Thinking about how Dependency Injection could work in Haskell similar to Angular.js, to allow for mocking in unit tests, therefore most easily testing even IO code or complex pure code.

dependency-injection.hs

fns = 
  [ ("a", ["print", "b"], \print b -> print b)
  , ("b", [], 12)
  , ("print", [], print)
  ]

main = assemble fns `lookup` "a"

spec = do 
  b' =? 3
  a' =? 12
  where
    b' = assemble ([("b", [], 3)] ++ fns) `lookup` "b"
    a' = assemble ([("print", [], id)] ++ fns) `lookup` "a"