SPINE - a (C) programmer's calculator

BUILDING AND INSTALLATION

Type make. This should build an executable file named spine in current directory. Building (uses bison and flex, as well as a C compiler) doesn't perform anything tricky.

spine consists of a single executable, so you can put it anywhere in your PATH.

I believe this will compile on almost any Unix-like operating system (linux, Solaris, NetBSD, etc), since I did most of the development under NetBSD and finished it under Linux.

UNARY OPERATORS

This constitutes one of the few tricks in the parser/lexer

Consider an expression like "12-9"

If a lex lexical analyzer has an integer constant recognizer line like this: "[0-9]+ { yylval.val = atoi(yytext); return CONST; }"

lexer returns the sequence of tokens {CONST, '-', CONST}

If the lexer has this: "-*[0-9]+ { yylval.val = atoi(yytext); return CONST; }"

lexer returns the sequence of tokens {CONST, CONST} for these cases.

In essence, this is adding the unary-minus-detection to the lexer.

A simple yacc grammar would have problems with one of these cases, and if you put in productions to do both, you'll probably introduce conflicts into some other part of the grammar. For bone-headed grammars:

Expressions: 12- 9; and 12 - 9; both work.

Expressions: 12-9; 12 -9; are syntax errors.

OPERATOR PRECEDENCE

Least binding

1. unary operators: -, +, ~ (unary minus, unary plus, 1's complement)
2. multiplicative: *, /, % (multiply, divide, modulo)
3. additive: +, -
4. bitwise shift: <<, >>
5. bitwise AND: &
6. bitwise XOR: ^
7. bitwise OR: |

Most binding

Parentheses group expressions "tighter" than operator precedence.

This directory has a grammar taken from the K&R 2nd ed ANSI-C grammar. It recognizes several unary operators, and does operator precedence in the productions. The lexer recognizes '-' and '[0-9]+' as seperate lexemes and returns them as such. The grammar doesn't build up a an explicit parse tree: it does all the operations as semantic actions of the productions, feeding values calculated back up the productions until it hits a "stmnt", at which point it prints out the ultimate value calculated.

MISCELLANEOUS

main() function (and other C functions) resides in the file gram2.y. It seemed acceptable to confuse the issue by putting yacc productions and C code in the .y file so as to have fewer source files.

Even though spine accepts binary (in the "base 2" sense) input values (7 == 111b) it does not print out in binary - all output done with printf(), so spine has whatever limitations printf() has.

You can type in trace; and help; during spine interaction to toggle evaluation tracing, and see the help-message over and over again.