Var parameters

The type of a parameter may be prefixed with the var keyword:

proc divmod(a, b: int; res, remainder: var int) =
  res = a div b
  remainder = a mod b
var
  x, y: int
divmod(8, 5, x, y) # modifies x and y
assert x == 1
assert y == 3

In the example, res and remainder are var parameters. Var parameters can be modified by the procedure and the changes are visible to the caller. The argument passed to a var parameter has to be an l-value. Var parameters are implemented as hidden pointers. The above example is equivalent to:

proc divmod(a, b: int; res, remainder: ptr int) =
  res[] = a div b
  remainder[] = a mod b
var
  x, y: int
divmod(8, 5, addr(x), addr(y))
assert x == 1
assert y == 3

In the examples, var parameters or pointers are used to provide two return values. This can be done in a cleaner way by returning a tuple:

proc divmod(a, b: int): tuple[res, remainder: int] =
  (a div b, a mod b)
var t = divmod(8, 5)
assert t.res == 1
assert t.remainder == 3

One can use tuple unpacking to access the tuple’s fields:

var (x, y) = divmod(8, 5) # tuple unpacking
assert x == 1
assert y == 3

Note: var parameters are never necessary for efficient parameter passing. Since non-var parameters cannot be modified the compiler is always free to pass arguments by reference if it considers it can speed up execution.