How to Design Programs: An Introduction to Programming and Computing

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WPI CS536 (F07) Homework 4 CS 536 Homework 4: On Laziness Due: October 4, hardcopy in class Problem 1 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 Dolittle, 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 from class (PLAI chapter 8), 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.
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  • Fall '07
  • Fisler
  • Lazy Interpreter, Doug Dolittle

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