Selector

Overview

Selector is a simple language created in April 2008 by User:EvincarOfAutumn.

It has ten signed unbounded integer data registers, one unbounded stack, basic looping, basic exception handling, and sufficient additive arithmetic operations to be considered Turing-complete. (Thanks to User:Ais523 for informing me that it is).

Each program consists of a series of blocks. All but one of these blocks are "locked" when execution begins. Unlocked blocks are executed in order from top to bottom, and the file is repeatedly executed until no unlocked blocks remain to execute. Only one register may be "selected" for write access at a time.

Source Requirements

Source is given in en-US.US_ASCII encoding. Commands are given in capital letters. All non-capital letters and whitespace are ignored in the source, as are the characters [ and ] and all intermediate characters. Noncapitals separate commands from their parameters and commands from one another.

Commands

ALL

ALL identifier

Begins a block named identifier.

Special Blocks

• KNOB Program entry point (short for "no block")
• NOSE Exception: No selection where selection needed
• BAD Exception: Block already deselected
• BASE Exception: Block already selected

PICK

PICK register

Selects the indicated register.

Registers

Each register has an initial value corresponding to its name.

• ZERO
• ONE
• TWO
• THREE
• FOUR
• FIVE
• SIX
• SEVEN
• EIGHT
• NINE

In addition, there is one special register named NOSE. The statement PICK NOSE deselects all registers. Its name is an abbreviation of "no selection". I swear.

MY

MY register

Subtracts the value of the indicated register from the selected one. Raises a NOSE exception if no register is selected.

YOUR

YOUR register

Adds the value of the indicated register to the selected one. Raises a NOSE exception if no register is selected.

GO

GO direction

Jumps in the given direction.

GO Directions

• FORWARD

If the selected register is zero, jumps to the matching GO BACK. Otherwise a nop.

• BACK

If the selected register is nonzero, jumps to the matching GO FORWARD. Otherwise a nop.

• ON

Jumps to the start of the next enabled block. This can wrap around the end of the program! Note that a GO ON is implicit at the end of every block.

• OFF

Jumps to the start of the previous enabled block. This can wrap around the start of the program!

MAKE

MAKE action

Performs a stack operation.

MAKE Actions

• PILE

Pushes the value of the selected register to the stack. If no register is selected, a value is pushed from standard input.

• HOLE

Pops the value at the head of the stack into the selected register. If no register is selected, an ASCII value is popped to standard output, modulo 256 if the value is greater than 255.

LESS

LESS identifier

Disables a named block. Raises a BAD exception if the block is already disabled.

MORE

MORE identifier

Enables a named block. Raises a BASE exception if the block is already disabled.

BECOME

BECOME identifier

Swaps the current block and the indicated one, and jumps to the start of the now-current block.

ESCAPE

ESCAPE

Returns to the start of the last block to execute. In the case of exception-handlers or blocks called with a BECOME statement, this is always the caller. Named subs right there!

Examples

These are almost the least-obfuscated possible programs. The interpreter supports all but the BECOME and ESCAPE statements, so the following programs are guaranteed to work. Not that the interpreter is getting posted in its unfinished state.

Cat

This is a simple cat program. It doesn't perform error checking of any kind, and it doesn't recognise EOFs. It just loops, outputting the lines that are input, until the program gets killed.

ALL KNOB GO FORWARD MAKE PILE MAKE HOLE GO BACK

Hello, world!

Anyway, I was too lazy to make a hello world, so the following program prints "Hello." instead.

ALL KNOB PICK NINE GO FORWARD PICK ZERO YOUR EIGHT PICK NINE MY ONE
PICK NINE GO BACK PICK ZERO MAKE PILE PICK NOSE MAKE HOLE PICK ZERO
YOUR EIGHT YOUR EIGHT YOUR EIGHT YOUR FIVE MAKE PILE PICK NOSE MAKE
HOLE PICK ZERO YOUR SEVEN MAKE PILE MAKE PILE PICK NOSE MAKE HOLE MAKE
HOLE PICK ZERO YOUR THREE MAKE PILE PICK NOSE MAKE HOLE PICK ZERO GO
FORWARD MY ONE GO BACK PICK FIVE GO FORWARD PICK ZERO YOUR EIGHT PICK
FIVE MY ONE GO BACK PICK ZERO YOUR SIX MAKE PILE PICK NOSE MAKE HOLE
LESS KNOB