Error with generic concepts

[andreaferretti]

For a minimal example, let me define a fake RNG, together with a function to get random float values from it:

type RNG = object

proc random(rng: var RNG): float = 1.0

Now, I define a RandomVar[A] as something that is able, given a RNG, to sample random instances of A. As a few concrete instances, I consider a constant, a uniform distribution, and a closure. I also define constructors for these types, as well as sample functions to make sure that they are instances of RandomVar[A]:

type
  RandomVar[A] = concept x
    var rng: RNG
    x.sample(rng) 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)

What I am trying to do is define operators on random variables. Here is a function that will lift a function A => B to a function RandomVar[A] => RandomVar[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

When I try to use lift1 I get a compile time error:

when isMainModule:
  proc sq(x: float): float = x * x

  let
    c = constant(3)
    u = uniform(2, 18)
    t = lift1[float, float](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)

The line t = lift1[float, float](sq, u) fails with

Error: type mismatch: got (RNG, RandomVar[system.float])
but expected one of: 
proc sample[A](rng: var RNG; c: Constant[A]): A
proc sample[A](rng: var RNG; c: ClosureVar[A]): A
proc sample(rng: var RNG; u: Uniform): float

To reproduce the error more easily, here is the whole thing:

type RNG = object

proc random(rng: var RNG): float = 1.0

type
  RandomVar[A] = concept x
    var rng: RNG
    x.sample(rng) 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

when isMainModule:
  proc sq(x: float): float = x * x

  let
    c = constant(3)
    u = uniform(2, 18)
    t = lift1[float, float](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)

[branpk]

Well the type error is because you have a typo in your concept:

type
  RandomVar[A] = concept x
    var rng: RNG
    x.sample(rng) is A

The x.sample(rng) is A should be rng.sample(x) is A.

As fate would have it, fixing this crashes the compiler.