ALL CAPS BASIC

ALL CAPS BASIC (tentative name, since it no longer requires the source code to be in all caps) is a compiler written in Awk. It compiles a statically typed, garbage-collected Basic-like programming language to native code through modern C. It is intended to be a kind of working model compiler each piece of which is transparent. For this reason all data formats used by the compiler are text-based.

Warning! This is very much a work in progress and incomplete. Please do not submit it to Hacker News, Reddit, etc.

The source code is compiled in three phases. Because Awk does not support data structures other than (non-nestable) dictionaries no parse tree is built.

Phase	Component	Input	Output
Lexical analysis	lexer.awk	ACB source code	Token file
Parsing	parser.awk	Token file	Intermediate representation (IR) file
Type checking and code generation	codegen.awk	IR file	C source code

Run

./run.sh -v test/test00.bas

in this repository to see the output at every stage.

This project was initially inspired by BaCon, Niklaus Wirth's Oberon-0 [pdf] and the Awk compiler for Mercat.

The syntax is largely taken from the manuals for Visual Basic 6 and Gambas. The syntax and the semantics may differ from those two languages, however, both intentionally and not (the developer has hardly used them).

Sample code

sub repeat(s as string, n) as string
    let result = ""
    for i = 1 to n
        let result = result & s
    end
    return result
end

sub main()
    let s = "Ha"
    print repeat(s, 3)
end

Dependencies

ACB should work with any POSIX-compatible Awk. It has been tested with nawk, mawk and GNU Awk. It does not rely on the GNU-specific extensions.

To produce native binaries you will need a C99-compatible C compiler like GCC or clang.

Language grammar

The parser is hand-written, so this may not be accurate. It does not account for comments; those can be thought of as removed at a separate, line-based parsing stage.

Below ? means zero or one occurrence, * means zero or more occurrences and /.../ means a regular expression.

file = (sub | sub_declaration)*
sub = "SUB", sub_header, statement_list, "end"
sub_declaration = "DECLARE", sub_header
sub_header = "SUB", ident_maybe_type, "(", (ident_maybe_type, ",")*, ")", ("AS", ident)?
ident = /[a-zA-Z0-9][a-zA-Z0-9_]*/
ident_maybe_type = ideat, ("AS", ident)?
statement_list = nl, ((statement, nl)*, statement)?
statement = assignment | "BREAK" | conditional | "CONTINUE" | declaration | expression | for_loop | return | subcall | while_loop
assignment = "LET", ident, "=", expression
conditional = "IF", expression, statement_list, "END"
declaration = "DIM", ident, "AS", ident
for_loop = "FOR", ident, "=", expression, "TO", expression, statement_list, "END"
return = "RETURN", expression
subcall = ident, "(", expression, (expression, ",")*, ")"
while_loop = "WHILE", expression, statement_list, "END"
expression = ("(", expression_core, ")") | expression_core
expression_core = ident | (unary_operator, expression) | ((ident | expression), binary_operator, (ident | expression))
binary_operator = "OR" | "AND" | "=" | "<>" | "<" | ">" | "<=" | ">=" | "+" | "-" | "*" | "/" | "%" | "&"
unary_operator = "NOT"
nl = nl_atom, nl_atom*
nl_atom = \n | ":"

IR opcodes

Legend

Symbol	Meaning
byref?	Either the text "BYREF" or nothing
func	Function identifier
op[N]	Either variable identifier or literal
label	Label identifier
n	Integer literal
res	Variable identifier
type	Type identifier
var	Variable identifier

List

ARG var type byref?
DECLARE
DIM var type
ENDSUB
INCR var n
JT/JF label ident
JMP label
LABEL label
PRINT op1 op2 ... opN
RETTYPE type
RETURN var
SET res op1
SET+ res op1 op2
SET- res op1 op2
SET/ res op1 op2
SET* res op1 op2
SET% res op1 op2
SET= res op1 op2
SET< res op1 op2
SET> res op1 op2
SET<> res op1 op2
SET<= res op1 op2
SET>= res op1 op2
SETAND res op1 op2
SETOR res op1 op2
SETXOR res op1 op2
SETAND_NUM res op1 op2
SETOR_NUM res op1 op2
SETXOR_NUM res op1 op2
SETFUNC res func op1 op2 op3 ... opN
SUB func