Warning: This is the manual of the legacy Guile 2.0 series. You may want to read the manual of the current stable series instead.
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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:
x->type == integer
.
x->value.integer
.
x->type == vector
.
x->value.vector.elts[0]
to refer to its first element.
x->value.pair.car
to extract its car.