►Programming Language Design (Fall 2009)
▼2 Assignments
►2.10 Laziness
On this page:
2.10.1 Java
2.10.2 Stricness Points
2.10.3 State
2.10.4 Application
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2.10 Laziness

Complete this assignment with Team One.

You must submit this in an archive named "wlazy.zip".

You must submit this in a file named "wlazy.pdf".

2.10.1 Java

Is Java eager or lazy? Write a Java program to determine the answer to this question. The same program, run under the two different regimes, should produce different results. You may use any Java features you want, but keep your program relatively short; we will penalize you for programs we consider excessively long or obfuscatory. (Tip: It’s possible to solve this problem with a program no more than a few dozen lines long.)

You must turn in both the source code to your program (in printed or written form) as well as an answer to the question of whether Java is eager or lazy, and an explanation of how your program determines this. That is, you should provide a brief and unambiguous answer (e.g., “Java is lazy.”) followed by a description of what result one would obtain under each regime, along with a brief explanation of why that regime would generate that result.

In general, it would be a good idea to discuss your plan of attack with the course staff. This will help you avoid falling into a trap of measuring the wrong entity, and will improve your understanding of eagerness and laziness.

2.10.2 Stricness Points

In our lazy interpreter, we identified three points in the language where we need to force evaluation of expression closures (by invoking strict): the function position of an application, the test expression of a conditional, and arithmetic primitives. Doug Oord, a fairly sedentary student, is rather taken with the idea of laziness. He suggests that we can reduce the amount of code we need to write by replacing all invocations of strict with just one. In the lazy interpreter, he removes all instances of strict and replaces
  [id (v) (lookup v env)]
with
  [id (v) (strict (lookup v env))]
Doug’s reasoning is that the only time the interpreter returns an expression closure is on looking up an identifier in the environment. If we force its evaluation right away, we can be sure no other part of the interpreter will get an expression closure, so removing those other invocations of strict will do no harm. Being lazy himself, however, Doug fails to reason in the other direction, namely whether this will result in an overly eager interpreter.

Is it possible to write a program that will produce different results under the original interpreter and Doug’s Let the interpreted language feature arithmetic, first-class functions, with, if0, and rec (even though these are not in our in-class lazy interpreter).

If so, hand in an example program and the result under each interpreter, and clearly identify which interpreter will produce each result. Be sure to compare this behavior against that of the lazy interpreter of the sort we’ve written in class.

If it’s not possible, defend why one cannot exist.

2.10.3 State

No lazy language in history has also had state operations (such as mutating the values in boxes, or assigning values to variables). Why not?

The best answer to this question would include two things: a short program (which we assume will evaluate in a lazy regime) that uses state, and a brief explanation of what problem the execution of this program illustrates. Please be sure to use the non-caching (ie, original) notion of laziness. If you present a sufficiently illustrative example (which needn’t be very long!), your explanation can be quite short.

2.10.4 Application

In 850 words or less, describe if and how your knowledge of lazy evaluation will help you in your future programming practice. Good answers might discuss how this concept can be applied in interesting programming environments or how knowledge of its subtleties clarifies or improves existing practices, techniques, tools, etc.

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