a temporary application folder
• copies the elm.json
• changes the source-directories from the original elm.json to point to the correct location
• adds the source-directory for the elm-review that contains the main application logic to the source-directories
• Changes the dependencies to include the ones that the main application logic requires

The last step is done using elm-json install stil4m/elm-syntax@7 <and a couple more...>.

The dependencies are added so that users don't have to do so themselves and get a hard to understand error when they forget to do so.

The reason for indicating the major version is that if one of these packages releases a new major version incompatible with the previous major version, we don't force the latest major version and cause all current elm-review configuration to become un-compilable and un-runnable.

In practice, elm-review doesn't run elm-json install stil4m/elm-syntax@7 ... (at least for this), but instead elm-json solve stil4m/elm-syntax@7 ... <path-to-original-elm.json> which allows getting (and caching) the list of dependencies so that we don't have to write the elm.json file and then run elm-json install ... in a second step.

elm-review still uses install with the major version constraint when initializing a new configuration. To avoid asking the user to confirm the install prompt, elm-json install supports a --yes flag.

Summary of requests

• Support a way to constrain a package to a major version
• Add a --yes option to the install command (or similar)
• (Potentially?) Support a way to compute dependencies without installing them directly like elm-json solve.

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