Programming Language for Old Timers

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Assignment

The assignment operator := is an infix operator macro whose right-hand side is an expression. The value of the operator call is the value of the right-hand side.

If the left-hand side is a name, the name's definition in the current scope must be assignable. The definition's value is changed to the value of the right-hand side. The macro's expansion is an assignment P-expression.

If the left-hand side is an invocation, the function being invoked must be a name. The macro's expansion is an invocation of the corresponding setter function, whose name is the original function's name suffixed with ":=" and whose context is the original function's context. The setter function takes one additional argument, which is the right-hand side, and must return the value of that argument. If the definition of the setter function is a macro rather than a function, the syntax accepted by the macro must be the syntax of a function call, with comma-separated arguments in parentheses. The macro is expanded and the result is the expansion of the := macro.

An invocation on the left-hand side can be used to assign to a slot or other functionally-accessed location. It can also be used for destructuring, when the left-hand side invokes a constructor. For example, corresponding to the list constructor is the list:= destructuring macro which expands into assignments of parts of the list to its arguments. See the Destructuring section for more discussion.

Examples:

f(x, y) := z

f:=(x, y, z)

x.y := z

y:=(x, z)

w.x(y) := z

x:=(w, y, z)

list(a, b, c) := d

block
  def temp is list = d
  a := temp[0]
  b := temp[1]
  c := temp[2]

list(pt.x, pt.y) := coords

block
  def temp is list = coords
  x:=(pt, temp[0])
  y:=(pt, temp[1])

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