never executed always true always false

    1 {-# LANGUAGE CPP #-}
    2 {-# OPTIONS_HADDOCK not-home #-}
    3 
    4 -- | Custom GHC "Prelude"
    5 --
    6 -- This module serves as a replacement for the "Prelude" module
    7 -- and abstracts over differences between the bootstrapping
    8 -- GHC version, and may also provide a common default vocabulary.
    9 
   10 -- Every module in GHC
   11 --   * Is compiled with -XNoImplicitPrelude
   12 --   * Explicitly imports GHC.Prelude
   13 
   14 module GHC.Prelude
   15   (module X
   16   ,module Bits
   17   ,shiftL, shiftR
   18   ) where
   19 
   20 
   21 -- We export the 'Semigroup' class but w/o the (<>) operator to avoid
   22 -- clashing with the (Outputable.<>) operator which is heavily used
   23 -- through GHC's code-base.
   24 
   25 {-
   26 Note [Why do we import Prelude here?]
   27 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
   28 The files ghc-boot-th.cabal, ghc-boot.cabal, ghci.cabal and
   29 ghc-heap.cabal contain the directive default-extensions:
   30 NoImplicitPrelude. There are two motivations for this:
   31   - Consistency with the compiler directory, which enables
   32     NoImplicitPrelude;
   33   - Allows loading the above dependent packages with ghc-in-ghci,
   34     giving a smoother development experience when adding new
   35     extensions.
   36 -}
   37 
   38 import Prelude as X hiding ((<>))
   39 import Data.Foldable as X (foldl')
   40 
   41 #if MIN_VERSION_base(4,16,0)
   42 import GHC.Bits as Bits hiding (shiftL, shiftR)
   43 # if defined(DEBUG)
   44 import qualified GHC.Bits as Bits (shiftL, shiftR)
   45 # endif
   46 
   47 #else
   48 --base <4.15
   49 import Data.Bits as Bits hiding (shiftL, shiftR)
   50 # if defined(DEBUG)
   51 import qualified Data.Bits as Bits (shiftL, shiftR)
   52 # endif
   53 #endif
   54 
   55 {- Note [Default to unsafe shifts inside GHC]
   56    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
   57 The safe shifts can introduce branches which come
   58 at the cost of performance. We still want the additional
   59 debugability for debug builds. So we define it as one or the
   60 other depending on the DEBUG setting.
   61 
   62 Why do we then continue on to re-export the rest of Data.Bits?
   63 If we would not what is likely to happen is:
   64 * Someone imports Data.Bits, uses xor. Things are fine.
   65 * They add a shift and get an ambigious definition error.
   66 * The are puzzled for a bit.
   67 * They either:
   68   + Remove the import of Data.Bits and get an error because xor is not in scope.
   69   + Add the hiding clause to the Data.Bits import for the shifts.
   70 
   71 Either is quite annoying. Simply re-exporting all of Data.Bits avoids this
   72 making for a smoother developer experience. At the cost of having a few more
   73 names in scope at all time. But that seems like a fair tradeoff.
   74 
   75 See also #19618
   76 -}
   77 
   78 -- We always want the Data.Bits method to show up for rules etc.
   79 {-# INLINE shiftL #-}
   80 {-# INLINE shiftR #-}
   81 shiftL, shiftR :: Bits.Bits a => a -> Int -> a
   82 #if defined(DEBUG)
   83 shiftL = Bits.shiftL
   84 shiftR = Bits.shiftR
   85 #else                    
   86 shiftL = Bits.unsafeShiftL
   87 shiftR = Bits.unsafeShiftR
   88 #endif