>The representation method outlined in section [http://mitpress.mit.edu/sicp/full-text/sicp/book/node118.htm...] solves the problem of implementing list structure, provided that we have an infinite amount of memory. With a real computer we will eventually run out of free space in which to construct new pairs.[http://mitpress.mit.edu/sicp/full-text/sicp/book/footnode.ht...] However, most of the pairs generated in a typical computation are used only to hold intermediate results. After these results are accessed, the pairs are no longer needed--they are garbage. For instance, the computation
constructs two lists: the enumeration and the result of filtering the enumeration. When the accumulation is complete, these lists are no longer needed, and the allocated memory can be reclaimed. If we can arrange to collect all the garbage periodically, and if this turns out to recycle memory at about the same rate at which we construct new pairs, we will have preserved the illusion that there is an infinite amount of memory.
>The representation method outlined in section [http://mitpress.mit.edu/sicp/full-text/sicp/book/node118.htm...] solves the problem of implementing list structure, provided that we have an infinite amount of memory. With a real computer we will eventually run out of free space in which to construct new pairs.[http://mitpress.mit.edu/sicp/full-text/sicp/book/footnode.ht...] However, most of the pairs generated in a typical computation are used only to hold intermediate results. After these results are accessed, the pairs are no longer needed--they are garbage. For instance, the computation
(accumulate + 0 (filter odd? (enumerate-interval 0 n)))
constructs two lists: the enumeration and the result of filtering the enumeration. When the accumulation is complete, these lists are no longer needed, and the allocated memory can be reclaimed. If we can arrange to collect all the garbage periodically, and if this turns out to recycle memory at about the same rate at which we construct new pairs, we will have preserved the illusion that there is an infinite amount of memory.