Better error messages for implicit contract with generated code

[dillonkearns]

The generated code in elm-pages has an implicit contract. It is implicit unlike the contracts we're used to in Elm because the user's code is imported and used by generated code, so error messages will point to generated files as the source of the error which is misleading because the fix is to change something in user-maintained code.

Examples:

• The type of RouteParams in a Page Module must have the expected record fields (all of type String), as used in the elm-pages add <My.Page.Route> template
• Page Modules must expose types called Model and Msg (with no type variables)

Since Elm and elm-pages strive to have friendly error messages that give you clear actionable feedback, it would be nice to improve this.

Considerations

• It would be great to avoid hacks and instead have robust, maintainable code to detect these problems
• The compilation step in the dev server in particular is performance critical as we want to do a hot reload and regenerate DataSources as fast as possible. So ideally this should not have any impact on how quickly we can do hot reloads in the dev server, and shouldn't slow down the elm-pages build command either

I think that we could achieve this functionality in a performant and maintainable way if we used elm-review to run these checks, but only ran it after running elm make and seeing that there is a compiler error pointing to a generated module. Ideally, users should never see an error message pointing to generated code since this isn't actionable for them. And we can avoid paying a performance penalty for all cases besides the ones we're interested in by checking that condition.

[dillonkearns]

I created a starting point for an elm-review rule:

• https://github.com/dillonkearns/elm-pages/blob/83e1aeddb0e90905773fea39639b073eb5c1db49/src/Pages/Review/NoContractViolations.elm
• https://github.com/dillonkearns/elm-pages/blob/83e1aeddb0e90905773fea39639b073eb5c1db49/tests/Pages/Review/NoContractViolationsTest.elm

This can be used as a standalone rule that gives users feedback in their editor if they use VS Code with elm-review configured to run inline. That will give us a way to iterate on the review rule and try it out before integrating it to run directly from elm-pages for the build and dev server.

Eventually, we can wire it in to run from elm-pages and use the JSON report format to wire in errors to display just as an Elm compiler error would (whether in the dev server browser overlays or in the CLI).

[dillonkearns]

I created a project board to track ideas and tasks: https://github.com/dillonkearns/elm-pages/projects/4.

[dillonkearns]

The wiring is now complete as of NPM package 2.1.9.