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 <ezyang at mit.edu> 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 +0000:
> > 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 ()
> >
> >     <interactive>: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: <interactive>: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: <interactive>: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)
> >
> >     <interactive>: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 constraints via an appeal to a TypeError constraint
> > instead. Or maybe I should just leave the identified contradictions
> > unsolved?
> >
> > Thanks. -Nick
> >
> > PS - A big thank you to Adam Gundry and Christiaan Baaij (and probably
> > others) for the useful wiki pages, blog posts, and publications --
> they've
> > been a huge help for my learning about the typechecker via plugins! I'm
> > very excited about this plugin, and I hope to share it soon, but I'd like
> > to understand it better before I do.
>
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