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9.2.1 A Simple Representation

The simplest way to represent Scheme values in C would be to represent each value as a pointer to a structure containing a type indicator, followed by a union carrying the real value. Assuming that SCM is the name of our universal type, we can write:

enum type { integer, pair, string, vector, ... };

typedef struct value *SCM;

struct value {
  enum type type;
  union {
    int integer;
    struct { SCM car, cdr; } pair;
    struct { int length; char *elts; } string;
    struct { int length; SCM  *elts; } vector;
  } value;

with the ellipses replaced with code for the remaining Scheme types.

This representation is sufficient to implement all of Scheme’s semantics. If x is an SCM value: