main:
	declare char
	declare isa
	declare isnota
	calc char peek
	calc isa eq char "a"
	calc isnota not isa
	if isa
		emit "Got a"
		emit eol
	/
	if isnota
		emit "Did not get a"
		emit eol
	/
	return
/

make it2
cat example0.lang0 | ./it2-out > example0.c
gcc example0.c -o example0
echo 'a' | ./example0
# Got a
echo 'b' | ./example0
# Did not get a

Formatting

Indentation is done using tabs.

Parsing

Pre declaration (functions)

Functions need to be pre-declared if you intend to use them before you have defined them:

func result/

...

func result:
	return result
/

Pre declaration (variables)

Variables will need to be declared before you can use them:

func:
	declare result
	set result "Result"
	return result
/

Variables

calc var func args*

Calls func with args and stores the output in var.

declare var

Pre-declares var.

set var "const"

Sets var to "const".

Flow control

break

Exits the deepest forever loop.

func args*

Calls the function name func with the given arguments. The result, if any, is ignored.

if arg

Evaluates arg. If found to be true, descends in the block. Otherwise, skips the block

forever

Repeats the block until break or return is called.

return var?

Returns the given value. You can not give an argument if your function is never used in call.

Builtins

check func [arg0..] : msg [arg0..]

Calls the given function with the given arguments. If the function's return value is not found to be true, outputs the given msg and halts the program.

trace

Writes the name and value of the variable passed to stderr if the TRACE environment variable is set.

Standard library functions

addstringchar a b

b is expected to have length 1.

Creates a new string with b appended to a.

emit arg

Writes the given string to standard output.

eq a b

Return true if the given strings are the same.

lt a b

a and b are expected to have length 1.

Return true if a would sort before b.

peek

Checks stdin for the next character and returns it.

skip

Advances stdin a single character.

stdincolno

Returns the column number for stdin (starting at 1)

stdinlineno

Returns the line number for stdin (starting at 1)

Typing

Every variable is of type string. Every function gets a number of strings as output, and returns another string. The if statement accept all non-empty strings as true, and the empty string as false.

Notes

You can declare variables and functions ahead of time. This is necessary for some interpreters.

Bootstrap stairs

The first iteration is written by hand, both the compiler and the compiler compiler. It targets python3 as an interpreter.

The second version only has the compile written by hand, with the compiler compiler being the output of iteration 1. It targets gcc as interpreter.

We compile every iteration 3 times, to ensure that the the compiler round trips properly.

How to run

Install python3 and make

make clean all

Get more info

make TRACE=1 clean all

You will now have a lang0 compiler in python for python in ./it1-out.py, and one written in C for C in ./it2-out.