Jump to instance definition and explain typeclass evidence for GHC 9

[wz1000]

Now that we have GHC 9, it is easy to implement "Jump to instance definition" as well as show exactly
the instances being used at a point on hover.

TODO

• Extend to references - UI is a bit tricky here
• Improve markdown rendering - @isovector did you figure out how to have inline syntax highlighting yet?
• Fix tests
• Clean up CPP?

[pepeiborra]

Not strictly related to this change, but I wonder if it's possible to show the instances in scope when hovering on a type?

[wz1000]

Not strictly related to this change, but I wonder if it's possible to show the instances in scope when hovering on a type?

Yes it is. In fact, local completions will already (incorrectly) suggest local instances in scope (i.e. those arising from type signature contexts and case splits) if you start completing using $d.... I will fix this now.

For top level instances, we can use the same mechanism used by GHCi for :instances.

[wz1000]

Some opportunities for bikeshedding:

• We can probably do away with the top level bullet point
• "defined using"/"constructed from"?
• "evidence of constraint" or simply "evidence" or even "dictionary of type"?
  • Maybe omit the words "evidence of constraint" etc. from all the lower level bullet points as it is just noise at that point

[pepeiborra]

Some opportunities for bikeshedding:

• We can probably do away with the top level bullet point
• "defined using"/"constructed from"?
• "evidence of constraint" or simply "evidence" or even "dictionary of type"?

"from instance", "via instance" or simply "instance"?

• Maybe omit the words "evidence of constraint" etc. from all the lower level bullet points as it is just noise at that point

+1

[wz1000]

"from instance", "via instance" or simply "instance"?

It is not always an instance, see for example:

[pepeiborra]

@wz1000 are you planning to land this soon or could someone else work on it?
@July541 would you be interested in wrapping this up?

[July541]

It's a very practical function! I'm glad to push it far more with @wz1000' willing.

BTW, have this feature been integrated into hiedb? Call hierarchy can also jump to the corresponding instance with this.

[jneira]

Needs a non trivial (for me :-P ) rebase

[wz1000]

@July541 yes, feel free to pick this up. hiedb to some extent will "just work" with these changes, but it may also need some special support somewhere depending on what exactly you need to do.

[July541]

@wz1000 Hello, do you mind explaining some details?

1. For Extend to references, what should it be? Should it remove irrelative instances?
2. What should improve in markdown?

[fendor]

Rebase and finished in #4392