Why referring to immutable variable is NOT side effect free in Nim?

Question

The code below won't compile because of the reference to the let one = 1.

Why? It seems like it's side effect free (and thread safe too) - as it's impossible to change the immutable data.

playground

let one = 1
func one_plus(v: int): int =
  one + v
  
echo one_plus(2)

Does code like that - referring to external immutable data - supposed to be written somehow differently in Nim to be considered side-effect free?

Answer 1

My guess is the problem is using the global namespace, wrapping your code into a proc makes it compile and run:

proc main() =
  let one = 1
  func one_plus(v: int): int =
    one + v
    
  echo one_plus(2)

main()

If you still want to use a global, you need to use const section:

const one = 1
func one_plus(v: int): int =
  one + v
  
echo one_plus(2)