forked from alex-eg/sex
56 lines
2.0 KiB
Scheme
56 lines
2.0 KiB
Scheme
(input)
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(output "direct: 120"
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"fac: 120 3628800"
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"fib: 55 6765")
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(return 0)
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;;; A fixed point built out of closures, which is the hardest thing to
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;;; ask of them: recursion with no recursive function anywhere, only
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;;; self-application.
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;;;
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;;; Self-application needs `x x' and so a recursive type, which is
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;;; spelled here by routing it through a named struct whose field is a
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;;; closure whose own signature mentions that struct. The generated
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;;; closure struct is written before `struct rec' is, so this only
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;;; compiles because a forward declaration is emitted ahead of both.
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;;;
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;;; Note what `fix' captures: a *pointer* to the knot, not the knot. A
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;;; closure is a code pointer beside N bytes of environment, so
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;;; capturing one by value would need N >= 8 + N. No budget makes that
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;;; true, and the static assertion says so rather than letting it
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;;; corrupt anything.
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(include stdio.h)
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(struct rec ((f (closure (((* (struct rec))) (int)) int))))
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;;; Takes a step that expects itself, returns an ordinary closure with
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;;; the self-application hidden inside
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(fn fix ((step (* (struct rec)))) (closure ((int)) int)
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(return (closure ((n int)) int (step)
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(return ((-> step f) step n)))))
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(pub fn main () int
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(var fac-knot (struct rec))
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(= (. fac-knot f)
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(closure ((self (* (struct rec))) (n int)) int ()
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(if (<= n 1) (return 1))
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(return (* n ((-> self f) self (- n 1))))))
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;; the knot applied to itself directly, without fix
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(printf "direct: %d\n" ((. fac-knot f) (& fac-knot) 5))
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(var fib-knot (struct rec))
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(= (. fib-knot f)
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(closure ((self (* (struct rec))) (n int)) int ()
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(if (< n 2) (return n))
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(return (+ ((-> self f) self (- n 1))
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((-> self f) self (- n 2))))))
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;; one combinator, two different recursions
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(var fac (closure ((int)) int) (fix (& fac-knot)))
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(var fib (closure ((int)) int) (fix (& fib-knot)))
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(printf "fac: %d %d\n" (fac 5) (fac 10))
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(printf "fib: %d %d\n" (fib 10) (fib 20))
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(return 0))
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