Uxnbal is a stack effect validator.

Program validation is done at compile-time by comparing a routine's stack effect, against the resulting balance of all stack changes occurring in the routine's code. Words that do not pass the stack-checker are generating a warning, and so essentially this defines a very basic and permissive type system that nevertheless catches some invalid programs and enables compiler optimizations. For more details, see Uxnbal.

The simplest case is when a piece of code does not have any branches or recursion, and merely pushes literals and calls words. The stack effect routines is always known statically from the declaration.

@add-eight ( a -- a+8 )
	#0008 ADD JMP2r

Working-stack imbalance of +1, in add-eight.

In the case of branching, each branch is evaluated and if an imbalance occurs inside one of the branches, the branch name is indicated:

@branching ( a* -- c )
	LDAk #01 EQU ?&one
	LDAk #02 EQU ?&two
	POP2 #ff JMP2r
	&one ( a* -- c ) POP2 #12 JMP2r
	&two ( a* -- c d ) ADD JMP2r

Working-stack imbalance of -1, in branching/two.

In the case of a recursion, the validator will use the stack effect instead of repeatedly walking through the body of the routine.

@print-string ( str* -- )
	LDAk DUP ?{ POP POP2 JMP2r }
	emit-letter 
	INC2 !print-string

For loops that exits without affecting the stack depth, a > prefixed label is used as a shorthand to reduce the need for extraneous stack effect definitions in cases where it can be inferred:

@many-times ( a -- )
	DUP
	&>l
		INC DUP ?&>l
	POP2 JMP2r

Routines that pull items from the stack beyond their allowed depth will also raise a warning, making the stack effect act a sort of boundary:

@shallow ( a -- )
	POP2 JMP2r

Working-stack depth error of 1, in shallow.

Lastly, a runtime specific solution to validate the stack state at any one point during the execution of a program, is to read the System/wst port and compare it against a given stack pointer byte value.

@on-reset ( -> )
	#abcd DUP2 
	.System/wst DEI #05 EQU ?{
		#01 .System/debug DEO }
	BRK

incoming: uxntal software