Description of the IMP language

Comments

• starts by // until the end of the line
• starts by /* and finishes at the first occurrence of */

Expressions (e)

• signed integers constants 
• booleans : true and false
• variables (x,y) : starts with a lowercase alphabetical character or the character _, possibly continues with alphabetical characters, digits or _ 
• binary infix operations (only +, * and <) e1 op e2
• function calls f(e1,..,en)
• expressions between parentheses (e)

Instructions (i)

• putchar(e) : print the character whose ASCII code is the value of e
• e; : evaluate the expression e
• x=e; put the value of expression e in variable x
• if (e) { li1 } else  { li2 } : conditional
• while (e) { li } : while loop
• return(e); : exit a function call with the value of e
• lists of instructions (li) are just sequences of instructions 

Programs

• A sequence of global variables declarations var id;
• A sequence of function declarations : includes formal parameters x1,..,xn, local variables y1,..yk and body as a list of instructions
  function f (x1,..,xn) {var y1; ... var yk; li } 
  A function called main (expecting 1 argument) will be executed when the program is called.

Example

var retour;
var espace;

function main(arg) {
  var i;
  i = 0;
  retour = 10;
  espace = 32;

  while (i < arg+1) {
    affiche_ligne(i, arg);
    putchar(retour);
    i = i + 1;
  }
}

function affiche_ligne(i, n) {
  var j;
  j = 0;
  while (j < n+1) {
    if (i*i + j*j < n*n) {
      putchar(46);
    } else {
      putchar(35);
    }
    putchar(espace);
    j = j+1;
  }
}