[GHC] #12524: RFC: Allow prefixing result evaluated at GHCi prompt

[GHC]

#12524: RFC: Allow prefixing result evaluated at GHCi prompt
-------------------------------------+-------------------------------------
           Reporter:  Iceland_jack   |             Owner:
               Type:  feature        |            Status:  new
  request                            |
           Priority:  lowest         |         Milestone:
          Component:  GHCi           |           Version:  8.0.1
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 Similar to `prompt`, `prompt-cont` (previously `prompt2`): allowing users
 to supply a prefix to evaluation results with `result` has two use cases I
 can think of

 {{{
 :set prompt      "    ghci> "
 :set prompt-cont "    ghci| "
 :set result      "    "
 }}}

 the desired outcome would be `↓` as indented code for IRC, also reddit
 code format:
 {{{
     ghci> 1 + 2 + 3
     6
     ghci> reverse "hello"
     "olleh"
 }}}

 ----

 The other case came from
 [https://twitter.com/rightfold/status/767301255140179968 this tweet]

 {{{
 :set prompt      ""
 :set result      "-- "
 }}}

 {{{
 1 + 2 + 3
 -- 6
 reverse "hello"
 -- "olleh"
 }}}

 This can be copied/pasted back to a separate GHCi session where the result
 is ignored.

 ----

 I haven't seen this feature in any other repl, I'm not sure if this would
 see much use.

 This wouldn't work with IO actions without some magic.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12524>
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