Re: [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
      Resolution:                    |             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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Description changed by pikajude:

Old description:
...
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
}}}
Including explicit signatures with `-XInstanceSigs` has no effect.
The aforementioned instances all yield output with GHC 7.10.3 (after
replacing `prettyCallStack` with `showCallStack`):
{{{
?loc, called at implicit.hs:9:28 in main:Main
}}}
New description:

 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

 {-
 CallStack (from ImplicitParams):
   fooHelper, called at implicit.hs:11:11 in main:Main
 -}
 }}}

 Including explicit signatures with `-XInstanceSigs` has no effect.

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

 {{{
 ?loc, called at implicit.hs:9:28 in main:Main
 }}}

--

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Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11298#comment:3
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