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README.md

A Python counterfeit

Piton is both a language mimicking the syntax and semantics of Python and a compiler for this language, implemented in Racket. While type annotations are optional in Python (see PEP 484 and 526), they are mandatory in Piton, which is statically and strongly typed. Piton does not implement concepts such as inheritance or classes. For now, the compiler only supports MIPS assembly, but x86 may be added some day.

Sample program and tests

The tests/ folder contains a fairly exhaustive demonstration of all features and errors supported. You will need Spim (a MIPS simulator) and Python 3.6 to run test_features.py, as this script will execute all the code in tests/features/ and compare Piton and Python output.

The samples/ folder contains a short program showcasing Piton features. You may compile it with:

src/piton.rkt samples/poetry_generator.py

and then run the resulting assembly in Spim with:

spim -f samples/poetry_generator.s

Features

The basics

Piton comes with scalar types int and bool, and not-so-scalar string. ints and bools are assigned by copy and strings by reference, all of them are immutable. The NoneType also exists internally, as well as a function type, but no function nor NoneType variable may be declared with the language. Ints and bools come with basic arithmetic, comparison and logical operators as well as the unary not operator. Strings can be tested for equality and concatenated with the + operator, and offer subscript access to individual characters. Supported controls structures are limited to if-(else) and while. Indentation is handled in the same fashion as in Python, it can vary in length and mix tabs and spaces. Comments are also supported.

Functions

Function calls are implemented, and Piton's standard library includes typed print and len routines. Defining new functions is also possible, including nested functions (declared in the body of an outer function). Functions names share the same namespace as variables. As said before, an internal function type does exists but cannot be used to declare variables nor function parameters, and function variables cannot be reassigned. Recursion is fully supported, without tail-call optimization just as in Python.

Scoping

Python's lexical scoping follows the LEGB rule: Local, Enclosed, Global, Built-in. In a Python file, symbols at the upper-most level are assigned to the global scope (not considering the case of modules). In contrast, Piton does not really have a proper global scope, but all upper-most symbols are located in a implicit main function, which scope acts a a kind of global scope enclosing everything else except built-ins. Apart from this, Piton follow the same LEGB order for symbol resolution and supports variable and function shadowing. Function bodies open a new scope, conditional branches do not, just as in Python.

Safety

While Python is highly permissive and dynamic, Piton aims to be a safe static language. Its type system allows arity checking and type checking (including function parameters and return values) at compile-time. A couple of run-time dynamic checks are also performed, such as bounds checking for subscript access and zero division.

Implementation

Inlining

To simplify compilation, all binary and unary operations are converted into internal function calls, as well as subscript accesses. In order to avoid the runtime overhead of numerous functions call, these internal functions (as well as the typed print functions) are inlined by the compiler. What makes it possible is that during the semantic analysis, all nested function calls are flattened so as to avoid trouble with return values needing to be passed to the enclosing call. For instance, code such as this one:

print_int(a + 2 * b)

will be transformed inside Piton's AST into:

auto_8493 = mul_ints(2, b)
auto_8494 = add_ints(a, auto_8493)
print(auto_8494)

Inlining then becomes much easier as all returned values are already explicitly pushed on the stack. Note that to avoid issues with nested calls inside conditional statements, a internal AST Block structure is used to enclose the flattened statements. Indeed, the additional "flat" statements cannot just be plainly inserted before the conditional statement, as they need to be reevaluated at every iteration in the case of a while loop.

Indentation parsing

Python-style indentation parsing rests on the emission of Indent/Dedents tokens (see Python 3 full grammar). This requires the ability to emit several tokens from one parsing rule, in order to handle multiple dedents performed in one line such as in the following example:

def greet(name: str):
    if name == "William":
        print_str("Hi Bill!")
    else:
        print_str("Hello " + name)
# this line unidents 2 levels at once
greet("Sarah")

Racket's yacc parser does not handle this situation out of the box. However, it is possible to wrap multiple tokens into one, and intercept all calls from the lexer to the parser so as to unwrap theses tokens and enqueue them, before dequeuing one and returning it to the lexer.

MIPS conventions

MIPS register $t9 is dedicated to storing the results of conditional expressions, as well as operations needed a temporary register immediately released, such as memory-to-memory moves. Registers $t8 and $t9 are used for the retrieval of enclosing scope variables (see following paragraph). Register $t4, $t5 and $t6 are available when we need to load memory values in registers to pass them to an instruction expecting register operands, or reciprocally when the result of a instruction has to be put in a register before being stored in memory.

Functions arguments are passed by pushing them on the stack before calling the function. Registers $a0 to $a3 are only used to pass arguments to inlined functions. On the other hand, the $v0 register is always used to pass back return values. The $sp register is actually used a a frame pointer and is never modified within a function, the compiler being in charge of tracking the size of the stack.

Free variables are resolved at runtime using activation links, as described on this page. The tricky part to understand is that while it is possible to know at compile-time to which scope belongs a variable referred by a non-local symbol, we do not know yet where it will lay in memory relatively to the current frame, as this depends on the execution flow:

name: str = input()
def greet(greeting: str):
    print_str(greeting + " " + name)
def greet_warmly():
    greet("Hello dear" + name)
if be_warm:
    greet_warmly()
else:
    greet("Hello")

In this example, the symbol name inside greet always refer to the same variable belonging to the immediate outer scope of greet. But depending on where greet is called from, the frame of this scope might be the previous one on the stack, or the one before.

Activation links build a chain allowing us to walk back up to the address of frame where the non-local variable is stored. At each function call, in addition to the return address and the arguments, the current frame pointer is pushed onto the stack. When accessing a non-local variable, the compiler knows how many times it has to walk up the activation link chain, because it knows the level of the relevant scope relatively to the current scope. Once the address the scope's frame is known, all that is left to do is to use the address of the variable relative to that frame.

What's missing

Piton is an academic project obviously missing many real-life features. For instance, memory management is inexistent and objects allocated on the heap are never released. This is anyway a bit out of the scope of this project. What could be interesting would be the implementation of generics, using any as a type placeholder. This would make Piton code closer to real Python by removing the need of typed versions of standard function such as print, and could also allow for the introduction of type-safe lists. Full type annotation of function variables would allow them to become first-class objects that one could pass around while preserving type safety. Finally, beyond float support, it could also be interesting to add a notion of inheritance for built-in types such as numbers, as in the Python language specification.