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Russel's paradox makes the simplifier diverge

This ticket is the Master Ticket for a long list of bug reports, when users have used contravariant self-application to make GHC's Simplifier diverge.

Here are the tickets. They should be labelled Russell paradox:

GHC's simplifier can go into a loop if you use fixpoints in a funny way, as documented here http://www.haskell.org/ghc/docs/latest/html/users_guide/bugs.html#bugs-ghc (12.2.1, third bullet). But GADTs provide new opportunities. This ticket records one, thanks to Ryan Ingram and Matthieu Sozeau:

{-# LANGUAGE GADTs #-}
module Contr where

newtype I f = I (f ())
data R o a where R :: (a (I a) -> o) -> R o (I a)

run :: R o (I (R o)) -> R o (I (R o)) -> o
run x (R f) = f x
rir :: (R o) (I (R o))
rir = R (\x -> run x x)

absurd :: a
absurd = run rir rir

Now the simplifier can loop. Look:

run rir rir

= {inline run}
  case rir of R f -> f rir

= {case of known constructor}
  let { f = \x -> run x x } in f rir

= {inline f}
  run rir rir
Trac metadata
Trac field Value
Version 6.12.1
Type Bug
TypeOfFailure OtherFailure
Priority normal
Resolution Unresolved
Component Compiler
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system
Architecture
Edited by Simon Peyton Jones
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