Re: [GHC] #11220: Stack overflow instead of type check failure in Servant route

14 Dec 2015

      #11220: Stack overflow instead of type check failure in Servant route
-------------------------------------+-------------------------------------
        Reporter:  sopvop            |                Owner:
            Type:  bug               |               Status:  new
        Priority:  high              |            Milestone:  8.0.1
       Component:  Compiler (Type    |              Version:  7.10.3
  checker)                           |
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Compile-time      |  Unknown/Multiple
  crash                              |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------

Comment (by goldfire):

 You have this definition:

 {{{
 type family IsSubList a b :: Constraint where
   IsSubList '[] b          = ()
   IsSubList '[x] (x ': xs) = ()
   IsSubList '[x] (y ': ys) = IsSubList '[x] ys
   IsSubList (x ': xs) y    = IsSubList '[x] y `And` IsSubList xs y
 }}}

 But `IsSubList '[Int] '[]` loops by triggering the fourth equation (and
 being apart from all previous ones). Note that the left conjunct of the
 `And` is the exact same thing that we started with.

 So I think GHC's behavior here is quite reasonable.

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11220#comment:6
GHC http://www.haskell.org/ghc/
The Glasgow Haskell Compiler