Lazy

Delay an expression

Given a file named “main.scm” with:

(import (scheme base) (scheme lazy))

(delay (write-u8 65))

When I successfully run stak main.scm

Then the stdout should contain exactly "".

Check if a value is a promise

Given a file named “main.scm” with:

(import (scheme base) (scheme lazy))

(write-u8 (if (promise? <value>) 65 66))

When I successfully run stak main.scm

Then the stdout should contain exactly “<output>”.

Examples

value	output
#f	B
(delay #f)	A
(make-promise #f)	A

Force a promise

Given a file named “main.scm” with:

(import (scheme base) (scheme lazy))

(force (delay (write-u8 65)))

When I successfully run stak main.scm

Then the stdout should contain exactly “A”.

Force a promise twice

Given a file named “main.scm” with:

(import (scheme base) (scheme lazy))

(define x (delay (write-u8 65)))

(force x)
(force x)

When I successfully run stak main.scm

Then the stdout should contain exactly “A”.

Force and delay an expression

Given a file named “main.scm” with:

(import (scheme base) (scheme lazy))

(delay-force (write-u8 65))

When I successfully run stak main.scm

Then the stdout should contain exactly "".

Force, delay, and force an expression

Given a file named “main.scm” with:

(import (scheme base) (scheme lazy))

(write-u8 (force (delay-force (make-promise (+ 60 5)))))

When I successfully run stak main.scm

Then the stdout should contain exactly “A”.

Force and delay expressions in a loop

Given a file named “main.scm” with:

(import (scheme base) (scheme lazy))

(write-u8
  (force
    (let loop ((i 10000))
      (if (zero? i)
        (make-promise 65)
        (delay-force (loop (- i 1)))))))

When I successfully run stak main.scm

Then the stdout should contain exactly “A”.

Make a promise

Given a file named “main.scm” with:

(import (scheme base) (scheme lazy))

(write-u8 (force (make-promise 65)))

When I successfully run stak main.scm

Then the stdout should contain exactly “A”.