peter-lang

Useless and barely functional toy programming language interpreter written in Haskell from scratch (without any research how to actually lay things out)

The missing link between C and Python

Syntax

C style syntax with Python like runtime interpretation and built in functions.

Main

Main function void main() {} is the entry point.

void main() {
  print("Hello, World!");
}

Alternately, statements are executed from top to bottom in case no main is found.

print("Hello, World!");

Primitives

• void
• bool
• int
• float
• str

Conversion

• str(1) -> "1"
• int("1") -> 1
• float("1.3") -> 1.3

Operations

There is no operator precedence

Operation	void	bool	int	float	str
+	x	x	✓	✓	✓
-	x	x	✓	✓	x
*	x	x	✓	✓	x
\	x	x	✓	x	x
%	x	x	✓	x	x
==	x	✓	✓	✓	✓
!=	x	✓	✓	✓	✓
<	x	✓	✓	✓	x
>	x	✓	✓	✓	x
<=	x	✓	✓	✓	x
>=	x	✓	✓	✓	x
&&	x	✓	✓	x	x
||	x	✓	✓	x	x

Flow Control

If:

if 1 < 2 {
  i = 1;
} else {
  i = -1;
}

while:

int j = 0;
while j < 10 {
  println(str(j));
  j = j + 1;
}

Functions

int f() {
  return 42;
}

void main() {
  int x = f();
}

Structs

struct T {
  int x;
  int y;
}

T t;
t.x = 2;

Built In

• print -> prints string without buffering
• println -> print + appends newline at the end
• input -> reads stdin and outputs str

Limitations & Issues

• left side of operation has to be atomic (because of left recursive grammar)
• no arrays (yet)
• many bugs
• bad error messages from parser and interpreter
• types are optional and currently not always strictly enforced

Missing/Planned Features

• Arrays
• Classes (or something similar)
• String manipulation
• Multiple files support
• Enums
• Pattern matching
• switch
• for
• define grammar

Installation

Native

Dependencies:

• stack
• ghc-9.4.7: stack --compiler ghc-9.4.7 setup

Docker

docker build --target exe -t peter .
docker run peter ./examples/print.mmm

Usage

> stack run -- --help
Usage: peter-exe [-i|--inline STRING] [PATH]

Available options:
  -i,--inline STRING       Inline string to parse and interpret
  -h,--help                Show this help text

stack run -- ./examples/main_hello_world.mmm
stack run -- -i "void main() { print(\"Hello, World\"); }"

stack run -- ./examples/short_hello_world.mmm
stack run -- -i "print(\"Hello, World\");"

Tests

• stack test
• cd ./test/E2E/Interpreter/examples/ && ./check_examples.sh

Formatting

ormolu

ormolu --mode inplace $(find . -name '*.hs')

CI

docker build . + GitHub Actions