Reading Symbols

Reading in symbols is fairly straight forward. We start by allocating some empty string, and then, for each character in the input which is valid as part of a symbol we append this character to the string.

  1. int lval_read_sym(lval* v, char* s, int i) {
  3.   /* Allocate Empty String */
  4.   char* part = calloc(1,1);
  6.   /* While valid identifier characters */
  7.   while (strchr(
  8.       "abcdefghijklmnopqrstuvwxyz"
  9.       "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
  10.       "0123456789_+-*\\/=<>!&", s[i]) && s[i] != '\0') {
  12.     /* Append character to end of string */
  13.     part = realloc(part, strlen(part)+2);
  14.     part[strlen(part)+1] = '\0';
  15.     part[strlen(part)+0] = s[i];
  16.     i++;
  17.   }

You’ll notice that we’re also reading in numbers with this code. So the next step is to check if this symbol is actually a number. To do this we just check if all of the characters are digits.

  1.   /* Check if Identifier looks like number */
  2.   int is_num = strchr("-0123456789", part[0]);
  3.   for (int i = 1; i < strlen(part); i++) {
  4.     if (!strchr("0123456789", part[i])) { is_num = 0; break; }
  5.   }

Finally, if the symbol is a number we convert it with similar code to the previous code we used when we were reading from the mpc AST, or otherwise we just use it as a normal symbol. Then we free the temporary string we allocated and return the new position in the input.

  1.   /* Add Symbol or Number as lval */
  2.   if (is_num) {
  3.     errno = 0;
  4.     long x = strtol(part, NULL, 10);
  5.     lval_add(v, errno != ERANGE ? lval_num(x) : lval_err("Invalid Number %s", part));
  6.   } else {
  7.     lval_add(v, lval_sym(part));
  8.   }
  10.   /* Free temp string */
  11.   free(part);
  13.   /* Return updated position in input */
  14.   return i;
  15. }

And we are done with reading symbols. Onto strings!