Using symbol before it is definied

Question

How is it that in line below on the right side of equation one could use symbol 'fibs' although it is not yet definied:

let fibs = 0 : 1 : zipWith (+) fibs (tail fibs)

Answer 1

The point is that the definition of fibs

fibs = 0 : 1 : zipWith (+) fibs (tail fibs)

is not evaluated until it is used somewhere else. Then the definition unfolds using the already known part. We begin with fibs = 0 : 1 : ???. Then if the third element is ever needed, the definition is evaluated one step further,

fibs = 0 : 1 : zipWith (+) (0 : 1 : ???) (tail (0 : 1 : ???))
     = 0 : 1 : zipWith (+) (0 : 1 : ???) (1 : ???)
     = 0 : 1 : (0 + 1) : zipWith (+) (1 : ???) (???)

but then the unknown part ??? has become partly known, it has been determined to be ??? = 1 : ????, so the unfolding can go on,

     = 0 : 1 : 1 : zipWith (+) (1 : 1 : ????) (1 : ????)
     = 0 : 1 : 1 : 2 : zipWith (+) (1 : ????) (????)
     -- now ???? is known to be 2:?????
     = 0 : 1 : 1 : 2 : zipWith (+) (1 : 2 : ?????) (2 : ?????)

etc.