itrs.nim

import typetraits

iterator foo(i: int): int {.closure.} =
  yield i + 1
  yield i + 2

let f = foo
echo f(1) # 2
echo f(2) # 4

when false:
    echo foo() # Error: attempting to call undeclared routine: 'foo'

echo type(f)   # iterator (i: int): int{.closure, noSideEffect, gcsafe, locks: 0.}
echo type(foo) # iterator (i: int): int{.closure, noSideEffect, gcsafe, locks: 0.}