(guile.info.gz) A Simple Representation

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(guile.info.gz) Data Representation in Scheme

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 18.1.1 A Simple Representation
 ------------------------------
 
 The simplest way to meet the above requirements 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:
    *   To test if X is an integer, we can write `X->type == integer'.
 
    *   To find its value, we can write `X->value.integer'.
 
    *   To test if X is a vector, we can write `X->type == vector'.
 
    *   If we know X is a vector, we can write
      `X->value.vector.elts[0]' to refer to its first element.
 
    *   If we know X is a pair, we can write   `X->value.pair.car' to
      extract its car.
 

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(guile.info.gz) Data Representation in Scheme

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