Maximal and minimal types, identity testing, and printing

In general.diesel:

The following types, classes, and/or objects are special, and predefined:

void is the type of methods that do not return a ``real'' result. void is implicitly a supertype of all other types. The void object can be returned explicitly from a method, if necessary.

predefined object void;

any is implicitly the supertype of all other non-void types. It can be used as the argument type of functions that work over any (non-void) object.

predefined abstract class any;

The none type is the result type of functions that never return to their callers, and the type of closure arguments that are never invoked. none is a subtype of all other types, and there is no object of this type.

predefined type none;

The dynamic type disables static type checking. It's used to explicitly declare the type of a variable that cannot be statically checked. A value of any type can be assigned to a variable of type dynamic, and vice versa. (The Diesel compiler will perform run-time typechecking to ensure type safety even in the face of uses of dynamic or of static type errors.)

predefined type dynamic;

extend module Stdlib;

module General;

All objects can be compared for identity. Two things that print out the same may not be ==, e.g. "abc" == "abc" may return false. By default, operations on collections that compare elements, such as finding an element in a list or adding an element to a set, use = (implemented by comparable objects) rather than ==, unless the collection's name includes identity_.

fun ==(l:any, r:any):bool;  - object identity test (use = instead for value equality testing)
fun !==(l:any, r:any):bool;  - not ==

Any object can be converted to a string, although the default version can be low-level and ugly. Most commonly-used objects override print_string to return something prettier. (A two-element version of print permits strings to be printed to files.)

fun print_string(x:any):string;  - return a string print version
fun print(x:any):void;  - print the print_string
fun print_line(x:any):void;  - print the print_string and a newline
fun print_line():void;  - just print a newline

The error method is the standard way to prematurely quit execution of a Diesel program.

fun error(msg:ordered_collection[`T]):none;  - quits with an error message; does not return

upcast[T] changes the static type of its argument to T from a

fun upcast[T](x:`S <= T):T;

end module General;

end module Stdlib;