[GHC] #10089: feature: warn about unused data definitions (with typeclass instances)

[GHC]

#10089: feature: warn about unused data definitions (with typeclass instances)
-------------------------------------+-------------------------------------
              Reporter:  slyfox      |             Owner:
                  Type:  feature     |            Status:  new
  request                            |         Milestone:
              Priority:  normal      |           Version:  7.8.4
             Component:  Compiler    |  Operating System:  Unknown/Multiple
              Keywords:              |   Type of failure:  None/Unknown
          Architecture:              |        Blocked By:
  Unknown/Multiple                   |   Related Tickets:
             Test Case:              |
              Blocking:              |
Differential Revisions:              |
-------------------------------------+-------------------------------------
 Let's consider an example:

 {{{#!hs

 -- M.hs:
 module M () where

 -- ghc knows about uselessness of those
 data D = D

 -- but doesn't about this:
 data E = E deriving (Show, Read)
 }}}

 '''ghc''' warns about useless '''D''', but not '''E''':
 {{{
 $ ghc-7.8.4 -Wall -c M.hs -fforce-recomp

 M.hs:5:1: Warning:
     Defined but not used: type constructor or class ‘D’

 M.hs:5:10: Warning: Defined but not used: data constructor ‘D’
 }}}

 There is no way to refer to '''E''' type outside or
 call it's unstances, correct?

 I would expect no code should be generated for this module.

 '''ghc''' could also warn about defined,
 but not used instances for types, that are not exported.

 Some background on where those stray types come from:
 1. programmer usually adds a set of data types into their program
    (say, 10-15 types to represent an AST)
 2. then annotates them with 'deriving (Read, Show)' convenience instances
 3. (months later) amends AST representation a bit and leaves
    leftover data types not used anywhere

 Does that make sense?

 Thanks!

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10089>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler