16.5.3 Immutability of the Scheme stack

Under Vicare machine words on the stack are immutable by definition; let’s see why. Scheme has two kinds of values: immediate and compound.

• Each instance of an immediate value (boolean, fixnum, character, …) is fully contained in a single machine word; immediate values are moved around in CPU registers and memory locations and they are immutable; immediate values are put on the stack if it is needed for the execution of a program.
• Each instance of a compound value (pair, vector, record, …) is composed of a memory block, allocated on the heap and actually holding the data, and a reference to it: a machine word being the tagged address of the first word in the memory block.

  If we mutate a vector with vector-set!: we mutate the memory block, the reference to such vector is untouched. References to compound values are immutable.

  Vicare always allocates compound values’ memory blocks on the heap, while references are moved around in CPU registers and memory locations; references are put on the stack if it is needed for the execution of a program.

Local variables can be mutated, special care is taken to implement them; Local variables.