enum IO_ERR
is
	TIMEOUT,
	BAD_STATE
end enum

interface istream
is
	proc read(in byte[] buffer, size count) returns size, excepts IO_ERR; /*Procedures are non-pure; they can modify state, and can access global variables*/
	func end_of_input() returns bool; /*functions are pure; they cannot modify state, and can only modify parameters that are passed to them (a reference to the class is passed as an input parameter)*/

end interface


interface ostream
is
	proc write(out byte[] buffer, size count) returns size, excepts IO_ERR;
	func end_of_output() returns bool;
end interface

interface seekable
is
	func get_position() returns position;
	proc seek(position x) excepts SEEK_ERR;
end interface

/*inerfaces can be compound*/

type iostream is istream ostream;
type ipond is istream seekable;
type opond is ostream seekable;
type iopond is ipond opond;

/*The following two classes are identical*/

class File1
	implements iopond /*compound interfaces are decomposed and duplicates are removed*/
is
	/*...*/
end class

class File2
	implements istream ostream seekable
is
	/*...*/
end class


/*interfaces can also inherit other interfaces*/

interface ipondx
	implements istream seekable
is
 	/*...*/
end interface

interface opondx
	implements istream seekable
is
 	/*...*/
end interface

interface iopondx
	implements ipondx opondx
is
	/*...*/
end interface

/*In the case of iopondx, the seek and get_position methods are assumed to have separate implementations for both ipondx and opondx. In the interface iopond, there can only be one implementation of the methods in the pond interface since it is only included once.*/

/*you can also have functions with unaliased compound types as parameters*/

func foo(in istream ostream pond a);

/*You can then pass objects of type File1 or File2 to foo, however:*/

class File3
	implements iopondx
is
	/*...*/
end class

/*It is not possible to pass objects of type File3 to foo, even though it implements the three interfaces. You must pass objects of a type that implements all three of those interfaces directly and not inherited from other interfaces (except with a cast, and only if single base class or implemented interface directly implements those interfaces)*/