[Hugs] #65: depth cutoff reports the wrong constraint

20 Apr 2007

      #65: depth cutoff reports the wrong constraint
-------------------------+--------------------------------------------------
 Reporter:  ross         |       Owner:  nobody
     Type:  defect       |      Status:  new   
 Priority:  major        |   Milestone:        
Component:  type system  |     Version:  200609
 Keywords:               |  
-------------------------+--------------------------------------------------
Given the following:
 {{{
 class C a where
         f :: a -> a

 instance (Eq a, C a) => C a

 test = f True
 }}}
 hugs -98 reports
 {{{
 ERROR "Silly.hs" -
 *** The type checker has reached the cutoff limit while trying to
 *** determine whether:
 ***     Eq Bool
 *** can be deduced from:
 ***     ()
 *** This may indicate that the problem is undecidable.  However,
 *** you may still try to increase the cutoff limit using the -c
 *** option and then try again.  (The current setting is -c40)
 }}}
 when in fact it is trying to solve `C Bool`.

-- 
Ticket URL: <http://hackage.haskell.org/trac/hugs/ticket/65>
Hugs <http://www.haskell.org/hugs/>
Hugs 98, an interpreter for Haskell