[filwit]

You prove to the compiler that the nilable var is not-nil via if statement. Eg:

  proc foobar(f:Foo not nil) =
    discard
  
  let f = Foo() # nilable ref
  let b: Foo not nil = f # Error, can't prove 'f' is not nil

  if f != nil:
    let b: Foo not nil = f # can assign non-nil vars to 'f'
    foobar(f) # or can pass 'f' directly

[phaylon]

Ah, good to know. So Nim does static analysis on conditionals where Rust would use an explicit pattern match to get at the contained value.

Might be a good example for http://nim-lang.org/manual.html#not-nil-annotation