Our realization of the Forklang language maintained a shared heap. Those program expressions that assess heap directly manipulated this shared heap. This design had the advantage that it avoided having to pass around the heap object (similar to environments). The main disadvantage of this design was that it was difficult to implement variations of heap semantics where parts of the program may potentially use a variation of the original heap.
Modify the Forklang interpreter so that an expression can explicitly control the heap that is available during evaluation of its subexpressions.