:info enhancements

:info is a very useful ghci command, but there are several improvements that could be made:

1. It would be useful if :i showed not only which module a name is defined in, but which module(s) it was imported from in the current session.
2. A variant of :i that just prints a definition, rather than a definition and all instances, would be useful in many cases. E.g. :i Show prints the definition of Show, but then it prints so many instances that the definition is scrolled way off the screen.
3. :i -> is a parse error; it should behave like :i (->), the way it works for other type-level operators.
4. hvr had some more suggestions: "finding out which package a module belongs to", "and what module an 'as'-alias is associated with". (Currently I think modules are out of :info's scope, but perhaps it would be a useful addition.)

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Sounds good to me.

1. I think this is pretty easy; this info is kept in the GlobalRdrEnv. Someone would have to figure out what to say if something was in scope in several different ways.
2. Even easier; but now you have to specify what the user should say to get the two different behaviours. Perhaps :info for the terse behaviour and :info! to add the instances? (Though that would be a change.) Oh, now I look I see that :info! is already used for something to do with instances. So maybe :info, :info!, and :info!!? Or we could drop instances altogether from :info, and instead have a new command :instances T.
3. Sounds plausible. "Just" a parser issue.
4. This is really :info for modules, isn't it? So perhaps it's a variant of :browse (which tells you stuff about modules)?

Anyway, if anyone wants to work on the specification, get agreement, and work on implementation, I can advise. Nothing technically difficult here I think.

Simon

A patch for point 2 (:instances) is in the works in #2986.

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Related	→ #2986

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