[GHC] #11298: Implicit call stack empty in instance declarations

27 Dec 2015

      #11298: Implicit call stack empty in instance declarations
-------------------------------------+-------------------------------------
           Reporter:  pikajude       |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  7.11
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  Incorrect result
  Unknown/Multiple                   |  at runtime
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 Given the following code:

 {{{#!hs
 {-# LANGUAGE ImplicitParams #-}

 import GHC.Stack

 class Foo a where
     foo :: a -> String

 main = putStrLn $ foo ()
 }}}

 In GHC 7.11.20151216, the following instances result in no output:

 {{{#!hs
 instance Foo () where
     foo () = prettyCallStack ?loc
 }}}

 {{{#!hs
 fooHelper = prettyCallStack ?loc

 instance Foo () where
     foo () = fooHelper
 }}}

 Though this one does:

 {{{#!hs
 fooHelper () = prettyCallStack ?loc

 instance Foo () where
     foo = fooHelper
 }}}

 The aforementioned instances all yield output with GHC 7.10.3 (after
 replacing `prettyCallStack` with `showCallStack`).

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11298
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