XXIIVV

Typed uxntal is an inference system based on routine definitions.

Type inference in Uxntal is done by checking the stack effect declarations of words before they can be run, against the cumulative stack state of each item in the definition of each word.

@routine ( a b -- c ) Ok.
	MUL
JMP2r

The simplest case is when a piece of code does not have any branches or recursion, and just pushes literals and calls words. Pushing a literal has stack effect ( -- x ). The stack effect of most words is always known statically from the declaration.

@add ( a* b* -- c* ) Warning: Imbalance in @add of +2
	DUP2 ADD2
JMP2r

Branch Type

Words that do not pass the stack-checker are generating a warning, and so essentially this defines a very simple and permissive type system that nevertheless catches some invalid programs and enables compiler optimizations. 

@routine ( a -- ) Ok.
	#01 ?&branch POP 
JMP2r
	&branch ( a ~- )
		POP
JMP2r

Fall-through Type

The tail-less fall-through type allows for routine to not return, and check the stack balance including that of the next routine in memory.

@falling ( a b c -+ c )
	POP
@next-routine ( a b -- res )
	ADD
JMP2r

Return-Stack Passing

Passing items to a branch through the return-stack, is done by prefixing the definition with a backtick.

@routine ( a -- b )
	STHk
	DUP #01 EQU ?&branch
	POPr
JMP2r
	&branch ( a `a ~- b )
		POP STHr
JMP2r