Error: got proc, but expected proc {.closure.}

[readysloth]

Hi all!

I want to test out closures in Nim, so I wrote this code (you cat [run 
it](https://play.nim-lang.org/#ix=2i83)): 
    
    
    import sugar
    
    proc calc(a: int, b: int): (int) -> int =
      let diff = a - b
      result = proc differentiate(c: int): int =
        result = c - diff
    
    
    let diff_proc = calc(5, 6)
    echo diff_proc(7)
    
    
    Run

But instead of 8 got 
    
    
    Error: type mismatch: got <type proc> but expected 'proc (i0: int): 
int{.closure.}'
    
    
    Run

Why? Adding pragma do not help...