Opened 3 years ago

Last modified 18 months ago

#12780 new bug

Calling "do nothing" type checker plugin affects type checking when it shouldn't

Reported by: clinton Owned by:
Priority: normal Milestone:
Component: Compiler Version: 8.0.1
Keywords: TypeCheckerPlugins Cc:
Operating System: Unknown/Multiple Architecture: Unknown/Multiple
Type of failure: GHC rejects valid program Test Case:
Blocked By: Blocking:
Related Tickets: #11525 Differential Rev(s):
Wiki Page:

Description (last modified by clinton)

The following has been compiled against ghc version 8.1.20161022, my apologies if this bug has already been fixed.

The following program compiles fine:

{-# LANGUAGE TypeFamilyDependencies #-}
{-# LANGUAGE AllowAmbiguousTypes #-}
{-# LANGUAGE ConstraintKinds #-}

main :: IO ()
main = return ()

type family F p = t | t -> p

type IsF p t = (C p, t ~ F p)

class C p where
  f :: (IsF p t) => t
  f = undefined

g :: (IsF p t) => t
g = f

But if we define a "do nothing" type checker plugin like so:

{-# OPTIONS_GHC -dynamic-too #-}

module MyPlugin (plugin) where

import Plugins (
  defaultPlugin, tcPlugin
import TcRnTypes (
  TcPlugin(TcPlugin), tcPluginInit, tcPluginSolve, tcPluginStop,
  TcPluginResult(TcPluginContradiction, TcPluginOk)

plugin :: Plugin
plugin = defaultPlugin { tcPlugin = const (Just myPlugin)  }

myPlugin :: TcPlugin
myPlugin =
      tcPluginInit  = return (),
      tcPluginSolve = const solver,
      tcPluginStop  = const (return ())

solver :: TcPluginSolver
solver _ _ _ = return $ TcPluginOk [] []

And call it from our original source file by adding:

{-# OPTIONS_GHC -fplugin MyPlugin #-}

We get the following error:

pluginbug.hs:18:5: error:
    • Could not deduce (C p0, t ~ F p0) arising from a use of ‘f’
      from the context: IsF p t
        bound by the type signature for:
                   g :: IsF p t => t
        at pluginbug.hs:17:1-19
      The type variable ‘p0’ is ambiguous
      Relevant bindings include g :: t (bound at pluginbug.hs:18:1)
    • In the expression: f
      In an equation for ‘g’: g = f

Note that not just calling my "do nothing" type checking plugin does this, to be sure, instead I called ghc-typelits-natnormalise like so:

{-# OPTIONS_GHC -fplugin GHC.TypeLits.Normalise #-}

And the same error results.

One solution is to remove (C p) from the IsF type declaration, and instead adding it to gs definition, like so:

--type IsF p t = (C p, t ~ F p)
  type IsF p t = (t ~ F p)

--g :: (IsF p t) => t
  g :: (C p, IsF p t) => t

In addition, either of the following two solutions will get the code to compile:

  1. Replacing the reference to IsF p t in the class signature with t ~ F p like so:
--  f :: (IsF p t) => t
  f :: (t ~ F p) => t
  1. Alternatively, expanding out IsF p t in the signature of g as follows:
--g :: (IsF p t) => t
g :: (C p, t ~ F p) => t

Note that is we remove the class C entirely (making f an ordinary function) the bug disappears, but if instead replace the class with a constraint C like so:

type family C t :: Constraint

the bug still appears.

So it seems this bug needs four things to all occur for it to appear:

  1. A type constraint synonym in the calling code.
  2. A type constraint synonym in the called code.
  3. The type constraint synonym to contain a class constraint, or at least a non-equality one.
  4. A type checking plugin to be called.

Note that the error requires a type checking plugin to be called, if one does not override defaultPlugin like so:

plugin = defaultPlugin { tcPlugin = const (Just myPlugin) }

and instead just does:

plugin = defaultPlugin

then the issue will not occur.

So in summary, it seems that calling a type checking plugin somehow affects ordinary type checking in particular circumstances. I don't think this should happen.

Attachments (2)

MyPlugin.hs (603 bytes) - added by clinton 3 years ago.
pluginbug.hs (517 bytes) - added by clinton 3 years ago.

Download all attachments as: .zip

Change History (7)

Changed 3 years ago by clinton

Attachment: MyPlugin.hs added

comment:1 Changed 3 years ago by clinton


comment:2 Changed 3 years ago by clinton

Description: modified (diff)

Changed 3 years ago by clinton

Attachment: pluginbug.hs added

comment:3 Changed 3 years ago by clinton


comment:4 Changed 3 years ago by darchon

comment:5 Changed 18 months ago by adamgundry

Keywords: TypeCheckerPlugins added

This is fixed in 8.2.1 and later, I think?

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