Hello,
On Mon, Dec 3, 2012 at 8:44 AM, Simon Marlow <marlo...@gmail.com> 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 <marlo...@gmail.com
>> <mailto:marlo...@gmail.com>> 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 <diatc...@galois.com
>> <mailto:diatc...@galois.com>>
>>
>> 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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