The all-time favorite: a desk calculator. This program reads arithmetic expressions on standard input, one per line, and prints their values. Here is the grammar definition:
/* File parser.mly */ %token <int> INT %token PLUS MINUS TIMES DIV %token LPAREN RPAREN %token EOL %left PLUS MINUS /* lowest precedence */ %left TIMES DIV /* medium precedence */ %nonassoc UMINUS /* highest precedence */ %start main /* the entry point */ %type <int> main %% main: expr EOL { $1 } ; expr: INT { $1 } | LPAREN expr RPAREN { $2 } | expr PLUS expr { $1 + $3 } | expr MINUS expr { $1 - $3 } | expr TIMES expr { $1 * $3 } | expr DIV expr { $1 / $3 } | MINUS expr %prec UMINUS { - $2 } ;
Here is the definition for the corresponding lexer:
(* File lexer.mll *) { open Parser (* The type token is defined in parser.mli *) exception Eof } rule token = parse [' ' '\t'] { token lexbuf } (* skip blanks *) | ['\n' ] { EOL } | ['0'-'9']+ as lxm { INT(int_of_string lxm) } | '+' { PLUS } | '-' { MINUS } | '*' { TIMES } | '/' { DIV } | '(' { LPAREN } | ')' { RPAREN } | eof { raise Eof }
Here is the main program, that combines the parser with the lexer:
(* File calc.ml *) let _ = try let lexbuf = Lexing.from_channel stdin in while true do let result = Parser.main Lexer.token lexbuf in print_int result; print_newline(); flush stdout done with Lexer.Eof -> exit 0
To compile everything, execute:
ocamllex lexer.mll # generates lexer.ml ocamlyacc parser.mly # generates parser.ml and parser.mli ocamlc -c parser.mli ocamlc -c lexer.ml ocamlc -c parser.ml ocamlc -c calc.ml ocamlc -o calc lexer.cmo parser.cmo calc.cmo