Ok basically I have a problem knowing whether option 1 or 2 applies in the following case:

naturals = 0 : map (+ 1) naturals

Where options are:
1. The execution is awful, everything is recomputed at each step:

naturals     = [0]
naturals'    = 0:map (+ 1) [0]          // == [0, 1]
naturals''   = 0:map (+ 1) [0, 1]       // == [0, 1, 2]
naturals'''  = 0:map (+ 1) [0, 1, 2]    // == [0, 1, 2, 3]
naturals'''' = 0:map (+ 1) [0, 1, 2, 3] // == [0, 1, 2, 3, 4]

2. The execution is not awful, the list is always infinite and map is applied once only

naturals     = 0:something
                                  |
naturals'    = 0:      map (+ 1) (0:      something)
                                    |
naturals''   = 0:1:    map (+ 1) (0:1:    something')
                                      |
naturals'''  = 0:1:2:  map (+ 1) (0:1:2:  something'')
                                        |
naturals'''' = 0:1:2:3:map (+ 1) (0:1:2:3:something''')

with | indicating where map is at in its execution.

I do know that answers may be only 1 or 2 but I'd appreciate some pointers to good explanations on co-recursion too to clear the last doubts :)

haskell recursion
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