[1] The word thunk was invented by an informal working group that was discussing the implementation of call-by-name in Algol 60. They observed that most of the analysis of (thinking about) the expression could be done at compile time; thus, at run time, the expression would already have been thunk about (Ingerman et al. 1960).
[2] This is analogous to the use of force on the delayed objects that were introduced in chapter 3 to represent streams. The critical difference between what we are doing here and what we did in chapter 3 is that we are building delaying and forcing into the evaluator, and thus making this uniform and automatic throughout the language.
[3] Lazy evaluation combined with memoization is sometimes referred to as call-by-need argument passing, in contrast to call-by-name argument passing. (Call-by-name, introduced in Algol 60, is similar to non-memoized lazy evaluation.) As language designers, we can build our evaluator to memoize, not to memoize, or leave this an option for programmers (exercise 4.23). As you might expect from chapter 3, these choices raise issues that become both subtle and confusing in the presence of assignments. (See exercises 4.19 and 4.21.) An excellent article by Clinger (1982) attempts to clarify the multiple dimensions of confusion that arise here.
[4] Notice that we also erase the env from the thunk once the expression's value has been computed. This makes no difference in the values returned by the interpreter. It does help save space, however, because removing the reference from the thunk to the env once it is no longer needed allows this structure to be garbage-collected and its space recycled Similarly, we could have allowed unneeded environments in the memoized delayed objects of section 3.5.1 to be garbage-collected, by having memo_fun do something like fun = null; to discard the function fun (which includes the environment in which the delay was evaluated) after storing its value.
[5] This exercise demonstrates that the interaction between lazy evaluation and side effects can be very confusing. This is just what you might expect from the discussion in chapter 3.
4.2.2 An Interpreter with Lazy Evaluation