Backhand

Backhand is an esoteric language with an unusual program flow, inspired by 2D languages, such as Befunge and ><>.

Backhand is a 1D language, so most programs are interpreted as a series of characters. The origin of the name comes from "Back and Forth", since to get the most out of the code, the instruction pointer has to go back and forth. Initially, the pointer will start at the first character and move 3 steps at a time. For example: 1  1  +  O  @ will add 1 + 1 and output 2 before terminating.

If the pointer is about to step out of bounds, it reverses direction. You can use this to edit the previous program to the shorter 1O.1+@. The program evaluation goes like so:

1  1 →   Bounce off end and go left
←O  +←   Bounce off start and go right
→ .  @   Output and terminate

This can lead to some very compressed programs folding in on themselves, like "ol!,ld elWHro" printing Hello, World!.

But if you want to decrease the step count of the pointer, you can with v or W. The program W"!dlroW ,olleH"H first changes the step count to 1 and then prints Hello, World!. If you want to go the other way, you can use ^ or M to increase the pointer steps. Note that if you decrease the step count to negative then not only will the pointer move in the other direction, directionals like <> will have the opposite effect.

Data structure

Similar to Brain-Flak, Backhand uses two separate stacks. You usually operate on the main stack, but you can pull/push to the other stack with ( or ), or even switch stacks with x. Attempting to pop from an empty stack will yield a 0 instead.

Operators

Group	Character(s)	Name	Action
Literals
	0-9a-f	Number	Pushs the appropriate hexadecimal number. The characters a to f push the values 10 to 15
	"	String	Turn on string mode, which pushes the ordinal value of each character it lands on until it reaches either another "
	'	Character	Push the ordinal value of the next instruction instead of executing it
Stack manipulation
	~	Pop	Pop and discard a
	$	Swap	Pop a then b and push a then b
	:	Dupe	Pop a then push a twice
	&	Register	If there is not a value in the register, pop a and store a in the register. Otherwise, push the value in the register and clear it
	r	Reverse	Reverse the stack
	l	Length	Push the length of the stack
	(	Pull	Pop a from the other stack and push a
	)	Push	Pop a and push a to the other stack
	x	Switch	Swap the main and other stack
Control Flow
	<	Left	Change direction to left
	>	Right	Change direction to right
	{	Step Left	Step left one
	}	Step Right	Step right one
	^	Increment Step	Increase the step value by 1
	M	Double Increment Step	Increase the step value by 2
	v	Decrement Step	Decrease the step value by 1
	W	Double Decrement Step	Decrease the step value by 2
	?	Random	Step left or right randomly
	j	Jump	Pop a and jump to the ath character, bouncing off the sides as usual. 0 represents the first character
	s	Skip	Pop a and skip forward a characters, bouncing off the sides as usual. This moves forward in the current direction
	@	Terminate	End the program
Branching
	_	Decision Step	Pop a and if a is zero step right, else step left
	|	Decision Change	Pop a and if a is not zero reverse direction
	!	Not	Pop a and push 1 if a is equal to 0, otherwise 0
	L	Less	Pop a then b and push 1 if a is less than b, otherwise 0
	G	Greater	Pop a then b and push 1 if a is greater than b, otherwise 0
	E	Equal	Pop a then b and push 1 if a is equal to b, otherwise 0
Arithmetic
	-	Subtraction	Pop two values a then b and push b-a
	+	Addition	Pop two values a then b and push b+a
	*	Multiplication	Pop two values a then b and push b*a
	/	Division	Pop two values a then b and push b//a, the integer division of b and a
	%	Modulo	Pop two values a then b and push b%a, where the sign of the result matches the sign of a
	[	Decrement	Pop a and push a-1
	]	Increment	Pop a and push a+1
Input/Output
	i	ASCII input	Push the ordinal value of the next character in input. If there is no input left, push -1
	o	ASCII output	Pop a and output a as a character
	I	Number input	Ignore input data until it reaches a number and push that number. If the number is preceeded by a - then the number is negative. If there is no input left, push -1
	O	Number output	Pop a and print a as a number
	\n	Newline	Print a newline
	H	Halt	Print the contents of the stack as a string and terminate the program
	h	Finish	Output the top of stack as a number and terminate the program

Any other character is ignored.

Example programs:

Countdown from 10

aO0{@|}}:
.O[.

Hello, World!

"ol!,ld elWHro"

Cat program

io

Truth Machine

I|@}: O

Count up forever:

]{O:.

Factorial:

1@ IO :~!{|{}: ([ *).

Quine:

"#v{<@^:[ba+0v|{$:o[}

Printing Backhand

"acdBkn"haH