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discard """
output: '''
true
true
true
3
18.0
324.0
'''
"""
type RNG = object
proc random(rng: var RNG): float = 1.0
type
RandomVar[A] = concept x
var rng: RNG
rng.sample(x) is A
Constant[A] = object
value: A
Uniform = object
a, b: float
ClosureVar[A] = proc(rng: var RNG): A
proc sample[A](rng: var RNG, c: Constant[A]): A = c.value
proc sample(rng: var RNG, u: Uniform): float = u.a + (u.b - u.a) * rng.random()
proc sample[A](rng: var RNG, c: ClosureVar[A]): A = c(rng)
proc constant[A](a: A): Constant[A] = Constant[A](value: a)
proc uniform(a, b: float): Uniform = Uniform(a: a, b: b)
proc lift1[A, B](f: proc(a: A): B, r: RandomVar[A]): ClosureVar[B] =
proc inner(rng: var RNG): B = f(rng.sample(r))
return inner
proc main() =
proc sq(x: float): float = x * x
let
c = constant(3)
u = uniform(2, 18)
t = lift1(sq, u)
var rng: RNG
echo(c is RandomVar[int])
echo(u is RandomVar[float])
echo(t is RandomVar[float])
echo rng.sample(c)
echo rng.sample(u)
echo rng.sample(t)
main()
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