ghc panic Simplifier ticks exhausted when trying UnfoldingDone x_alB

The following code causes a panic. It's the Y combinator.

module Y where

newtype F a = F { unF :: F a -> a }

y :: (a -> a) -> a
y = \f -> (\x -> x (F x)) (\x -> f (unF x x))

bash$ ~/src/ghc-7.10.0.20150123/installed/bin/ghc -c Y.hs
ghc: panic! (the 'impossible' happened)
  (GHC version 7.10.0.20150123 for x86_64-unknown-linux):
	Simplifier ticks exhausted
  When trying UnfoldingDone x_alB
  To increase the limit, use -fsimpl-tick-factor=N (default 100)
  If you need to do this, let GHC HQ know, and what factor you needed
  To see detailed counts use -ddump-simpl-stats
  Total ticks: 4722

Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug

However it works fine in GHCi

bash$ ~/src/ghc-7.10.0.20150123/installed/bin/ghci Y.hs
GHCi, version 7.10.0.20150123: http://www.haskell.org/ghc/  :? for help
[1 of 1] Compiling Y                ( Y.hs, interpreted )
Ok, modules loaded: Y.
*Y> take 10 (y (1:))
[1,1,1,1,1,1,1,1,1,1]

I have tried this code with 7.6.3, 7.8.3 and 7.10.1-rc2. Same result.

At least in 7.10.1-rc2 it doesn't work even with -fsimpl-tick-factor=100000 (that's 10^5^)

Trac metadata

Trac field	Value
Version	7.10.1-rc2
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system
Architecture

changed weight to 5

added Tbug Trac import labels

In case you didn't get it, I installed 7.10.1-rc2 in prefix ~/src/ghc-7.10.0.20150123/installed/

Yes indeed: see the https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/bugs.html user manual]. At least it no longer loops!

Simon

Oh yes. Actually I knew that before but in this case it just says "panic" rather than looping so I thought it was irrlevant.

Maybe fix the docs?

Also an idea: Maybe we can do positivity analysis on data types just like Agda. We don't have to refuse it, just add a check when compiling functions that deal with it (and maybe a warning?)

mentioned in commit d2e6a3b5

That's great! thanks

closed

Trac metadata

Trac field	Value
Resolution	Unresolved → ResolvedDuplicate
Related	- → #10372 (closed)

added Pnormal label