[GHC] #1770: GHCi should know about itself, so that .ghci can be portable

[GHC]

#1770: GHCi should know about itself, so that .ghci can be portable
------------------------------+---------------------------------------------
  Reporter:  guest            |          Owner:         
      Type:  feature request  |         Status:  new    
  Priority:  normal           |      Milestone:         
 Component:  GHCi             |        Version:  6.6.1  
  Severity:  normal           |       Keywords:         
Difficulty:  Unknown          |             Os:  Unknown
  Testcase:                   |   Architecture:  Unknown
------------------------------+---------------------------------------------
 i've recently found myself writing .ghci files for ghc head which, with a
 few changes, can be made to work with 6.6.1 as well. then someone reported
 that they produce errors on his system (apparently, some people still have
 to use 6.4.1, for whatever reason). there are now two versions of the
 .ghci file: one for 6.4.1, one for 6.6.1 and later.

 it would be useful if ghci could be queried about itself. since the query
 results need to be processed further, queries should be functions. i
 propose an implicitly loaded namespace GHCi. for such queries. examples:
 {{{
 GHCi.version :: [Int] -- version number
 GHCi.info :: [(String,String)] -- the results of +RTS --info on the
 current ghci
 GHCi.docdir :: String -- the result of --print-docdir
 GHCi.libdir :: String -- the result of --print-libdir
 GHCi.exists :: String -> Bool -- commands defined, flags available
 GHCi.has :: String -> Bool -- features availabe
 }}}
 there are an increasing number of ghc options that provide information not
 easily available in ghci (calling ":!ghc --version" gives no guarantee of
 reaching the right version of ghc! using Config.cProjectVersion was not
 available in 6.4.1, and requires loading -package ghc, which might leave
 residues/instances for the rest of the session). all of those information
 flags should have ghci equivalents.

 functions GHCi.exists and GHCi.has are inspired by established practice in
 vim:
 the former gives information about specific items (commands defined, flags
 accepted), the latter gives information about more general feature sets
 (debugger available? threads available?).
 {{{
 GHCi.exists "-fno-print-bind-result" -- false in 6.4.1
 GHCi.exists ":edit" -- true from 6.6.1 on
 GHCi.exists ":cmd" -- true from somewhere in 6.7.x onwards
 GHCi.has "debugger" -- true from somewhere in 6.7.x onwards
 }}}
 no, we can't backpatch 6.4.1 or 6.6.1, but 6.8.1 will still be around when
 7.x is in standard use, so the sooner the better.

 (related to #1765, which was not by me;-)

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/1770>
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