Re: [commit: ghc] master: Add ":info!" to GHCi. This shows all instances without filtering first. (2ec32a8)

Iavor Diatchki <[email protected]> Mon, 3 Dec 2012 12:13:49 -0800
Newsgroups	gmane.comp.lang.haskell.cvs.ghc
Message-ID	<CAGK9nupnqNKSP07EX8s9_j=AM3nJYUcrTKZ7zufj5qCVfp569w@mail.gmail.com>
Hello,

On Mon, Dec 3, 2012 at 8:44 AM, Simon Marlow <[email protected]> wrote:

> OI know that Accessor has an Applicative instace because the operations
>>
>> work.  I'd like to find the instance, so I can see how it works, so I try:
>>
>
> Ok, so this is a bug: you shouldn't be able to use the instance because it
> isn't in scope.  If this was a source file, then GHC would complain that
> the instance was not in scope.
>
> The bug is (sort of) documented in the "Known Bugs" section of the user
> guide, although the documentation incorrectly says that it also affects
> --make, which it doesn't (I'll fix it).
>

I don't think that this is the bug to blame: the instance is in scope, it
is just that it is being filtered by ":info".
Here is an example:

module Test where

import Control.Applicative (pure)
import Control.Lens

example :: Accessor () ()
example = pure ()

This works just fine.  I think the issue is as follows.  The `Applicative`
instance for `Accessor` is like this:

instance Monoid r => Applicative (Accessor r)

Now, on the GHCi command line `Accessor` and `Applicative` are in scope but
`Monoid` is not.  However, there are instances of `Monoid` for various
datatypes (e.g., ()) that are also in scope, so that instance is actually
usable.

As far as I understand, the current plausiblity check filters out any
instances that contain tycons that are not in-scope, which is why this
particular instance does not show up.  It looks like in some cases this is
too aggressive.

So I don't really object to having this "feature", as long as we say
> clearly in the documentation that it doesn't have a well-specified
> behaviour, and the instances it shows may or may not actually be available.
> (if we fix the bug, many of them won't be available, but it might be useful
> to find out where to get them from).
>
> Would you mind updating the docs, and close #5998?
>

Yeah, I'd be happy to do that.  Which documentation should I update?

-Iavor





>
> Cheers,
>         Simon
>
>
>
>  Prelude Control.Applicative Control.Lens> :i Accessor
>> newtype Accessor r a
>>    = Control.Lens.Internal.Accessor {Control.Lens.Internal.**runAccessor
>> :: r}
>> -- Defined in `Control.Lens.Internal'
>> instance Functor (Accessor r) -- Defined in `Control.Lens.Internal'
>> instance Gettable (Accessor r)
>>    -- Defined in `Control.Lens.Internal’
>>
>> Weird, it doesn’t show up, so what are the instances of `Applicative`?
>>
>> Prelude Control.Applicative Control.Lens> :i Applicative
>> class Functor f => Applicative f where
>>    pure :: a -> f a
>>    (<*>) :: f (a -> b) -> f a -> f b
>>    (*>) :: f a -> f b -> f b
>>    (<*) :: f a -> f b -> f a
>> -- Defined in `Control.Applicative'
>> instance Applicative [] -- Defined in `Control.Applicative'
>> instance Applicative ZipList -- Defined in `Control.Applicative'
>> instance Monad m => Applicative (WrappedMonad m)
>>    -- Defined in `Control.Applicative'
>> instance Applicative Maybe -- Defined in `Control.Applicative'
>> instance Applicative IO -- Defined in `Control.Applicative'
>> instance Applicative (Either e) -- Defined in `Control.Applicative'
>> instance Applicative ((->) a) -- Defined in `Control.Applicative'
>> instance Applicative Mutator -- Defined in `Control.Lens.Internal'
>> instance Applicative (Bazaar a b)
>>    -- Defined in `Control.Lens.Internal’
>>
>> It does not show up, but I'm sure that there is an instance as the
>> operations seem to work! It turns out that the only way to find the
>> instance is to not only already know that there is one and import the
>> appropriate module, but to also import the modules used in the context.
>> But if I already knew all of this I wouldn't have asked GHCi.
>>
>> Prelude Control.Applicative Control.Lens> import Data.Monoid
>> Prelude Control.Applicative Control.Lens Data.Monoid> :i Accessor
>> newtype Accessor r a
>>    = Control.Lens.Internal.Accessor {Control.Lens.Internal.**runAccessor
>> :: r}
>> -- Defined in `Control.Lens.Internal'
>> instance Functor (Accessor r) -- Defined in `Control.Lens.Internal'
>> *instance Monoid r => Applicative (Accessor r)*
>>
>>    -- Defined in `Control.Lens.Internal'
>> instance Gettable (Accessor r)
>>    -- Defined in `Control.Lens.Internal'
>>
>> In contrast, with ":info!" we get everything that GHCi knows about, so
>> it is quite easy to figure out what's going on.
>>
>> -Iavor
>>
>>
>>
>>
>>
>> On Fri, Nov 30, 2012 at 3:42 AM, Simon Marlow <[email protected]
>> <mailto:[email protected]>> wrote:
>>
>>     On 30/11/12 02:03, Iavor Diatchki wrote:
>>
>>         Repository : ssh://darcs.haskell.org//srv/_**_darcs/ghc<http://darcs.haskell.org//srv/__darcs/ghc>
>>         <http://darcs.haskell.org//**srv/darcs/ghc<http://darcs.haskell.org//srv/darcs/ghc>
>> >
>>
>>         On branch  : master
>>
>>         http://hackage.haskell.org/__**trac/ghc/changeset/__**
>> 2ec32a8e1cb323b230b0c228dbee31**__3647892bf4<http://hackage.haskell.org/__trac/ghc/changeset/__2ec32a8e1cb323b230b0c228dbee31__3647892bf4>
>>         <http://hackage.haskell.org/**trac/ghc/changeset/**
>> 2ec32a8e1cb323b230b0c228dbee31**3647892bf4<http://hackage.haskell.org/trac/ghc/changeset/2ec32a8e1cb323b230b0c228dbee313647892bf4>
>> >
>>
>>             ------------------------------**
>> __----------------------------**--__---
>>
>>
>>         commit 2ec32a8e1cb323b230b0c228dbee31**__3647892bf4
>>
>>         Author: Iavor S. Diatchki <[email protected]
>>         <mailto:[email protected]>>
>>
>>         Date:   Thu Nov 29 17:14:48 2012 -0800
>>
>>               Add ":info!" to GHCi.  This shows all instances without
>>         filtering first.
>>
>>               The default behavior of :info is to show only those
>>         instances of
>>               for a type, where all relevant type constructor names are
>>         in scope.
>>               This keeps down the number of instances shown to the user.
>>
>>               In some cases, it is nice to be able to see all instances
>>         for a type.
>>               This patch implements this with the :info! command.
>>
>>
>>     Was there some discussion about this?  The last I remember was this:
>>
>>     http://hackage.haskell.org/__**trac/ghc/ticket/5998<http://hackage.haskell.org/__trac/ghc/ticket/5998>
>>
>>     <http://hackage.haskell.org/**trac/ghc/ticket/5998<http://hackage.haskell.org/trac/ghc/ticket/5998>
>> >
>>
>>     where we decided not to implement this because it is essentially a
>>     random UI: the behaviour can't be described sensibly because it
>>     depends on which interface files GHC happens to have seen so far.
>>
>>     There's also this:
>>
>>     http://hackage.haskell.org/__**trac/ghc/ticket/3080<http://hackage.haskell.org/__trac/ghc/ticket/3080>
>>
>>     <http://hackage.haskell.org/**trac/ghc/ticket/3080<http://hackage.haskell.org/trac/ghc/ticket/3080>
>> >
>>
>>     which looks like a duplicate (I just closed it).
>>
>>     Cheers,
>>              Simon
>>
>>
>>
>>
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>> >
>>
>>
>>
>
>

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