Created
December 21, 2021 19:42
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| import types | |
| the_empty_stream = () | |
| def cons(x, y): | |
| return (x, y) | |
| def cons_stream(x, y): | |
| assert isinstance(y, types.LambdaType) | |
| return cons(x, y) | |
| def force(s): | |
| return s() | |
| def head(x): | |
| return x[0] | |
| def tail(x): | |
| return force(x[1]) | |
| def empty_stream(s): | |
| return s == the_empty_stream | |
| def filter(pred, s): | |
| if empty_stream(s): | |
| return the_empty_stream | |
| elif pred(head(s)): | |
| return cons_stream(head(s), lambda: filter(pred, tail(s))) | |
| else: | |
| return filter(pred, tail(s)) | |
| def first(n, s): | |
| if n == 0 or empty_stream(s): | |
| return the_empty_stream | |
| else: | |
| return cons_stream(head(s), lambda: first(n-1, tail(s))) | |
| def for_each(proc, s): | |
| if empty_stream(s): | |
| pass | |
| else: | |
| proc(head(s)) | |
| for_each(proc, tail(s)) | |
| def print_stream(s): | |
| for_each(lambda x: print(x, end="\t"), first(100, s)) | |
| print() | |
| def integers_from(n): | |
| return cons_stream(n, lambda: integers_from(n+1)) | |
| def divisible(n, d): | |
| return n % d == 0 | |
| def sieve(s): | |
| return cons_stream( | |
| head(s), | |
| lambda: sieve( | |
| filter( | |
| lambda x: not divisible(x, head(s)), | |
| tail(s) | |
| ) | |
| ) | |
| ) | |
| integers = integers_from(1) | |
| primes = sieve(integers_from(2)) |
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