On 29/01/2013, at 10:59 PM, Junior White wrote:
So this is a problem in lazy evaluation language, it will not appear in python or erlang, am i right?
Wrong. Let's take Erlang: [f(X, Y) || X <- g(), Y <- h()] Does the order of the generators matter here? You _bet_ it does. First off, in all of these languages, it affects the order of the results. Let's take a toy case: g() -> [1,2]. h() -> [a,b]. % constants f(X, Y) -> {X,Y}. % a pair [f(X, Y) || X <- g(), Y <- h()] yields [{1,a},{1,b},{2,a},{2,b}] [f(X, Y) || Y <- h(), X <- g()] yields [{1,a},{2,a},{1,b},{2,b}] Now let's change it by giving g/0 and h/0 (benign) side effects. g() -> io:write('g called'), io:nl(), [1,2]. h() -> io:write('h called'), io:nl(), [a,b]. Generating X before Y yields 'g called' 'h called' 'h called' [{1,a},{1,b},{2,a},{2,b}] Generating Y before X yields 'h called' 'g called' 'g called' [{1,a},{2,a},{1,b},{2,b}] If a function call may yield side effects, then the compiler must not re-order or coalesce calls to that function. This applies to both Erlang and Python (and to SETL, which had set and tuple comprehensions before Erlang, Python, or Haskell were conceived).