Side effects for fun and profit

Consider this infinite Fibonacci sequence:
       var fib = fcn(ab){a,b:=ab; ab.del(0)+(a+b); a}.fp(L(0,1));

Ask it for a Fibonacci number and you get the first one. Ask again and you get the next one and so on:
       > do(15){print(fib(),”,”)} → 0,1,1,2,3,5,8,13,21,34,55,89,144,233,377
It never restarts, it just continues the same sequence:
       > fib() → 610
       > fib() → 987

How does it do this? By using a closure over a mutable list that holds two Fibonacci numbers. This list is the parameter to a function that uses it like a shift register: the value-to-be-returned is popped off the front of the list and sum of the two is appended (del(0) is the pop and +(a+b) is the append). The list and the function are bound together in a partial application (the fp()) so that every time the function runs, the parameter is the next number(s) in the sequence.

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