lang0
=====

The base of language; a language designed to be just a compiler for other languages.

And you probably won't write _your_ custom language in lang0, but in a system language built in lang0.

The language
---

lang0 is designed for building small compilers. As such, it has a number of primitives specific for that.

### Examples

See `example0.lang0`:
```
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
/
```

```sh
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 return value 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. If you don't give something to return, the function will return the empty string.

### 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 constants

#### backslash

Backslash character.

#### eof

End of file character, zero byte.

#### eol

End of line character.

#### quote

Double quote character.

### Standard library functions

#### add a b

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.

#### intinc

Assumes `a` is a string containing solely a decimal integer.

Returns a increased by one.

#### mapclear mapname

Maps are global and can be used from any function.

Clears all values set in the map named `mapname`.

#### mapgetkey mapname key

Maps are global and can be used from any function.

Looks up `key` in the map named `mapname` and returns it.
If not found, returns an empty string.

#### mapsetkey mapname key value

Maps are global and can be used from any function.

Adds a mapping from `key` to `value` to the map named `mapname`.

#### not a

Returns "1" if a is empty; otherwise, returns "".

#### 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)

#### strlen a

Returns a string with the length of a.

### 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.

![A graph of the build setup](t-diagram.png)

How to run
---

Install python3 and make

```sh
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`.
