[GHC] #16169: Unused variable warning affects compositionality when generating code

[GHC]

#16169: Unused variable warning affects compositionality when generating code
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           Reporter:  mpickering     |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.6.3
           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:
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 If I'm generating a program using template haskell, I might define
 a combinator to help me generate lambdas.

 {{{
 type Code a = Q (TExp a)

 _lam :: (Code a -> Code b) -> Code (a -> b)
 _lam f = [|| \a -> $$(f [|| a ||]) ||]
 }}}

 However, if I now pass a constant function into `_lam`, the generated code
 contains an unused variable `a` that I can do nothing about and desire not
 to do anything about.

 {{{
 c5 :: Code (a -> Int)
 c5 = _lam (const [|| 5 ||])
 }}}

 However, GHC decides that it's wise to warn me about this when I splice it
 in.

 {{{
 > $$c5
 F2.hs:6:5: warning: [-Wunused-matches] Defined but not used: ‘a’
   |
 6 | q = $$c5
   |     ^^^
 }}}

 As Ryan will tell you, I'm against emitting warnings from generated code
 as it breaks the abstraction. The code that is generated is guaranteed to
 be type and scope correct but any aesthetic warning is irrelevant to the
 consumer.

 I see it in precisely the same way as warning if we use a function that
 contains an unused variable in a library. Once the definition is exported
 from the library, its definition is opaque. The same principle should be
 applied to code generated by template haskell.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/16169>
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