[GHC] #5461: Reconfigurable pretty-printing in GHCi

[GHC]

#5461: Reconfigurable pretty-printing in GHCi
---------------------------------+------------------------------------------
    Reporter:  simonpj           |       Owner:  bravit          
        Type:  feature request   |      Status:  patch           
    Priority:  normal            |   Milestone:  7.6.1           
   Component:  Compiler          |     Version:  7.2.1           
    Keywords:                    |          Os:  Unknown/Multiple
Architecture:  Unknown/Multiple  |     Failure:  None/Unknown    
  Difficulty:                    |    Testcase:                  
   Blockedby:                    |    Blocking:                  
     Related:                    |  
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Changes (by bravit):

  * status:  new => patch


Comment:

 Feature request is implemented in this way:

 1) User can specify flag `-interactive-print=<Module-name.function-name>`
 in command line or set it via `:set` command in ghci. This function's type
 should be of a form `C a => a -> IO ()`, though this isn't checked prior
 to actual call.

 2) We search for this function in currently loaded modules and registered
 packages. Errors, if any, are reported.

 3) The function is invoked after evaluating of given expression.

 For example, user has this special printing function:
 {{{
 module SpecPrinter where
 import System.IO

 sprint a = putStrLn $ show a ++ "!"
 }}}
 Running command:
 {{{
 ghc -e "[1,2,3]" -interactive-print=SpecPrinter.sprint SpecPrinter
 }}}
 gives output
 {{{
 [1,2,3]!
 }}}

 In GHCi:
 {{{
 Prelude> [1,2,3]
 [1,2,3]
 Prelude> :load SpecPrinter
 *SpecPrinter> :set -interactive-print=SpecPrinter.sprint
 *SpecPrinter> [1,2,3]
 [1,2,3]!
 }}}

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/5461#comment:10>
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