LiquidHaskell interaction with optimized Haskell

last.hs

-- | Extract the last element of a list, which must be finite and non-empty.
last                    :: [a] -> a
#ifdef USE_REPORT_PRELUDE
last [x]                =  x
last (_:xs)             =  last xs
last []                 =  errorEmptyList "last"
#else
-- Use foldl to make last a good consumer.
-- This will compile to good code for the actual GHC.List.last.
-- (At least as long it is eta-expaned, otherwise it does not, #10260.)
last xs = foldl (\_ x -> x) lastError xs
{-# INLINE last #-}
-- The inline pragma is required to make GHC remember the implementation via
-- foldl.
lastError :: a
lastError = errorEmptyList "last"
#endif