|The Hyper Programming Language|
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All parameters are passed by reference. This means that when you pass the value of an expression as a parameter to a procedure or a constructor, the value is not copied. The callee gets the address of the value. This is the same as using a reference for a parameter in C++.
Contrary to what you might think, this convention can not be a source of bugs. The value of a parameter can not be accidentally modified from inside a procedure, because parameters are by default input parameters, and those are read-only.
An input parameter is the default for a parameter that does not specify otherwise. It is not marked by a keyword (but such an optional keyword could still be added in the future).
This kind of parameter is used for passing a value without allowing the callee to modify any aspect of it. This means that the entire type is automatically marked as const. An example:
procedure abc(x : int, y :  int, z : *  * int) var s : int = x # OK x = 7 # ERROR : x is const y = s # ERROR : the array y is const z =$ new  * int # ERROR : the pointer z is const z =$ new int(10) # ERROR : the array z is const z = 99 # ERROR : the int that is pointed to by z is const end
To avoid confusion with input/output parameters, it is actually forbidden to use the const keyword in the type of an input parameter:
procedure p(d : const int) # ERROR : const not allowed for an input parameter # (...) end
An input/output parameter allows the callee to modify the values it receives. The keyword inout is used to mark such a parameter. Any aspect that is not to be modified must be explicitly marked as const. Example:
procedure abc(inout x : int, inout y :  int, inout z : const *  * int) var s : int = x # OK x = 7 # OK y = s # OK z =$ new  * int # ERROR : the pointer z is const z =$ new int(10) # OK z = 99 # OK end