An alternative syntax for C, part 3: literals, return, and assignment

README.md

Blog 2019/10/15

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An alternative syntax for C, part 3: literals, return, and assignment

<- part 2 | part 4 ->

This version of the transpiler adds support for:

• float, string, and boolean literals
• the return statement
• the assignment statement
• the combined variable declaration / assignment statement

This:

func main(argc: int, argv: pointer<pointer<char>>) -> int:
    a: int = 42
    a = 43
    b: float = -3.14
    c: bool = true
    return 0

transpiles to this:

int main(int argc, char **argv) {
    int a = 42;
    a = 43;
    float b = -3.14;
    bool c = true;
    return 0;
}

TL;DR: What is Cy?

Cy is an alternative syntax for C which features indentation-based scoping and intuitive type declarations.

func main(argc: int, argv: pointer<pointer<char>>) -> int:
    printf("Hello, world!")
    return 0

But why tho?

To teach myself how to write a transpiler! 🤩🤩🤩

cy.py

#!/usr/bin/env python
# -*- coding: utf-8 -*-

# cy.py: a traspiler for an alternate syntax for C.
# Note: this is an incomplete work-in-progress.


# Data structures:

# A token is a tuple.
# First element is a string 'token'.
# Second element is a string which is the token type (e.g. 'OBRACKET').
# Third element is a string which is the source text of the token (e.g. '[').
# e.g. ('token', 'OBRACKET', '[')
# e.g. ('token', 'IDENTIFIER', 'foo')
# e.g. ('token', 'WS', '   ')


# An AST node is a tuple.
# First element is a string 'ast'.
# Second element is a string which is the AST node type (e.g. 'vardecl').
# Third element is an array which are the child tuples (AST nodes or tokens).
# e.g. ('ast', 'type', [('token', 'IDENTIFIER', 'int')])


# Grammar:
#
#     program = statement [WS] { statement [WS] }
#   statement = declassign | assign | fundecl | vardecl | scope | return
#     fundecl = FUNC WS IDENTIFIER fundeclargs [ fundeclret ] scope
# fundeclargs = OPAREN [ [WS] vardecl { COMMA WS vardecl } [WS] ] CPAREN
#  fundeclret = WS ARROW WS type
#     vardecl = IDENTIFIER COLON WS type
#       scope = COLON INDENT statement { DENT statement } DEDENT
#      return = RETURN [ expr ]
#  declassign = vardecl WS EQUAL WS expr
#      assign = IDENTIFIER WS EQUAL WS expr
#        expr = INTLIT | FLOATLIT | STRINGLIT | BOOLLIT
#        type = POINTER LT type GT
#             | ARRAY LT type GT
#             | FUNCTION LT type GT
#             | ARRAY OBRACKET INTLIT CBRACKET LT type GT
#             | IDENTIFIER
#
# Note: lower case are AST nodes.
# Note: ALL CAPS are tokens (terminals).


# Token data structure utils:

def is_token(token):
    """Returns whether the arg is a token."""
    return isinstance(token, tuple) and len(token) > 0 and token[0] == 'token'


def token_type(token):
    """Returns the type of the token (e.g. 'IDENTIFIER')."""
    assert is_token(token)
    return token[1]


def token_text(token):
    """Returns the source text of the token (e.g. 'foo' for a 'IDENTIFIER' token)."""
    assert is_token(token)
    return token[2]


def is_toktype(token, toktype):
    """
    Returns whether the token is of the given type.
    _toktype_ is e.g. 'IDENTIFIER', 'COLON', etc.
    """
    assert is_token(token)
    return token_type(token) == toktype



# Tokenizer implementation:

def load_tokendefs(fpath):
    """
    Load the token definitions from the file at fpath.

    Returns a list of tuples of the form (<token type>, <compiled regex>).

    The format of the tokendefs file should be pairs of lines:
    - a line which is the token type (e.g. 'IDENTIFIER', 'OBRACKET', etc),
    - followed by a line which is the regex which recognizes that token.

    Example tokendefs.txt:
    IDENTIFIER
    [a-zA-Z][a-zA-Z0-9]*
    OPAREN
    \(
    CPAREN
    )
    """
    import re
    tokendefs = []
    with open(fpath) as f:
        for line in f:
            token_name = line.rstrip('\n')
            pattern = f.next().rstrip('\n')
            regex = re.compile(pattern)
            tokendef = (token_name, regex)
            tokendefs.append(tokendef)
    return tokendefs


def offside_rule(token, previous, indent):
    """
    Implement the "offside rule" indentation-based scoping mechanism.
    See https://en.wikipedia.org/wiki/Off-side_rule

    If this is a WS (containing a newline) following a COLON and indentation has increased,
      sub in an INDENT token.
    If this is a WS (containing a newline) and indentation has decreased,
      sub in a DEDENT token.
    If this is a WS (containing a newline) and indentation hasn't changed,
      sub in a DENT token.
    Otherwise, return the token unmodified.
    """
    assert is_token(token)
    assert is_token(previous) or previous is None
    text = token_text(token)
    if is_toktype(token, 'WS') and '\n' in text:
        indentation = text.split('\n')[-1]
        if is_toktype(previous, 'COLON') and len(indentation) > indent:
            token = ('token', 'INDENT', text)
            indent += 1
        elif len(indentation) < indent:
            token = ('token', 'DEDENT', text)
            indent -= 1
            assert indent >= 0
        else:
            if indent > 0:
                token = ('token', 'DENT', text)
    return (token, indent)


def tokenize(tokendefs, input, indent=0):
    """Uses tokendefs to tokenize the 'input' string and return a list of tokens"""
    tokens = []
    offset = 0
    previous = None
    while offset < len(input):
        for (token_name, regex) in tokendefs:
            m = regex.match(input, offset)
            if m is not None:
                matched_text = m.group(0)
                offset += len(matched_text)
                token = ('token', token_name, matched_text)
                (token, indent) = offside_rule(token, previous, indent)
                if token is not None:
                    tokens.append(token)
                    previous = token
                break
        else:
            raise Exception("Couldn't tokenize starting at '%s...'" % input[offset:offset+16])
    return tokens



# AST node data structure utils:

def is_ast(ast):
    """Returns whether the arg is an AST node."""
    return isinstance(ast, tuple) and len(ast) > 0 and ast[0] == 'ast'


def is_asttype(ast, asttype):
    """
    Returns whether the AST node is of the given type.
    _asttype_ is e.g. 'vardecl', 'statement', etc.
    """
    assert is_ast(ast)
    return ast[1] == asttype


def ast_children(ast):
    """Return the child nodes of the given AST."""
    assert is_ast(ast)
    return ast[2]


# Parser implementation:

# Note: all parse_x functions share a similar format:
# - Their first argument is the list of tokens.
# - They return a tuple of (<parsed result>, <remaining tokens>).
#   - <parsed result> is either an AST node or a token (a terminal).
#   - <remaining tokens> is the list of unconsumed tokens.
# - If the parse fails, (None, <tokens>) is returned, where <tokens>
#   is the list of tokens which was passed in.

def alternation(tokens, funcs):
    """Try each of the parsing functions until one succeeds."""
    failure = (None, tokens)
    for f in funcs:
        (ast, tokens) = f(tokens)
        if ast is not None:
            return (ast, tokens)
    return failure


def parse_terminal(tokens, toktype):
    """
    Attempts to parse a terminal node of type _toktype_.
    Note that the parsed result of a terminal is the token itself, not an AST node.
    """
    failure = (None, tokens)
    if len(tokens) > 0 and is_toktype(tokens[0], toktype):
        return (tokens[0], tokens[1:])
    else:
        return failure


# fundecl examples:
# func f():
# func f(a: int):
# func f() -> int:
# func f(a: int) -> int:
# func f(a: int, b: int):
# func f(a: int, b: int) -> int:
def parse_fundecl(tokens):
    """
    Attempts to parse a function declaration.
    
    Grammar:
    fundecl = FUNC WS IDENTIFIER fundeclargs [ fundeclret ] scope
    """
    failure = (None, tokens)
    (ast, tokens) = parse_terminal(tokens, 'FUNC')
    if ast is None:
        return failure
    (ast, tokens) = parse_terminal(tokens, 'WS')
    if ast is None:
        return failure
    (identifier_token, tokens) = parse_terminal(tokens, 'IDENTIFIER')
    if identifier_token is None:
        return failure
    (args_ast, tokens) = parse_fundeclargs(tokens)
    if args_ast is None:
        return failure
    (ret_ast, tokens) = parse_fundeclret(tokens)
    (scope_ast, tokens) = parse_scope(tokens)
    if scope_ast is None:
        return failure
    ast = ('ast', 'fundecl', [identifier_token, args_ast, ret_ast, scope_ast])
    return (ast, tokens)


def parse_fundeclargs(tokens):
    """
    Attempts to parse a list of function declaration arguments.

    Grammar:
    fundeclargs = OPAREN [ [WS] vardecl { COMMA WS vardecl } [WS] ] CPAREN
    """
    failure = (None, tokens)
    (ast, tokens) = parse_terminal(tokens, 'OPAREN')
    if ast is None:
        return failure

    args = []
    (_, tokens) = parse_terminal(tokens, 'WS')
    (arg, tokens) = parse_vardecl(tokens)
    if arg is not None:
        args.append(arg)
        while True:
            (ast, tokens) = parse_terminal(tokens, 'COMMA')
            if ast is None:
                break
            (ast, tokens) = parse_terminal(tokens, 'WS')
            if ast is None:
                break
            (arg, tokens) = parse_vardecl(tokens)
            if ast is None:
                break
            args.append(arg)
    
    (ast, tokens) = parse_terminal(tokens, 'CPAREN')
    if ast is None:
        return failure
    
    ast = ('ast', 'fundeclargs', args)
    return (ast, tokens)


def parse_fundeclret(tokens):
    """
    Attempts to parse a function declaration return value.

    Grammar:
    fundeclret = WS ARROW WS type
    """
    failure = (None, tokens)
    (ast, tokens) = parse_terminal(tokens, 'WS')
    if ast is None:
        return failure
    (ast, tokens) = parse_terminal(tokens, 'ARROW')
    if ast is None:
        return failure
    (ast, tokens) = parse_terminal(tokens, 'WS')
    if ast is None:
        return failure
    (ast, tokens) = parse_type(tokens)
    if ast is None:
        return failure
    
    ast = ('ast', 'fundeclret', [ast])
    return (ast, tokens)


def parse_scope(tokens):
    """
    Attempts to parse a scoped sequence of statements.

    Grammar:
    scope = COLON INDENT statement { DENT statement } DEDENT
    """
    failure = (None, tokens)
    (ast, tokens) = parse_terminal(tokens, 'COLON')
    if ast is None:
        return failure
    (ast, tokens) = parse_terminal(tokens, 'INDENT')
    if ast is None:
        return failure

    statements = []
    (ast, tokens) = parse_statement(tokens)
    if ast is None:
        return failure
    statements.append(ast)
    while True:
        (ast, tokens) = parse_terminal(tokens, 'DENT')
        if ast is None:
            break
        (ast, tokens) = parse_statement(tokens)
        if ast is None:
            break
        statements.append(ast)

    (ast, tokens) = parse_terminal(tokens, 'DEDENT')
    if ast is None:
        return failure
    
    ast = ('ast', 'scope', statements)
    return (ast, tokens)


def parse_vardecl(tokens):
    """
    Attempts to parse a vardecl AST node.

    Grammar:
    vardecl = IDENTIFIER COLON WS type
    """
    failure = (None, tokens)
    (identifier_token, tokens) = parse_terminal(tokens, 'IDENTIFIER')
    if identifier_token is None:
        return failure
    (colon_token, tokens) = parse_terminal(tokens, 'COLON')
    if colon_token is None:
        return failure
    (wspace_token, tokens) = parse_terminal(tokens, 'WS')
    if wspace_token is None:
        return failure
    (type_ast, tokens) = parse_type(tokens)
    if type_ast is None:
        return failure
    vardecl_ast = ('ast', 'vardecl', [identifier_token, type_ast])
    return (vardecl_ast, tokens)


def parse_type(tokens):
    """
    Attemps to parse a type declaration.

    Grammar:
    type = POINTER LT type GT
         | ARRAY LT type GT
         | FUNCTION LT type GT
         | ARRAY OBRACKET INTLIT CBRACKET LT type GT
         | IDENTIFIER
    """

    def parse_type1(tokens, toktype):
        """
        Grammar fragments:
        POINTER LT type GT
        ARRAY LT type GT

        'array<int>' becomes:
        ('ast', 'type', [
            ('token', 'ARRAY', 'array'),
            ('ast', 'type', [
                ('token', 'IDENTIFIER', 'int')
            ])
        ])

        'pointer<char>' becomes:
        ('ast', 'type', [
            ('token', 'POINTER', 'pointer'),
            ('ast', 'type', [
                ('token', 'IDENTIFIER', 'char')
            ])
        ])

        'array<pointer<char>>' becomes:
        ('ast', 'type', [
            ('token', 'ARRAY', 'array'),
            ('ast', 'type', [
                ('token', 'POINTER', 'pointer'),
                ('ast', 'type', [
                    ('token', 'IDENTIFIER', 'char')
                ])
            ])
        ])
        """
        failure = (None, tokens)
        (identifier_token, tokens) = parse_terminal(tokens, toktype)
        if identifier_token is None:
            return failure
        (lessthan, tokens) = parse_terminal(tokens, 'LT')
        if lessthan is None:
            return failure
        (subtype_ast, tokens) = parse_type(tokens)
        if subtype_ast is None:
            return failure
        (greaterthan, tokens) = parse_terminal(tokens, 'GT')
        if greaterthan is None:
            return failure
        ast = ('ast', 'type', [identifier_token, subtype_ast])
        return (ast, tokens)

    def parse_type2(tokens):
        """
        Grammar fragment:
        ARRAY OBRACKET INTLIT CBRACKET LT type GT

        'array[8]<int>' becomes:
        ('ast', 'type', [
            ('token', 'ARRAY', 'array'),
            ('token', 'INTLIT', '8'),
            ('ast', 'type', [
                ('token', 'IDENTIFIER', 'int')
            ])
        ])
        """
        failure = (None, tokens)
        (identifier_token, tokens) = parse_terminal(tokens, 'ARRAY')
        if identifier_token is None:
            return failure
        (obracket, tokens) = parse_terminal(tokens, 'OBRACKET')
        if obracket is None:
            return failure
        (intlit, tokens) = parse_terminal(tokens, 'INTLIT')
        if intlit is None:
            return failure
        (cbracket, tokens) = parse_terminal(tokens, 'CBRACKET')
        if obracket is None:
            return failure
        (lessthan, tokens) = parse_terminal(tokens, 'LT')
        if lessthan is None:
            return failure
        (subtype_ast, tokens) = parse_type(tokens)
        if subtype_ast is None:
            return failure
        (greaterthan, tokens) = parse_terminal(tokens, 'GT')
        if greaterthan is None:
            return failure
        ast = ('ast', 'type', [identifier_token, intlit, subtype_ast])
        return (ast, tokens)

    def parse_type3(tokens):
        """
        Grammar fragment:
        IDENTIFIER

        'int' becomes:
        ('ast', 'type', [('token', 'IDENTIFIER', 'int')])

        'char' becomes:
        ('ast', 'type', [('token', 'IDENTIFIER', 'char')])
        """
        failure = (None, tokens)
        (identifier_token, tokens) = parse_terminal(tokens, 'IDENTIFIER')
        if identifier_token is None:
            return failure
        type_ast = ('ast', 'type', [identifier_token])
        return (type_ast, tokens)

    failure = (None, tokens)
    (ast, tokens) = parse_type1(tokens, 'POINTER')
    if ast is not None:
        return (ast, tokens)
    (ast, tokens) = parse_type1(tokens, 'ARRAY')
    if ast is not None:
        return (ast, tokens)
    (ast, tokens) = parse_type1(tokens, 'FUNCTION')
    if ast is not None:
        return (ast, tokens)
    (ast, tokens) = parse_type2(tokens)
    if ast is not None:
        return (ast, tokens)
    (ast, tokens) = parse_type3(tokens)
    if ast is not None:
        return (ast, tokens)
    return failure


def parse_return(tokens):
    """
    Attempts to parse a return statement.

    Grammar:
    return = RETURN [ WS expr ]
    """
    failure = (None, tokens)
    (ast, tokens) = parse_terminal(tokens, 'RETURN')
    if ast is None:
        return failure
    return_ast = ('ast', 'return', [])
    (ast, tokens) = parse_terminal(tokens, 'WS')
    if ast is None:
        return return_ast
    (expr_ast, tokens) = parse_expr(tokens)
    if expr_ast is not None:
        return_ast = ('ast', 'return', [expr_ast])
    return (return_ast, tokens)


def parse_expr(tokens):
    """
    Attempts to parse an expression.

    Grammar:
    expr = INTLIT | FLOATLIT | STRINGLIT | BOOLLIT
    """
    failure = (None, tokens)
    (token, tokens) = parse_terminal(tokens, 'INTLIT')
    if token is not None:
        ast = ('ast', 'expr', [token])
        return (ast, tokens)
    (token, tokens) = parse_terminal(tokens, 'FLOATLIT')
    if token is not None:
        ast = ('ast', 'expr', [token])
        return (ast, tokens)
    (token, tokens) = parse_terminal(tokens, 'STRINGLIT')
    if token is not None:
        ast = ('ast', 'expr', [token])
        return (ast, tokens)
    (token, tokens) = parse_terminal(tokens, 'BOOLLIT')
    if token is not None:
        ast = ('ast', 'expr', [token])
        return (ast, tokens)
    return failure


def parse_declassign(tokens):
    """
    Attempts to parse a declaration-assignment statement.

    Grammar:
    declassign = vardecl WS EQUAL WS expr
    """
    failure = (None, tokens)
    (vardecl_ast, tokens) = parse_vardecl(tokens)
    if vardecl_ast is None:
        return failure
    (ast, tokens) = parse_terminal(tokens, 'WS')
    if ast is None:
        return failure
    (ast, tokens) = parse_terminal(tokens, 'EQUAL')
    if ast is None:
        return failure
    (ast, tokens) = parse_terminal(tokens, 'WS')
    if ast is None:
        return failure
    (expr_ast, tokens) = parse_expr(tokens)
    if expr_ast is None:
        return failure
    ast = ('ast', 'declassign', [vardecl_ast, expr_ast])
    return (ast, tokens)


def parse_assign(tokens):
    """
    Attempts to parse an assignment statement.

    Grammar:
    assign = IDENTIFIER WS EQUAL WS expr
    """
    failure = (None, tokens)
    (identifier_token, tokens) = parse_terminal(tokens, 'IDENTIFIER')
    if identifier_token is None:
        return failure
    (ast, tokens) = parse_terminal(tokens, 'WS')
    if ast is None:
        return failure
    (ast, tokens) = parse_terminal(tokens, 'EQUAL')
    if ast is None:
        return failure
    (ast, tokens) = parse_terminal(tokens, 'WS')
    if ast is None:
        return failure
    (expr_ast, tokens) = parse_expr(tokens)
    if ast is None:
        return failure
    ast = ('ast', 'assign', [identifier_token, expr_ast])
    return (ast, tokens)


def parse_statement(tokens):
    """
    Attempts to parse a statement.

    Grammar:
    statement = declassign | assign | fundecl | vardecl | scope | return
    """
    failure = (None, tokens)
    (ast, tokens) = alternation(tokens, [
        parse_declassign,
        parse_assign,
        parse_fundecl,
        parse_vardecl,
        parse_scope,
        parse_return
    ])
    if ast is None:
        return failure
    statement_ast = ('ast', 'statement', [ast])
    return (statement_ast, tokens)


def parse_program(tokens):
    """
    Attempts to parse a program.
    
    Grammar:
    statement [WS] { statement [WS] }
    """
    failure = (None, tokens)
    statement_asts = []
    (ast, tokens) = parse_statement(tokens)
    if ast is None:
        return failure
    statement_asts.append(ast)
    (ast, tokens) = parse_terminal(tokens, 'WS')
    while True:
        (ast, tokens) = parse_statement(tokens)
        if ast is None:
            break
        statement_asts.append(ast)
        (ast, tokens) = parse_terminal(tokens, 'WS')
    ast = ('ast', 'program', statement_asts)
    return (ast, tokens)


def parse(tokens):
    """Attempts to parse the tokens into an AST."""
    (ast, tokens) = parse_program(tokens)
    if ast is None:
        raise Exception("Couldn't parse starting at %s" % tokens[:4])
    if len(tokens) > 0:
        raise Exception("Leftover tokens: %s" % tokens)
    return ast



# Output generator implementation:

# Note: all generate_x functions share a similar format:
# - Their first argument is as AST.
# - They return a string (of C code).
# If the passed AST is valid, then errors are not possible.
# Exceptions are thrown otherwise.


def generate_type(ast, identifier=""):
    """
    Generate a C type declaration.
    
    If identifier is not given, this is an abstract type declaration (e.g. a cast).
    """
    def did_switch_direction(previous, current):
        """
        Detect a change in 'direction' while intepreting the type stack.

        C type declaration operator precedence requires that we "go right" first.
        i.e. 'int *foo[]' is "array of pointer to int", not "pointer to array of int".
        In order to express "pointer to array of int", we have to use parenthesis
        to change overcome operator precedence, i.e. 'int (*foo)[]'.
        Any time we need to "change direction", we need to wrap in parenthesis.
        See http://unixwiz.net/techtips/reading-cdecl.html
        """
        lefts = ['pointer']
        rights = ['array', 'function']
        return (previous in lefts and current in rights) \
            or (previous in rights and current in lefts)

    assert is_asttype(ast, 'type')
    types = type_ast_as_list(ast)
    output = identifier
    previous = None
    for t in types[:-1]:
        if t == 'pointer':
            if did_switch_direction(previous, t):
                output = "*(%s)" % output
            else:
                output = "*%s" % output
        elif t == 'array':
            if did_switch_direction(previous, t):
                output = "(%s)[]" % output
            else:
                output = "%s[]" % output
        elif t == 'function':
            if did_switch_direction(previous, t):
                output = "(%s)()" % output
            else:
                output = "%s()" % output
        elif t.startswith('array:'):
            array_size = int(t.split(':')[1])
            if did_switch_direction(previous, 'array'):
                output = "(%s)[%i]" % (output, array_size)
            else:
                output = "%s[%i]" % (output, array_size)
        else:
            raise Exception("generate_type: unexpected type '%s'." % t)
        
        if t.startswith('array:'):
            previous = 'array'
        else:
            previous = t
    
    base_type = types[-1]
    if identifier == "":
        output = "%s%s" % (base_type, output)
    else:
        output = "%s %s" % (base_type, output)
    return output


def type_ast_as_list(ast):
    """
    Return the type AST as a list of types.

    e.g.
    - 'int' results in ['int']
    - 'pointer<array<char>>' results in ['pointer', 'array', 'char']
    - 'array[8]<int>' results in ['array:8', 'int']
    """
    assert is_asttype(ast, 'type')
    children = ast_children(ast)
    if len(children) == 1:
        # if there is only one child, it is the "base" type (e.g. int, char, etc).
        assert is_token(children[0])
        token = children[0]
        assert is_toktype(token, 'IDENTIFIER')
        # return e.g. ['int'], ['char'], etc.
        return [token_text(token)]
    elif len(children) == 2:
        # if there are 2 children, this is either array or pointer or function.
        assert is_toktype(children[0], 'ARRAY') \
            or is_toktype(children[0], 'POINTER') \
            or is_toktype(children[0], 'FUNCTION')
        token = children[0]
        assert is_ast(children[1])
        sub_ast = children[1]
        assert is_asttype(ast, 'type')
        # return e.g. ['array', <recursive call>]
        return [token_text(token)] + type_ast_as_list(sub_ast)
    elif len(children) == 3:
        # if there are three children, this is a dimensioned array (e.g. array[8]).
        assert is_toktype(children[0], 'ARRAY')
        assert is_toktype(children[1], 'INTLIT')
        intlit_token = children[1]
        array_size = int(token_text(intlit_token))
        assert is_ast(children[2])
        sub_ast = children[2]
        assert is_asttype(sub_ast, 'type')
        # return e.g. ['array:8', <recursive call>]
        return ["array:%s" % (array_size)] + type_ast_as_list(sub_ast)
    else:
        raise Exception("type_ast_as_list: type AST node with more than 3 children!")


def generate_vardecl(ast):
    """
    Generate a C variable declaration.
    
    Note: we don't append the trailing ';' here because this is also used for function args.
    """
    assert is_asttype(ast, 'vardecl')
    children = ast_children(ast)

    identifier_token = children[0]
    assert is_toktype(identifier_token, 'IDENTIFIER')
    var_name = token_text(identifier_token)

    type_ast = children[1]
    assert is_asttype(type_ast, 'type')

    # e.g. 'int a', 'char* b', 'int c[8]', 'char** d', 'int* d[3]', etc.
    return "%s" % generate_type(type_ast, var_name)


def generate_fundecl(ast):
    """Generate a C function declaration."""
    assert is_asttype(ast, 'fundecl')

    identifier_token = ast_children(ast)[0]
    assert is_toktype(identifier_token, 'IDENTIFIER')
    identifier = token_text(identifier_token)

    fundeclargs_ast = ast_children(ast)[1]
    assert is_asttype(fundeclargs_ast, 'fundeclargs')
    arg_asts = ast_children(fundeclargs_ast)
    args = []
    for vardecl_ast in arg_asts:
        arg = generate_vardecl(vardecl_ast)
        args.append(arg)
    args_output = ", ".join(args)

    fundeclret_ast = ast_children(ast)[2]
    if fundeclret_ast is None:
        ret = "void"
    else:
        ret_ast = ast_children(fundeclret_ast)[0]
        ret = generate_type(ret_ast)

    scope_ast = ast_children(ast)[3]
    scope = generate_scope(scope_ast)

    output = "%s %s(%s)%s" % (ret, identifier, args_output, scope)
    return output


def generate_scope(ast, indent=0):
    """Generate a C scope."""
    assert is_ast(ast)
    assert is_asttype(ast, 'scope')
    output = " {"
    for statement_ast in ast_children(ast):
        statement = generate_statement(statement_ast)
        line = "\n" + (" " * 4 * (indent+1)) + statement
        output += line
    line = "\n" + (" " * 4 * (indent)) + "}"
    output += line
    return output


def generate_expr(ast):
    """Generate a C expression."""
    assert is_asttype(ast, 'expr')
    child = ast_children(ast)[0]
    if is_token(child):
        token = child
        if is_toktype(token, 'INTLIT') \
            or is_toktype(token, 'FLOATLIT') \
            or is_toktype(token, 'STRINGLIT') \
            or is_toktype(token, 'BOOLLIT') \
        :
            output = token_text(token)
            return output
    raise Exception("generate_expr: expected an INTLIT, got %s" % (ast,))


def generate_return(ast):
    """Generate a C return statement."""
    assert is_asttype(ast, 'return')
    children = ast_children(ast)
    if len(children) == 0:
        return "return;"
    else:
        expr_ast = children[0]
        return "return %s;" % generate_expr(expr_ast)


def generate_declassign(ast):
    """Generate a C declaration-assignment statement."""
    assert is_asttype(ast, 'declassign')
    children = ast_children(ast)
    vardecl_ast = children[0]
    assert is_asttype(vardecl_ast, 'vardecl')
    vardecl = generate_vardecl(vardecl_ast)
    expr_ast = children[1]
    assert is_asttype(expr_ast, 'expr')
    expr = generate_expr(expr_ast)
    return "%s = %s;" % (vardecl, expr)


def generate_assign(ast):
    """Generate a C assignment statement."""
    assert is_asttype(ast, 'assign')
    children = ast_children(ast)
    identifier_token = children[0]
    assert is_toktype(identifier_token, 'IDENTIFIER')
    identifier = token_text(identifier_token)
    expr_ast = children[1]
    assert is_asttype(expr_ast, 'expr')
    expr = generate_expr(expr_ast)
    return "%s = %s;" % (identifier, expr)


def generate_statement(ast):
    """Generate a C statement."""
    assert is_asttype(ast, 'statement')
    sub_ast = ast_children(ast)[0]
    assert is_ast(sub_ast)
    if is_asttype(sub_ast, 'fundecl'):
        return generate_fundecl(sub_ast)
    elif is_asttype(sub_ast, 'vardecl'):
        return generate_vardecl(sub_ast) + ';'
    elif is_asttype(sub_ast, 'scope'):
        return generate_scope(sub_ast)
    elif is_asttype(sub_ast, 'return'):
        return generate_return(sub_ast)
    elif is_asttype(sub_ast, 'declassign'):
        return generate_declassign(sub_ast)
    elif is_asttype(sub_ast, 'assign'):
        return generate_assign(sub_ast)
    else:
        raise Exception("generate_statement: don't know how to generate %s" % sub_ast)


def generate_program(ast):
    """Generate a C program."""
    assert is_asttype(ast, 'program')
    outputs = []
    for statement_ast in ast_children(ast):
        outputs.append(generate_statement(statement_ast))
    return '\n'.join(outputs) + '\n'


def generate(ast):
    """Generate C code from the given AST."""
    return generate_program(ast)


if __name__ == "__main__":
    import sys
    import pprint
    tdefs = load_tokendefs("tokendefs.txt")
    input = None
    if len(sys.argv) > 1:
        input = open(sys.argv[-1]).read()
    else:
        input = sys.stdin.read()
    tokens = tokenize(tdefs, input)
    if '--tokens' in sys.argv:
        pprint.pprint(tokens)
        sys.exit(0)
    ast = parse(tokens)
    if '--ast' in sys.argv:
        pprint.pprint(ast)
        sys.exit(0)
    output = generate(ast)
    sys.stdout.write(output)

test-output.txt

👉 test13.cy
  input:
    a: pointer<int>
    
    func area(x: float, y: float) -> int:
        b: array<array<pointer<char>>>
    
    c: array[8]<char>
  output:
    int *a;
    int area(float x, float y) {
        char *(b[][]);
    }
    char c[8];
✅ test13.cy

👉 test14.cy
  input:
    func main(argc: int, argv: pointer<pointer<char>>) -> int:
        return 0
  output:
    int main(int argc, char **argv) {
        return 0;
    }
✅ test14.cy

👉 test15.cy
  input:
    func main(argc: int, argv: pointer<pointer<char>>) -> float:
        return -3.14
  output:
    float main(int argc, char **argv) {
        return -3.14;
    }
✅ test15.cy

👉 test16.cy
  input:
    func main(argc: int, argv: pointer<pointer<char>>) -> pointer<char>:
        return "I said \"Hello!\" to the butcher."
  output:
    char* main(int argc, char **argv) {
        return "I said \"Hello!\" to the butcher.";
    }
✅ test16.cy

👉 test17.cy
  input:
    func main(argc: int, argv: pointer<pointer<char>>) -> bool:
        return true
  output:
    bool main(int argc, char **argv) {
        return true;
    }
✅ test17.cy

👉 test18.cy
  input:
    func foo():
        a: int
        a = 42
  output:
    void foo() {
        int a;
        a = 42;
    }
✅ test18.cy

👉 test19.cy
  input:
    func foo():
        a: int = 42
  output:
    void foo() {
        int a = 42;
    }
✅ test19.cy

👉 test20.cy
  input:
    func main(argc: int, argv: pointer<pointer<char>>) -> int:
        a: int = 42
        a = 43
        b: float = -3.14
        c: bool = true
        return 0
  output:
    int main(int argc, char **argv) {
        int a = 42;
        a = 43;
        float b = -3.14;
        bool c = true;
        return 0;
    }
✅ test20.cy

test.sh

#!/bin/bash

# run all of the tests

set -e

for f in test*.cy
do
    echo "👉 $f"

    echo "  input:"
    cat $f | sed 's/^/    /'

    outfile=`mktemp`
    ./cy.py $f > $outfile
    echo "  output:"
    cat $outfile | sed 's/^/    /'

    cfile="`basename $f .cy`.c"
    if diff -q $cfile $outfile >/dev/null
    then
        echo "✅ $f"
    else
        echo "❌ $f"
        diff -urN --color=auto $cfile $outfile
    fi

    echo
done

test20.c

int main(int argc, char **argv) {
    int a = 42;
    a = 43;
    float b = -3.14;
    bool c = true;
    return 0;
}

test20.cy

func main(argc: int, argv: pointer<pointer<char>>) -> int:
    a: int = 42
    a = 43
    b: float = -3.14
    c: bool = true
    return 0

tokendefs.txt

POINTER
pointer
ARRAY
array
FUNCTION
function
FUNC
func
RETURN
return
FLOATLIT
-?\d+\.\d+
INTLIT
-?\d+
STRINGLIT
"([^"\\]|\\[\s\S])*"
BOOLLIT
(true)|(false)
COLON
:
EQUAL
=
COMMA
,
OBRACKET
\[
CBRACKET
]
OPAREN
\(
CPAREN
\)
ARROW
->
LT
<
GT
>
WS
\s+
IDENTIFIER
[a-zA-Z][a-zA-Z0-9_]*