[GHC] #9995: :info enhancements

[GHC]

#9995: :info enhancements
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        Reporter:  shachaf           |                   Owner:
            Type:  feature request   |                  Status:  new
        Priority:  normal            |               Milestone:
       Component:  GHCi              |                 Version:  7.8.4
      Resolution:                    |                Keywords:
Operating System:  Unknown/Multiple  |            Architecture:
 Type of failure:  None/Unknown      |  Unknown/Multiple
      Blocked By:                    |               Test Case:
 Related Tickets:                    |                Blocking:
                                     |  Differential Revisions:
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Old description:

> `:info` is a very useful ghci command, but there are several improvements
> that could be made:
>
> * 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.
> * 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.
> * `:i ->` is a parse error; it should behave like `:i (->)`, the way it
> works for other type-level operators.
> * 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.)

New description:

 `: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.)

--

Comment (by simonpj):

 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

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9995#comment:1>
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