Skip to content

Instantly share code, notes, and snippets.

@cellularmitosis cellularmitosis/README.md

Last active Oct 22, 2019
Embed
What would you like to do?
An alternative syntax for C, part 6: "if" statements

Blog 2019/10/17

<- previous | index | next ->

An alternative syntax for C, part 6: "if" statements

<- part 5 | part 7 ->

Int this installment:

  • Support for if-statements has been added
  • A DEDENT bugfix was added (previously, multiple decreases in indentation resulted in only one DEDENT token, which broke multiply-nested scopes)
  • An output indentation bug was fixed
if 1:
    return 1
elif foo():
    return 2
else:
    if (foo() || bar()):
        if a:
            return
    else:
        return

Note:

I have realized there is an issue with the parser:

if a parsing branch fails, the parser isn't smart enough to backtrack through a previously successful parsing branch.

i.e, this fails to parse:

if 1:
    return
    a = 1

(ignore for the moment that it doesn't make sense to have a statement after a return statement -- I'm just using this example to illustrate the problem)

It gets the tokenization correct:

$ ./cy.py --tokens 9.cy 
[('token', 'IF', 'if'),
 ('token', 'WS', ' '),
 ('token', 'INTLIT', '1'),
 ('token', 'COLON', ':'),
 ('token', 'INDENT', '\n    '),
 ('token', 'RETURN', 'return'),
 ('token', 'WS', '\n    '),
 ('token', 'IDENTIFIER', 'a'),
 ('token', 'WS', ' '),
 ('token', 'EQ', '='),
 ('token', 'WS', ' '),
 ('token', 'INTLIT', '1'),
 ('token', 'DEDENT', '\n')]

but it fails to parse:

$ ./cy.py 9.cy 
Traceback (most recent call last):
  File "./cy.py", line 1577, in <module>
    ast = parse(tokens)
  File "./cy.py", line 1006, in parse
    raise Exception("Couldn't parse starting at %s" % tokens[:8])
Exception: Couldn't parse starting at [('token', 'IF', 'if'), ('token', 'WS', ' '), ('token', 'INTLIT', '1'), ('token', 'COLON', ':'), ('token', 'INDENT', '\n    '), ('token', 'RETURN', 'return'), ('token', 'WS', '\n    '), ('token', 'IDENTIFIER', 'a')]

The problem is that the return statement ends up consuming RETURN WS IDENTIFIER. That is, it thinks the statement is return a. This causes it to blow up when it tries to parse = 1 as a statement.

The parser will try alternatives if a parsing branch fails, but it won't go back on previously successful parsing branches to try alternatives.

I.e. because return a was considered successful, it will never go back far enough to try return as a stand-alone statement and then proceed with a = 1.

For reference, this what a successful parse derivation would look like:

  • program
  • statement WS statement WS
  • return WS statement WS
  • RETURN WS statement WS
  • RETURN WS assign WS
  • RETURN WS IDENTIFIER WS aoperator WS expr WS
  • RETURN WS IDENTIFIER WS EQ WS expr WS
  • RETURN WS IDENTIFIER WS EQ WS INTLIT WS

hmm, rather than implementing back-tracking, I wonder if I can solve this by getting more specific about what "whitespace" means?

Note 2:

Let's describe a simpler scenario which illustrates this problem:

Consider this grammar:

  program = statement { statement }
statement = foo | bar
      foo = 'a' ['b']
      bar = 'b' 'b'
  • A program is a series of statements
  • A statement is a foo or a bar
  • A foo is either "a" or "ab"
  • A bar is "bb"

The problem comes in when parsing "abb".

This should be parsed as:

  • program
  • statement statement
  • foo("a") bar("bb")

But if foo consumes "ab" and succeeds, then the parse will fail because nothing matches "b".

The issue is, once a foo has already succeeded, how do we later backtrack across it and proceed as though it had failed?

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! 🤩🤩🤩

#!/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 | return | funcall | if | scope
# 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 { WS statement } DEDENT
# return = RETURN [ expr ]
# declassign = vardecl WS EQ WS expr
# assign = IDENTIFIER WS aoperator WS expr
# aoperator = PLUSEQ | MINUSEQ | STAREQ | SLASHEQ | PERCENTEQ | EQ
# unary = uoperator OPAREN expr CPAREN
# uoperator = MINUS | AMP | STAR | BANG
# binary = OPAREN expr WS boperator WS expr CPAREN
# boperator = PLUS | MINUS | STAR | SLASH | PERCENT
# | LT | LTEQ | GT | GTEQ | EQEQ | BANGEQ
# | AMPAMP | BARBAR | BANG
# | AMP | BAR | LTLT | GTGT | TILDE | CARAT
# if = IF WS expr scope { elif } [else]
# elif = ELIF WS expr scope
# else = ELSE scope
# expr = funcall | binary | unary | IDENTIFIER
# | INTLIT | FLOATLIT | STRINGLIT | BOOLLIT
# funcall = IDENTIFIER OPAREN [ [WS] expr { COMMA WS expr } [WS] ] CPAREN
# type = POINTER LT type GT
# | ARRAY LT type GT
# | FUNCTION LT type GT
# | ARRAY OBRACKET INTLIT CBRACKET LT type GT
# | IDENTIFIER
# The above EBNF notation is:
#
# Concatenation: rule1 = rule2 rule3 rule4
# Alternation: rule1 = rule2 | rule3 | rule4
# Repetition: rule1 = rule2 { rule3 }
#
# Note: lowercase are AST nodes.
# Note: ALLCAPS 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 load_keywords(fpath):
"""
Load the keyword definitions from the file at the path.
Returns a dict which maps keywords to toktypes.
The format of the keywords file should be pairs of lines:
- a keywork (e.g. 'true'),
- followed by a line which is the toktype of that keyword (e.g. 'BOOLLIT').
Example keywords.txt file:
true
BOOLLIT
false
BOOLLIT
array
ARRAY
return
RETURN
"""
keyword_map = {}
with open(fpath) as f:
for line in f:
keyword = line.rstrip('\n')
toktype = f.next().rstrip('\n')
keyword_map[keyword] = toktype
return keyword_map
INDENT_UNIT=4
def offside_rule(token, previous, indent_level):
"""
Implement the "offside rule" indentation-based scoping mechanism.
See https://en.wikipedia.org/wiki/Off-side_rule
Args:
- token: the current token
- previous: the previous token
- indent_level: the indent level before encounting this token
If this is a WS (containing a newline) following a COLON and indentation has increased,
return an INDENT token.
If this is a WS (containing a newline) and indentation has decreased,
return (possibly multiple) DEDENT token(s).
Otherwise,
return the token unmodified.
"""
tokens2 = [token]
def get_indent_level(text):
"""
Determine the indentation level of the text.
TODO: this implementation is pretty naive.
"""
indentation = text.split('\n')[-1]
assert len(indentation) % INDENT_UNIT == 0
return len(indentation) / INDENT_UNIT
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:
new_level = get_indent_level(text)
if is_toktype(previous, 'COLON') and new_level > indent_level:
tokens2 = [('token', 'INDENT', text)]
indent_level = new_level
elif new_level < indent_level:
# emit as many DEDENT tokens as needed.
tokens2 = []
for _ in range(indent_level - new_level):
tokens2.append(('token', 'DEDENT', text))
indent_level = new_level
return (tokens2, indent_level)
def keyword_specialcases(token, keyword_map):
"""
Add a special-case which turns IDENTIFIER tokens into keyword tokens.
e.g.:
- 'array' changes from an IDENTIFIER token to an ARRAY token.
- 'true' changes from an IDENTIFIER token to a BOOLLIT token.
"""
if is_toktype(token, 'IDENTIFIER'):
text = token_text(token)
if text in keyword_map.keys():
toktype = keyword_map[text]
return ('token', toktype, text)
return token
def tokenize(tokendefs, keyword_map, 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)
# special-case: distinguish keywords from identifiers
token = keyword_specialcases(token, keyword_map)
# special-case: implement "offside-rule"
(tokens2, indent) = offside_rule(token, previous, indent)
tokens += tokens2
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 ast_type(ast):
"""Returns the type of the AST (e.g. 'vardecl', 'statement', etc.)"""
assert is_ast(ast)
return ast[1]
def is_asttype(ast, atype):
"""
Returns whether the AST node is of the given type.
atype is e.g. 'vardecl', 'statement', etc.
"""
return ast_type(ast) == atype
def ast_children(ast):
"""Return the child nodes of the given AST."""
assert is_ast(ast)
return ast[2]
# Parser helpers:
# See https://en.wikipedia.org/wiki/Extended_Backus%E2%80%93Naur_form#Table_of_symbols
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 concatenation(tokens, funcs):
"""
All of the parsing functions must succeed as a whole.
Returns (<array of AST's>, <remaining tokens>).
"""
failure = (None, tokens)
asts = []
for f in funcs:
(ast, tokens) = f(tokens)
if ast is None:
return failure
asts.append(ast)
return (asts, tokens)
# Parsing functions:
# 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.
# 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)
(asts, tokens) = concatenation(tokens, [
parse_FUNC,
parse_WS,
parse_IDENTIFIER,
parse_fundeclargs
])
if asts is None:
return failure
identifier_token = asts[2]
args_ast = asts[3]
(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)
def list_of_args(tokens):
"""
Grammar: [WS] vardecl { COMMA WS vardecl } [WS]
Returns (<list of args>, <remaining tokens>)
"""
args = []
(_, tokens) = parse_WS(tokens)
(vardecl_ast, tokens) = parse_vardecl(tokens)
if vardecl_ast is None:
return (args, tokens)
args.append(vardecl_ast)
while True:
(asts, tokens) = concatenation(tokens, [
parse_COMMA,
parse_WS,
parse_vardecl
])
if asts is None:
break
vardecl_ast = asts[2]
args.append(vardecl_ast)
(_, tokens) = parse_WS(tokens)
return (args, tokens)
(ast, tokens) = parse_OPAREN(tokens)
if ast is None:
return failure
(args, tokens) = list_of_args(tokens)
(ast, tokens) = parse_CPAREN(tokens)
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)
(asts, tokens) = concatenation(tokens, [
parse_WS,
parse_ARROW,
parse_WS,
parse_type
])
if asts is None:
return failure
type_ast = asts[3]
ast = ('ast', 'fundeclret', [type_ast])
return (ast, tokens)
def parse_scope(tokens):
"""
Attempts to parse a scoped sequence of statements.
Grammar:
scope = COLON INDENT statement { WS statement } DEDENT
"""
failure = (None, tokens)
statements = []
(asts, tokens) = concatenation(tokens, [
parse_COLON,
parse_INDENT,
parse_statement
])
if asts is None:
return failure
statements.append(asts[2])
while True:
(asts, tokens) = concatenation(tokens, [
parse_WS,
parse_statement
])
if asts is None:
break
statements.append(asts[1])
(ast, tokens) = parse_DEDENT(tokens)
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)
(asts, tokens) = concatenation(tokens, [
parse_IDENTIFIER,
parse_COLON,
parse_WS,
parse_type
])
if asts is None:
return failure
identifier_token = asts[0]
type_ast = asts[3]
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
"""
failure = (None, tokens)
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
(asts, tokens) = concatenation(tokens, [
parse_LT,
parse_type,
parse_GT
])
if asts is None:
return failure
subtype_ast = asts[1]
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)
(asts, tokens) = concatenation(tokens, [
parse_ARRAY,
parse_OBRACKET,
parse_INTLIT,
parse_CBRACKET,
parse_LT,
parse_type,
parse_GT
])
if asts is None:
return failure
identifier_token = asts[0]
intlit_token = asts[2]
subtype_ast = asts[5]
ast = ('ast', 'type', [identifier_token, intlit_token, 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)
(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', [])
(asts, tokens) = concatenation(tokens, [
parse_WS,
parse_expr
])
if asts is not None:
expr_ast = asts[1]
return_ast = ('ast', 'return', [expr_ast])
return (return_ast, tokens)
def parse_expr(tokens):
"""
Attempts to parse an expression.
Expressions result in a value.
Grammar:
expr = funcall | binary | unary | IDENTIFIER
| INTLIT | FLOATLIT | STRINGLIT | BOOLLIT
"""
failure = (None, tokens)
(ast, tokens) = alternation(tokens, [
parse_funcall,
parse_binary,
parse_unary,
parse_IDENTIFIER,
parse_INTLIT,
parse_FLOATLIT,
parse_STRINGLIT,
parse_BOOLLIT
])
if ast is None:
return failure
expr_ast = ('ast', 'expr', [ast])
return (expr_ast, tokens)
def parse_funcall(tokens):
"""
Attempts to parse a funcion call.
Grammar:
funcall = IDENTIFIER OPAREN [ [WS] expr { COMMA WS expr } [WS] ] CPAREN
"""
failure = (None, tokens)
def list_of_exprs(tokens):
"""
Grammar: [WS] expr { COMMA WS expr } [WS]
Returns (<list of exprs>, <remaining tokens>)
"""
exprs = []
(_, tokens) = parse_WS(tokens)
(expr_ast, tokens) = parse_expr(tokens)
if expr_ast is None:
return (exprs, tokens)
exprs.append(expr_ast)
while True:
(asts, tokens) = concatenation(tokens, [
parse_COMMA,
parse_WS,
parse_expr
])
if asts is None:
break
expr_ast = asts[2]
exprs.append(expr_ast)
(_, tokens) = parse_WS(tokens)
return (exprs, tokens)
(asts, tokens) = concatenation(tokens, [
parse_IDENTIFIER,
parse_OPAREN
])
if asts is None:
return failure
identifier_token = asts[0]
(exprs, tokens) = list_of_exprs(tokens)
(ast, tokens) = parse_CPAREN(tokens)
if ast is None:
return failure
funcall_ast = ('ast', 'funcall', [identifier_token] + exprs)
return (funcall_ast, tokens)
def parse_declassign(tokens):
"""
Attempts to parse a declaration-assignment statement.
Grammar:
declassign = vardecl WS EQ WS expr
"""
failure = (None, tokens)
(asts, tokens) = concatenation(tokens, [
parse_vardecl,
parse_WS,
parse_EQ,
parse_WS,
parse_expr
])
if asts is None:
return failure
vardecl_ast = asts[0]
expr_ast = asts[4]
ast = ('ast', 'declassign', [vardecl_ast, expr_ast])
return (ast, tokens)
def parse_assign(tokens):
"""
Attempts to parse an assignment statement.
Grammar:
assign = IDENTIFIER WS aoperator WS expr
"""
failure = (None, tokens)
(asts, tokens) = concatenation(tokens, [
parse_IDENTIFIER,
parse_WS,
parse_aoperator,
parse_WS,
parse_expr
])
if asts is None:
return failure
identifier_token = asts[0]
op_ast = asts[2]
expr_ast = asts[4]
ast = ('ast', 'assign', [identifier_token, op_ast, expr_ast])
return (ast, tokens)
def parse_aoperator(tokens):
"""
Attempts to parse an assignment operator.
Grammar:
aoperator = PLUSEQ | MINUSEQ | STAREQ | SLASHEQ | PERCENTEQ | EQ
"""
failure = (None, tokens)
(token, tokens) = alternation(tokens, [
parse_PLUSEQ,
parse_MINUSEQ,
parse_STAREQ,
parse_SLASHEQ,
parse_PERCENTEQ,
parse_EQ
])
if token is None:
return failure
ast = ('ast', 'aoperator', [token])
return (ast, tokens)
def parse_unary(tokens):
"""
Attempts to parse a unary expression.
Grammar:
unary = uoperator OPAREN expr CPAREN
"""
failure = (None, tokens)
(asts, tokens) = concatenation(tokens, [
parse_uoperator,
parse_OPAREN,
parse_expr,
parse_CPAREN
])
if asts is None:
return failure
op = asts[0]
expr = asts[2]
ast = ('ast', 'unary', [op, expr])
return (ast, tokens)
def parse_uoperator(tokens):
"""
Attempts to parse a unary operator.
Grammar:
uoperator = MINUS | AMP | STAR | BANG
"""
failure = (None, tokens)
(token, tokens) = alternation(tokens, [
parse_MINUS,
parse_AMP,
parse_STAR,
parse_BANG
])
if token is None:
return failure
ast = ('ast', 'uoperator', [token])
return (ast, tokens)
def parse_binary(tokens):
"""
Attempts to parse a binary expression.
Grammar:
binary = OPAREN expr WS boperator WS expr CPAREN
"""
failure = (None, tokens)
(asts, tokens) = concatenation(tokens, [
parse_OPAREN,
parse_expr,
parse_WS,
parse_boperator,
parse_WS,
parse_expr,
parse_CPAREN
])
if asts is None:
return failure
expr1 = asts[1]
op = asts[3]
expr2 = asts[5]
ast = ('ast', 'binary', [expr1, op, expr2])
return (ast, tokens)
def parse_boperator(tokens):
"""
Attempts to parse a binary operator.
Grammar:
boperator = PLUS | MINUS | STAR | SLASH | PERCENT
| LT | LTEQ | GT | GTEQ | EQEQ | BANGEQ
| AMPAMP | BARBAR | BANG
| AMP | BAR | LTLT | GTGT | TILDE | CARAT
"""
failure = (None, tokens)
(token, tokens) = alternation(tokens, [
parse_PLUS,
parse_MINUS,
parse_STAR,
parse_SLASH,
parse_LT,
parse_LTEQ,
parse_GT,
parse_GTEQ,
parse_EQEQ,
parse_BANGEQ,
parse_AMPAMP,
parse_BARBAR,
parse_BANG,
parse_AMP,
parse_BAR,
parse_LTLT,
parse_GTGT,
parse_TILDE,
parse_CARAT,
])
if token is None:
return failure
ast = ('ast', 'boperator', [token])
return (ast, tokens)
def parse_statement(tokens):
"""
Attempts to parse a statement.
Grammar:
statement = declassign | assign | fundecl | vardecl | return | funcall | if | scope
"""
failure = (None, tokens)
(ast, tokens) = alternation(tokens, [
parse_declassign,
parse_assign,
parse_fundecl,
parse_vardecl,
parse_return,
parse_funcall,
parse_if,
parse_scope
])
if ast is None:
return failure
statement_ast = ('ast', 'statement', [ast])
return (statement_ast, tokens)
def parse_if(tokens):
"""
Attempts to parse an if statement.
Grammar:
if = IF WS expr scope { elif } [else]
"""
failure = (None, tokens)
def parse_elif(tokens):
"""
Attempts to parse an elif statement.
Grammar:
elif = ELIF WS expr scope
"""
failure = (None, tokens)
(asts, tokens) = concatenation(tokens, [
parse_ELIF,
parse_WS,
parse_expr,
parse_scope
])
if asts is None:
return failure
ast = ('ast', 'elif', [asts[2], asts[3]])
return (ast, tokens)
def parse_else(tokens):
"""
Attempts to parse an else statement.
Grammar:
else = ELSE scope
"""
failure = (None, tokens)
(asts, tokens) = concatenation(tokens, [
parse_ELSE,
parse_scope
])
if asts is None:
return failure
ast = ('ast', 'else', [asts[1]])
return (ast, tokens)
(asts, tokens) = concatenation(tokens, [
parse_IF,
parse_WS,
parse_expr,
parse_scope
])
if asts is None:
return failure
sub_asts = [asts[2], asts[3]]
elifs = []
while True:
(ast, tokens) = parse_elif(tokens)
if ast is None:
break
elifs.append(ast)
sub_asts += elifs
(else_ast, tokens) = parse_else(tokens)
if else_ast is not None:
sub_asts.append(else_ast)
if_ast = ('ast', 'if', sub_asts)
return (if_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)
(_, tokens) = parse_WS(tokens)
while True:
(ast, tokens) = parse_statement(tokens)
if ast is None:
break
statement_asts.append(ast)
(_, tokens) = parse_WS(tokens)
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[:8])
if len(tokens) > 0:
raise Exception("Leftover tokens: %s" % tokens)
return ast
# Terminal parsing: parsing individual tokens.
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
def parse_FUNC(tokens):
return parse_terminal(tokens, 'FUNC')
def parse_ARRAY(tokens):
return parse_terminal(tokens, 'ARRAY')
def parse_COMMA(tokens):
return parse_terminal(tokens, 'COMMA')
def parse_WS(tokens):
return parse_terminal(tokens, 'WS')
def parse_ARROW(tokens):
return parse_terminal(tokens, 'ARROW')
def parse_IDENTIFIER(tokens):
return parse_terminal(tokens, 'IDENTIFIER')
def parse_COLON(tokens):
return parse_terminal(tokens, 'COLON')
def parse_INTLIT(tokens):
return parse_terminal(tokens, 'INTLIT')
def parse_FLOATLIT(tokens):
return parse_terminal(tokens, 'FLOATLIT')
def parse_STRINGLIT(tokens):
return parse_terminal(tokens, 'STRINGLIT')
def parse_BOOLLIT(tokens):
return parse_terminal(tokens, 'BOOLLIT')
def parse_OPAREN(tokens):
return parse_terminal(tokens, 'OPAREN')
def parse_CPAREN(tokens):
return parse_terminal(tokens, 'CPAREN')
def parse_OBRACKET(tokens):
return parse_terminal(tokens, 'OBRACKET')
def parse_CBRACKET(tokens):
return parse_terminal(tokens, 'CBRACKET')
def parse_MINUS(tokens):
return parse_terminal(tokens, 'MINUS')
def parse_AMP(tokens):
return parse_terminal(tokens, 'AMP')
def parse_STAR(tokens):
return parse_terminal(tokens, 'STAR')
def parse_BANG(tokens):
return parse_terminal(tokens, 'BANG')
def parse_PLUS(tokens):
return parse_terminal(tokens, 'PLUS')
def parse_SLASH(tokens):
return parse_terminal(tokens, 'SLASH')
def parse_LT(tokens):
return parse_terminal(tokens, 'LT')
def parse_LTEQ(tokens):
return parse_terminal(tokens, 'LTEQ')
def parse_GT(tokens):
return parse_terminal(tokens, 'GT')
def parse_GTEQ(tokens):
return parse_terminal(tokens, 'GTEQ')
def parse_EQ(tokens):
return parse_terminal(tokens, 'EQ')
def parse_EQEQ(tokens):
return parse_terminal(tokens, 'EQEQ')
def parse_BANGEQ(tokens):
return parse_terminal(tokens, 'BANGEQ')
def parse_AMPAMP(tokens):
return parse_terminal(tokens, 'AMPAMP')
def parse_BARBAR(tokens):
return parse_terminal(tokens, 'BARBAR')
def parse_BAR(tokens):
return parse_terminal(tokens, 'BAR')
def parse_LTLT(tokens):
return parse_terminal(tokens, 'LTLT')
def parse_GTGT(tokens):
# So, I'm not super happy about this, but here's why this has to be special-cased:
# If we just treated this like all of the other tokenization and parsing functions,
# we would have a token for '>>', and this would be a one-liner:
# return parse_terminal(tokens, 'GTGT')
# However, that would break `pointer<pointer<char>>`, because the last two
# characters would be a single GTGT token, rathe than two separate GT tokens.
# So we have to implement parse_GTGT by looking for two distinct GT tokens.
failure = (None, tokens)
(gt_tokens, tokens) = concatenation(tokens, [parse_GT, parse_GT])
if gt_tokens is None:
return failure
token = ('token', 'GTGT', '>>')
def parse_TILDE(tokens):
return parse_terminal(tokens, 'TILDE')
def parse_CARAT(tokens):
return parse_terminal(tokens, 'CARAT')
def parse_PLUSEQ(tokens):
return parse_terminal(tokens, 'PLUSEQ')
def parse_MINUSEQ(tokens):
return parse_terminal(tokens, 'MINUSEQ')
def parse_STAREQ(tokens):
return parse_terminal(tokens, 'STAREQ')
def parse_SLASHEQ(tokens):
return parse_terminal(tokens, 'SLASHEQ')
def parse_PERCENTEQ(tokens):
return parse_terminal(tokens, 'PERCENTEQ')
def parse_INDENT(tokens):
return parse_terminal(tokens, 'INDENT')
def parse_DEDENT(tokens):
return parse_terminal(tokens, 'DEDENT')
def parse_IF(tokens):
return parse_terminal(tokens, 'IF')
def parse_ELIF(tokens):
return parse_terminal(tokens, 'ELIF')
def parse_ELSE(tokens):
return parse_terminal(tokens, 'ELSE')
# 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 indent_text(text):
"""Add indentation to the given block of text."""
indent = " " * INDENT_UNIT
indented_lines = [indent + line for line in text.splitlines()]
return "\n".join(indented_lines)
def generate_scope(ast):
"""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)
output += ("\n" + indent_text(statement))
line = "\n}"
output += line
return output
def generate_funcall(ast):
"""Generate a C function call."""
assert is_asttype(ast, 'funcall')
children = ast_children(ast)
identifier_token = children[0]
assert is_toktype(identifier_token, 'IDENTIFIER')
identifier = token_text(identifier_token)
exprs = [generate_expr(a) for a in children[1:]]
return "%s(%s)" % (identifier, ", ".join(exprs))
def generate_expr(ast):
"""Generate a C expression."""
assert is_asttype(ast, 'expr')
child = ast_children(ast)[0]
if is_ast(child):
if is_asttype(child, 'funcall'):
return generate_funcall(child)
if is_asttype(child, 'binary'):
return generate_binary(child)
elif is_asttype(child, 'unary'):
return generate_unary(child)
elif 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') \
or is_toktype(token, 'IDENTIFIER') \
:
output = token_text(token)
return output
else:
raise Exception("generate_expr: unexpcted ast %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)
op_ast = children[1]
assert is_asttype(op_ast, 'aoperator')
op_token = ast_children(op_ast)[0]
assert is_token(op_token)
op = token_text(op_token)
expr_ast = children[2]
assert is_asttype(expr_ast, 'expr')
expr = generate_expr(expr_ast)
return "%s %s %s;" % (identifier, op, expr)
def generate_unary(ast):
"""Generate a unary-operator C statement."""
assert is_asttype(ast, 'unary')
children = ast_children(ast)
op_ast = children[0]
assert is_asttype(op_ast, 'uoperator')
op_token = ast_children(op_ast)[0]
assert is_token(op_token)
op = token_text(op_token)
expr_ast = children[1]
assert is_asttype(expr_ast, 'expr')
expr = generate_expr(expr_ast)
return "%s(%s)" % (op, expr)
def generate_binary(ast):
"""Generate a binary-operator C statement."""
assert is_asttype(ast, 'binary')
children = ast_children(ast)
expr1_ast = children[0]
assert is_asttype(expr1_ast, 'expr')
expr1 = generate_expr(expr1_ast)
op_ast = children[1]
assert is_asttype(op_ast, 'boperator')
op_token = ast_children(op_ast)[0]
assert is_token(op_token)
op = token_text(op_token)
expr2_ast = children[2]
assert is_asttype(expr2_ast, 'expr')
expr2 = generate_expr(expr2_ast)
return "(%s %s %s)" % (expr1, op, expr2)
def generate_if(ast):
"""Generate a C if statement"""
assert is_asttype(ast, 'if')
children = ast_children(ast)
predicate_ast = children[0]
assert is_asttype(predicate_ast, 'expr')
predicate = generate_expr(predicate_ast)
output = "if (%s) " % (predicate)
scope_ast = children[1]
assert is_asttype(scope_ast, 'scope')
output += generate_scope(scope_ast)
for ast in children[2:]:
assert is_ast(ast)
if ast_type(ast) == 'elif':
elif_children = ast_children(ast)
predicate_ast = elif_children[0]
assert is_asttype(predicate_ast, 'expr')
predicate = generate_expr(predicate_ast)
output += " else if (%s) " % (predicate)
scope_ast = elif_children[1]
assert is_asttype(scope_ast, 'scope')
output += generate_scope(scope_ast)
elif ast_type(ast) == 'else':
else_children = ast_children(ast)
scope_ast = else_children[0]
assert is_asttype(scope_ast, 'scope')
output += " else " + generate_scope(scope_ast)
else:
raise Exception("generate_if: unexpected AST type %s" % ast_type(ast))
return output
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, 'declassign'):
return generate_declassign(sub_ast)
elif is_asttype(sub_ast, 'assign'):
return generate_assign(sub_ast)
elif 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, 'return'):
return generate_return(sub_ast)
elif is_asttype(sub_ast, 'funcall'):
return generate_funcall(sub_ast) + ';'
elif is_asttype(sub_ast, 'if'):
return generate_if(sub_ast)
elif is_asttype(sub_ast, 'scope'):
return generate_scope(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):
statement = generate_statement(statement_ast)
if statement.endswith('\n}'):
statement += '\n' # extra newline after function bodies.
outputs.append(statement)
output = '\n'.join(outputs)
if not output.endswith('\n'):
output += '\n' # always include a trailing newline.
return output
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")
keywords = load_keywords("keywords.txt")
input = None
if len(sys.argv) > 1 and not sys.argv[-1].startswith('--'):
input = open(sys.argv[-1]).read()
else:
input = sys.stdin.read()
tokens = tokenize(tdefs, keywords, 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)
pointer
POINTER
array
ARRAY
function
FUNCTION
func
FUNC
return
RETURN
true
BOOLLIT
false
BOOLLIT
if
IF
elif
ELIF
else
ELSE
👉 test26.cy
input:
func main():
if 1:
return 1
elif foo():
return 2
else:
if (foo() || bar()):
if a:
return
else:
return
output:
void main() {
if (1) {
return 1;
} else if (foo()) {
return 2;
} else {
if ((foo() || bar())) {
if (a) {
return;
}
} else {
return;
}
}
}
✅ test26.cy
#!/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
void main() {
if (1) {
return 1;
} else if (foo()) {
return 2;
} else {
if ((foo() || bar())) {
if (a) {
return;
}
} else {
return;
}
}
}
func main():
if 1:
return 1
elif foo():
return 2
else:
if (foo() || bar()):
if a:
return
else:
return
IDENTIFIER
[a-zA-Z][a-zA-Z0-9_]*
FLOATLIT
-?\d+\.\d+
INTLIT
-?\d+
STRINGLIT
"([^"\\]|\\[\s\S])*"
ARROW
->
LTEQ
<=
GTEQ
>=
EQEQ
==
BANGEQ
!=
PLUSEQ
\+=
MINUSEQ
-=
STAREQ
\*=
SLASHEQ
/=
PERCENTEQ
%=
AMPAMP
&&
BARBAR
\|\|
LTLT
<<
TILDE
~
CARAT
\^
COLON
:
COMMA
,
PLUS
\+
MINUS
-
STAR
\*
SLASH
/
PERCENT
%
LT
<
GT
>
EQ
=
AMP
&
BAR
\|
BANG
!
OBRACKET
\[
CBRACKET
]
OPAREN
\(
CPAREN
\)
WS
\s+
Sign up for free to join this conversation on GitHub. Already have an account? Sign in to comment
You can’t perform that action at this time.