How to chain prodecure calls that mutate a reference object ?

[ducdetronquito]

Hi there !

I am trying to chain procedures that mutate a reference object, but I can't 
managed to do it.

Here is a minimal example:
    
    
    type
        Child* = ref object
            value: int
        
        Parent* = ref object
            child: Child
    
    
    proc set_value*(self: var Child, value: int): Child {.discardable.} =
        self.value = value
        return self
    
    
    proc get_child*(self: var Parent): Child {.discardable.} =
        var child = Child()
        self.child = child
        return child
    
    
    var parent = Parent()
    
    parent.get_child().set_value(10)
    
    
    
    Run

This example triggers the following compile error, but I dont understand why I 
can't mutate the reference return by the get_child procedure.
    
    
    Error: type mismatch: got <Child, int literal(10)>
    
    but expected one of:
    proc set_value(self: var Child; value: int): Child
      first type mismatch at position: 1
      required type for self: var Child
      but expression 'get_child(parent)' is immutable, not 'var'
    
    expression: set_value(get_child(parent), 10)
    
    
    Run

Any ideas ?

Thanks :)