EOPL - 3.2 - Exercise 3.5

Exercise 3.5 asks you to add a print primitive to the language, which prints its argument and returns the value 1. The relevant changes are simple. In module Eopl_3 only the definition of type Primitive and function apply_primitive are different:

// in eopl_3.fs (module Eopl_3)
type Primitive = AddPrim | SubtractPrim | MultPrim | IncrPrim 
               | DecrPrim | PrintPrim

let apply_primitive prim args = 
    match (prim, args) with
      (AddPrim, [a; b]) -> a + b
    | (SubtractPrim, [a; b]) -> a - b
    | (MultPrim, [a; b]) -> a * b
    | (IncrPrim, [a]) -> a + 1
    | (DecrPrim, [a]) -> a - 1
    | (PrintPrim, [a]) -> (print_endline (string_of_int a); 1)
    | _ -> failwith "Incorrect argument list for primitive" 

In the lexer specification a token for the print primitive is added:

rule token = parse
| whitespace { token lexbuf }
| '%' [^ '\n']* { token lexbuf } 
| newline       { record_newline lexbuf; token lexbuf }
| "+"         { PLUS }
| "-"         { MINUS }
| "*"  { MULT }
| "("           { LPAREN }
| ")"         { RPAREN }
| ","           { COMMA  }
| "add1" { INCR }
| "sub1" { DECR }
| "print"       { PRINT }
| id         { ID(lexeme lexbuf) }
| ['-']?digit+  { LIT (Int32.Parse(lexeme lexbuf)) }
| eof   { EOF }

And, in the parser, this token must be declared and dealt with:

%token PRINT INCR DECR LPAREN RPAREN PLUS MINUS MULT COMMA EOF

Prim: PLUS                        { AddPrim }
    | MINUS                       { SubtractPrim }
    | MULT                        { MultPrim }
    | INCR                        { IncrPrim }
    | DECR                        { DecrPrim }
    | PRINT                       { PrintPrim }

The remaining parts of these three files are the same as the previous two posts.