Re: How does GHC handle TcPluginContradiction?

[Nicolas Frisby]

I also just found issue #22 in Adam's uom github repository; it seems
related.

"""
The plugin is correctly reporting the constraint Base "s" ~~ Base "m" ^: 2 as
insoluble (note the use of ~~, which is a type family provided by
uom-plugin that
is a pseudo-synonym of ~). However, it turns out that the GHC
error-reporting machinery

requires
a subtle invariant, namely that only certain kinds of constraint can be
regarded as insoluble. Equalities are included, but not irreducible
predicates such as ~~. Failure to respect this invariant means that errors
are silently ignored.
"""

The insoluble constraints in my actual plugin is a genuine class, so this
isn't an obvious perfect fit for my case.

On Sat, Aug 5, 2017 at 7:46 PM Nicolas Frisby 
wrote:

> Yes, thanks! I agree with you.
>
> I just realized on my dog walk that, according to
> https://ghc.haskell.org/trac/ghc/wiki/Plugins/TypeChecker, "If the plugin
> finds a contradiction amongst the givens, it should return
> TcPluginContradiction containing the contradictory constraints. These will
> turn into inaccessible code errors."
>
> And the brief demo in my previous email seems to indicate that
> "inaccessible code errors" are only realized as deferred type errors, with
> no static manifestation.
>
> Thanks. -Nick
>
>
> On Sat, Aug 5, 2017 at 7:27 PM Edward Z. Yang  wrote:
>
>> Hi Nicolas,
>>
>> While I am not 100% sure, I belive this is related to
>> the fact that currently inaccessible branches (a local
>> contradiction indicates inaccessibility) are not
>> being reported in GHC.  I reported this in
>> https://ghc.haskell.org/trac/ghc/ticket/12694
>> whose root cause was suggested might be
>> https://ghc.haskell.org/trac/ghc/ticket/12466
>>
>> Edward
>>
>> Excerpts from Nicolas Frisby's message of 2017-08-06 01:27:53 +:
>> > My TC plugin is identifying a contradiction, but that is not preventing
>> the
>> > module from type checking, nor does GHC even raising a warning (with
>> > -Wall). This is not the behavior I was expecting. How confused am I?
>> >
>> > I've distilled the behavior that's confusing me down to the following
>> > example
>> >
>> > > module Test where
>> > >
>> > > class C a where f :: a -> ()
>> >
>> > paired with a very silly plugin that always claims every constraint it
>> sees
>> > is a contradiction.
>> >
>> > > module Contrarian (plugin) where
>> > >
>> > > import Plugins (Plugin(..),defaultPlugin)
>> > > import TcPluginM (tcPluginIO)
>> > > import TcRnTypes (TcPlugin(..),TcPluginResult(TcPluginContradiction))
>> > >
>> > > plugin :: Plugin
>> > > plugin = defaultPlugin { tcPlugin = const (Just contrarianPlugin) }
>> > >
>> > > contrarianPlugin :: TcPlugin
>> > > contrarianPlugin = TcPlugin {
>> > > tcPluginInit = return ()
>> > >   ,
>> > > tcPluginSolve = \() given derived wanted -> do
>> > >   tcPluginIO $ putStrLn "Alternative facts!"
>> > >   return $ TcPluginContradiction $ given ++ derived ++ wanted
>> > >   ,
>> > > tcPluginStop  = \() -> return ()
>> > >   }
>> >
>> > Please review the following GHCi session. We start *without* the plugin.
>> >
>> > $ ghc --interactive Test.hs
>> > GHCi, version 8.2.1: http://www.haskell.org/ghc/  :? for help
>> > [1 of 1] Compiling Test ( Test.hs, interpreted )
>> > Ok, 1 module loaded.
>> > *Test> f ()
>> >
>> > :1:1: error:
>> > * No instance for (C ()) arising from a use of `f'
>> > * In the expression: f ()
>> >   In an equation for `it': it = f ()
>> >
>> > That behavior meets my expectations. Now let's add the plugin to see
>> what
>> > contradictions cause to happen --- my naive guess is that we should see
>> the
>> > same thing. (I don't really know what contradictions *should* cause GHC
>> to
>> > do *differently* than the above.) The following is a continuation of the
>> > same session.
>> >
>> > *Test> :set -fplugin=Contrarian
>> > *Test> f ()
>> > Alternative facts!
>> > Alternative facts!
>> > Alternative facts!
>> > Alternative facts!
>> > *** Exception: :3:1: error:
>> > * No instance for (C ()) arising from a use of `f'
>> > * In the expression: f ()
>> >   In an equation for `it': it = f ()
>> > (deferred type error)
>> > *Test> :t f
>> > Alternative facts!
>> > f :: a -> ()
>> >
>> > Interesting: claiming the constraint is a contradiction seems to convert
>> > the type error to a deferred type error. Let's see if we can disable
>> that
>> > (since it's not the behavior I want).
>> >
>> > *Test> :set -fno-defer-type-errors
>> > *Test> :t f
>> > Alternative facts!
>> > f :: a -> ()
>> > *Test> f ()
>> > Alternative facts!
>> > Alternative facts!
>> > Alternative facts!
>> > Alternative facts!
>> > *** Exception: :3:1: error:
>> > * No instance 

Re: How does GHC handle TcPluginContradiction?

[Nicolas Frisby]

Yes, thanks! I agree with you.

I just realized on my dog walk that, according to
https://ghc.haskell.org/trac/ghc/wiki/Plugins/TypeChecker, "If the plugin
finds a contradiction amongst the givens, it should return
TcPluginContradiction containing the contradictory constraints. These will
turn into inaccessible code errors."

And the brief demo in my previous email seems to indicate that
"inaccessible code errors" are only realized as deferred type errors, with
no static manifestation.

Thanks. -Nick

On Sat, Aug 5, 2017 at 7:27 PM Edward Z. Yang  wrote:

> Hi Nicolas,
>
> While I am not 100% sure, I belive this is related to
> the fact that currently inaccessible branches (a local
> contradiction indicates inaccessibility) are not
> being reported in GHC.  I reported this in
> https://ghc.haskell.org/trac/ghc/ticket/12694
> whose root cause was suggested might be
> https://ghc.haskell.org/trac/ghc/ticket/12466
>
> Edward
>
> Excerpts from Nicolas Frisby's message of 2017-08-06 01:27:53 +:
> > My TC plugin is identifying a contradiction, but that is not preventing
> the
> > module from type checking, nor does GHC even raising a warning (with
> > -Wall). This is not the behavior I was expecting. How confused am I?
> >
> > I've distilled the behavior that's confusing me down to the following
> > example
> >
> > > module Test where
> > >
> > > class C a where f :: a -> ()
> >
> > paired with a very silly plugin that always claims every constraint it
> sees
> > is a contradiction.
> >
> > > module Contrarian (plugin) where
> > >
> > > import Plugins (Plugin(..),defaultPlugin)
> > > import TcPluginM (tcPluginIO)
> > > import TcRnTypes (TcPlugin(..),TcPluginResult(TcPluginContradiction))
> > >
> > > plugin :: Plugin
> > > plugin = defaultPlugin { tcPlugin = const (Just contrarianPlugin) }
> > >
> > > contrarianPlugin :: TcPlugin
> > > contrarianPlugin = TcPlugin {
> > > tcPluginInit = return ()
> > >   ,
> > > tcPluginSolve = \() given derived wanted -> do
> > >   tcPluginIO $ putStrLn "Alternative facts!"
> > >   return $ TcPluginContradiction $ given ++ derived ++ wanted
> > >   ,
> > > tcPluginStop  = \() -> return ()
> > >   }
> >
> > Please review the following GHCi session. We start *without* the plugin.
> >
> > $ ghc --interactive Test.hs
> > GHCi, version 8.2.1: http://www.haskell.org/ghc/  :? for help
> > [1 of 1] Compiling Test ( Test.hs, interpreted )
> > Ok, 1 module loaded.
> > *Test> f ()
> >
> > :1:1: error:
> > * No instance for (C ()) arising from a use of `f'
> > * In the expression: f ()
> >   In an equation for `it': it = f ()
> >
> > That behavior meets my expectations. Now let's add the plugin to see what
> > contradictions cause to happen --- my naive guess is that we should see
> the
> > same thing. (I don't really know what contradictions *should* cause GHC
> to
> > do *differently* than the above.) The following is a continuation of the
> > same session.
> >
> > *Test> :set -fplugin=Contrarian
> > *Test> f ()
> > Alternative facts!
> > Alternative facts!
> > Alternative facts!
> > Alternative facts!
> > *** Exception: :3:1: error:
> > * No instance for (C ()) arising from a use of `f'
> > * In the expression: f ()
> >   In an equation for `it': it = f ()
> > (deferred type error)
> > *Test> :t f
> > Alternative facts!
> > f :: a -> ()
> >
> > Interesting: claiming the constraint is a contradiction seems to convert
> > the type error to a deferred type error. Let's see if we can disable that
> > (since it's not the behavior I want).
> >
> > *Test> :set -fno-defer-type-errors
> > *Test> :t f
> > Alternative facts!
> > f :: a -> ()
> > *Test> f ()
> > Alternative facts!
> > Alternative facts!
> > Alternative facts!
> > Alternative facts!
> > *** Exception: :3:1: error:
> > * No instance for (C ()) arising from a use of `f'
> > * In the expression: f ()
> >   In an equation for `it': it = f ()
> > (deferred type error)
> >
> > :7:1: error:
> > * No instance for (Show (a0 -> ())) arising from a use of `print'
> > (maybe you haven't applied a function to enough arguments?)
> > * In a stmt of an interactive GHCi command: print it
> > *Test>
> >
> > I seems we can't disable this behavior. That makes it seem pretty
> > fundamental. So I'm guessing I'm confused about something, probably what
> > TcContradiction achieves in terms of the user experience.
> >
> > The behavior I want is for my plugin to identify (relevant)
> contradictions
> > and incur a static type error as soon as possible. I thought
> contradictions
> > would achieve that, but I seem to be wrong.
> >
> > What I should be expecting regarding contradictions?
> >
> > How should I achieve my desired behavior? Perhaps I should try "solving"
> > the contradiction