[GHC] #12137: Warning about “INLINE binder is (non-rule) loop breaker” with `-dcore-lint`

31 May 2016

      #12137: Warning about “INLINE binder is (non-rule) loop breaker” with `-dcore-lint`
-------------------------------------+-------------------------------------
           Reporter:  Iceland_jack   |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           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:
-------------------------------------+-------------------------------------
 With this code using `lens`

 {{{#!hs
 {-# Language TemplateHaskell #-}

 import Control.Lens

 data Config   = Config   { _companyName :: String }
 data AppState = AppState { _asConfig    :: Config }

 makeClassy ''Config

 instance HasConfig AppState where
   config = undefined
 }}}

 gives

 {{{
 $ ghci -ignore-dot-ghci -dcore-lint /tmp/tvQq.hs
 GHCi, version 8.0.1: http://www.haskell.org/ghc/  :? for help
 [1 of 1] Compiling Main             ( /tmp/tvQq.hs, interpreted )
 *** Core Lint warnings : in result of Simplifier ***
 /tmp/tvQq.hs:10:10: warning:
     [RHS of $ccompanyName_a7EP :: Lens' AppState String]
     INLINE binder is (non-rule) loop breaker: $ccompanyName_a7EP

 *** Core Lint warnings : in result of Simplifier ***
 /tmp/tvQq.hs:10:10: warning:
     [RHS of $ccompanyName_a7EP :: Lens' AppState String]
     INLINE binder is (non-rule) loop breaker: $ccompanyName_a7EP

 Ok, modules loaded: Main.
 *Main>
 }}}

 ----

 `makeClass` actually dumps

 {{{
 <interactive>:64:54-72: Splicing declarations
     makeClassy ''Config
   ======>
     class HasConfig c_axo7 where
       config :: Lens' c_axo7 Config
       companyName :: Lens' c_axo7 String
       {-# INLINE companyName #-}
       companyName = (.) config companyName
     instance HasConfig Config where
       {-# INLINE companyName #-}
       config = id
       companyName = iso (\ (Config x_axo8) -> x_axo8) Config
 }}}

 Using that to create a small example

 ----

 {{{#!hs
 data Config = Config { name :: String }
 class HasConfig a where
   config :: a -> Config
   companyName :: a -> String
   {-# INLINE companyName #-}
   companyName a = name (config a)

 instance HasConfig Config where
   {-# INLINE companyName #-}
   config = id
   companyName = name . config
 }}}

 which works perfectly fine without `-dcore-lint` but fails with

 {{{
 $ ghci -ignore-dot-ghci  -dcore-lint /tmp/tvQq.hs
 GHCi, version 8.0.1: http://www.haskell.org/ghc/  :? for help
 [1 of 1] Compiling Main             ( /tmp/tvQq.hs, interpreted )
 *** Core Lint warnings : in result of Simplifier ***
 /tmp/tvQq.hs:11:3: warning:
     [RHS of $ccompanyName_a18Q :: Config -> String]
     INLINE binder is (non-rule) loop breaker: $ccompanyName_a18Q

 *** Core Lint warnings : in result of Simplifier ***
 /tmp/tvQq.hs:11:3: warning:
     [RHS of $ccompanyName_a18Q :: Config -> String]
     INLINE binder is (non-rule) loop breaker: $ccompanyName_a18Q

 Ok, modules loaded: Main.
 }}}

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12137
GHC http://www.haskell.org/ghc/
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