Bar.hs

module Bar where
myReverse [] = []
myReverse (x:xs) = myReverse xs ++ [x]

Foo.hs

module Foo where
myReverse = foldl (flip (:)) []

Hoge.cabal

Name: Hoge
Version: 0.0.0
License: BSD3
Cabal-Version: >= 1.2
Build-Type: Simple

Executable foo
  Main-is: Main.hs
  Build-Depends: base
  CPP-Options: "-DUSE_FOO"

Executable bar
  Main-is: Main.hs
  Build-Depends: base
  CPP-Options: "-DUSE_BAR"

Main.hs

{-# LANGUAGE CPP #-}
module Main where

#if defined(USE_FOO)
import Foo
#elif defined(USE_BAR)
import Bar
#endif

main = print $ myReverse [1..10]