[GHC] #11049: Allow CallStacks to be hidden or cut

[GHC]

#11049: Allow CallStacks to be hidden or cut
-------------------------------------+-------------------------------------
        Reporter:  nomeata           |                Owner:  gridaphobe
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  7.10.2
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #11035            |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by nomeata):

 Sure.

 Assume we are given a function
 {{{#!hs
 readFile :: ?callstack :: CallStack => FilePath -> IO ()
 }}}
 that prints a stack trace with every IO error that happens (`bar` in the
 description).

 Now assume we want to implement a function that uses the above, for
 example
 {{{#!hs
 readConfig :: IO Config
 readConfig = do
   s <- readFile "config.txt" -- line 42
   return (parseConfig s)
 }}}
 and we deliberately do not want that function have a `?_::CallStack`
 constraint. What will happen when `config.txt` will be missing? `readFile`
 will raise an exception that includes a call stack that tells the user
 that this was raised due to `readFile` being called in line 42. But as the
 author of the `readConfig` function, I do not want this information (which
 is not very helpful to the user) to be omitted. This is the first use
 case.

 The second is related. I might now allow `readConfig` to have a
 `?_::CallStack` constraint, e.g.
 {{{#!hs
 readConfig :: ?callstack::CallStack => IO Config
 readConfig = do
   s <- readFile "config.txt" -- line 42
   return (parseConfig s)
 }}}
 But I do want to provide a polished API, and not leak any information to
 the users of my API about my internals. So _do_ want the `?callstack` to
 be passed on to `readFile` and be included in the exception, but I _don’t_
 want it to mention line 42; instead it should end with the (for the user
 relevant) information where `readConfig` was called. This is the second
 use case.


 So now to the suggested implementation: In both cases, I want to insert a
 marker into the callstack that makes the call stack printer ignore
 anything “below” or “after” it. This is the suggested `rootCallStack`
 value, and it allows me to write
 {{{#!hs
 readConfig :: IO Config
 readConfig = do
   s <- let ?callstack = rootCallStack in readFile "config.txt" -- line 42
   return (parseConfig s)
 }}}
 resp.
 {{{#!hs
 readConfig :: ?callstack::CallStack => IO Config
 readConfig = do
   s <- let ?callstack = rootCallStack `pushCallStack` ?callstack in
 readFile "config.txt" -- line 42
   return (parseConfig s)
 }}}
 to implement the above.

 The implementation sketch is ad-hoc, and there might be more elegant
 variants.


 This is not not necessarily an advocation of such “hiding internals”, but
 I think it should be possible.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11049#comment:3>
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