[GHC] #15468: in ghci -fbreak-on-exception omits exception info

[GHC]

#15468: in ghci -fbreak-on-exception omits exception info
-------------------------------------+-------------------------------------
        Reporter:  George            |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.8.1
       Component:  GHCi              |              Version:  8.4.3
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Poor/confusing    |  Unknown/Multiple
  error message                      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------
Description changed by George:

Old description:

> In ghc 8.4.3
>
> Without -fbreak-on-exception you get a good error message:
>
> {{{
> ghci -ignore-dot-ghci
> GHCi, version 8.4.3: http://www.haskell.org/ghc/  :? for help
> Prelude> let 2 + 2 = 5
> Prelude> 2 + 2
> 5
> Prelude> 3 + 3
> *** Exception: <interactive>:1:5-13: Non-exhaustive patterns in function
> +
> }}}
>
> With -fbreak-on-exception there is no info about the exception:
>
> {{{
>  ghci -ignore-dot-ghci -fbreak-on-exception
> GHCi, version 8.4.3: http://www.haskell.org/ghc/  :? for help
> Prelude> let 2 + 2 = 5
> Prelude> 2 + 2
> 5
> Prelude> 3 + 3
> Stopped in <exception thrown>, <unknown>
> _exception :: e = _
> [<unknown>] [<unknown>] Prelude>
> }}}
>
> In addition, contrary to the documentation, it doesn't seem possible to
> get ghci to print out the value of the exception
>
> {{{
> <unknown>] [<unknown>] Prelude> :print _exception
> _exception = (_t1::e)
> [<unknown>] [<unknown>] Prelude> :info _exception
> _exception :: e         -- Defined in ‘interactive:Ghci3’
> [<unknown>] [<unknown>] Prelude>
> }}}
>
> Finally the fact that you get now warning about
>
> {{{
> let 2 + 2 = 5
> }}}
>
> or for
> {{{
> 2 = 3
> }}}
>
> makes me think I should file an ER to add the following to the default
> warnings for ghci
>
> {{{
>  ghci -ignore-dot-ghci -Wname-shadowing -Wunused-pattern-binds
> GHCi, version 8.4.3: http://www.haskell.org/ghc/  :? for help
> Prelude> let 2 + 2 = 5
>
> <interactive>:1:7: warning: [-Wname-shadowing]
>     This binding for ‘+’ shadows the existing binding
>       imported from ‘Prelude’ (and originally defined in ‘GHC.Num’)
> Prelude> 2 = 3
>
> <interactive>:2:1: warning: [-Wunused-pattern-binds]
>     This pattern-binding binds no variables: 2 = 3
> Prelude>
> }}}
>
> I noticed the second and when googling to see if it had been reported I
> came across the following which documented the first:
> https://code.likeagirl.io/2-2-5-and-why-compiler-warnings-are-good-
> eaadef327146

New description:

 In ghc 8.4.3

 Without -fbreak-on-exception you get a good error message:

 {{{
 ghci -ignore-dot-ghci
 GHCi, version 8.4.3: http://www.haskell.org/ghc/  :? for help
 Prelude> let 2 + 2 = 5
 Prelude> 2 + 2
 5
 Prelude> 3 + 3
 *** Exception: <interactive>:1:5-13: Non-exhaustive patterns in function +
 }}}

 With -fbreak-on-exception there is no info about the exception:

 {{{
  ghci -ignore-dot-ghci -fbreak-on-exception
 GHCi, version 8.4.3: http://www.haskell.org/ghc/  :? for help
 Prelude> let 2 + 2 = 5
 Prelude> 2 + 2
 5
 Prelude> 3 + 3
 Stopped in <exception thrown>, <unknown>
 _exception :: e = _
 [<unknown>] [<unknown>] Prelude>
 }}}

 In addition, contrary to the documentation, it doesn't seem possible to
 get ghci to print out the value of the exception

 {{{
 <unknown>] [<unknown>] Prelude> :print _exception
 _exception = (_t1::e)
 [<unknown>] [<unknown>] Prelude> :info _exception
 _exception :: e         -- Defined in ‘interactive:Ghci3’
 [<unknown>] [<unknown>] Prelude>
 }}}

 Finally the fact that you get no warning about

 {{{
 let 2 + 2 = 5
 }}}

 or for
 {{{
 2 = 3
 }}}

 makes me think I should file an ER to add the following to the default
 warnings for ghci

 {{{
  ghci -ignore-dot-ghci -Wname-shadowing -Wunused-pattern-binds
 GHCi, version 8.4.3: http://www.haskell.org/ghc/  :? for help
 Prelude> let 2 + 2 = 5

 <interactive>:1:7: warning: [-Wname-shadowing]
     This binding for ‘+’ shadows the existing binding
       imported from ‘Prelude’ (and originally defined in ‘GHC.Num’)
 Prelude> 2 = 3

 <interactive>:2:1: warning: [-Wunused-pattern-binds]
     This pattern-binding binds no variables: 2 = 3
 Prelude>
 }}}

 I noticed the second and when googling to see if it had been reported I
 came across the following which documented the first:
 https://code.likeagirl.io/2-2-5-and-why-compiler-warnings-are-good-
 eaadef327146

--

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15468#comment:1>
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