CUP stands for Construction of Useful Parsers and is an LALR parser generator for Java. It was developed by
C. Scott Ananian, Frank Flannery, Dan Wang,
Andrew W. Appel and Michael Petter. It implements
standard LALR(1) parser generation. As a parser author, you specify the symbols of Your grammar (terminal T1,T2; non terminal N1, N2;), as well as the productions (LHS :== RHS1 | RHS2 ;). If you provide each production alternative with action code ({: RESULT = myfunction(); :}), the
parser will call this action code after performing a reduction with the particular production. You can use these callbacks to assemble an AST (Abstract Syntax Tree) or for arbitrary purposes. You should also have a look at the scanner generator JFlex, which is suited particularly well for collaboration with CUP.
CUP generates parsers from specifications, that you provide in a special file, whose syntax is quite similar to YACC:
/* Simple +/-/* expression language; parser evaluates constant expressions on the fly*/ import java_cup.runtime.*; parser code {: // Connect this parser to a scanner! scanner s; Parser(scanner s){ this.s=s; } :} /* define how to connect to the scanner! */ init with {: s.init(); :}; scan with {: return s.next_token(); :}; /* Terminals (tokens returned by the scanner). */ terminal SEMI, PLUS, MINUS, TIMES, UMINUS, LPAREN, RPAREN; terminal Integer NUMBER; // our scanner provides numbers as integers /* Non terminals */ non terminal expr_list; non terminal Integer expr; // used to store evaluated subexpressions /* Precedences */ precedence left PLUS, MINUS; precedence left TIMES; precedence left UMINUS; /* The grammar rules */ expr_list ::= expr_list expr:e SEMI {: System.out.println(e);:} | expr:e SEMI {: System.out.println(e);:} ; expr ::= expr:e1 PLUS expr:e2 {: RESULT = e1+e2; :} | expr:e1 MINUS expr:e2 {: RESULT = e1-e2; :} | expr:e1 TIMES expr:e2 {: RESULT = e1*e2; :} | MINUS expr:e {: RESULT = -e; :} %prec UMINUS | LPAREN expr:e RPAREN {: RESULT = e; :} | NUMBER:n {: RESULT = n; :} ;
This will produce two files, parser.java and sym.java, which can be included in arbitrary Java projects.
You can find more detailed information in CUP's manual:
Drop a patch to Michael Petter - if it is interesting enough, he will integrate it into the main sources.
git clone https://github.com/DrMichaelPetter/cup.git
or have a look at our github.