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Cpsc 201 Problem Set 1

# Cpsc 201 Problem Set 1 - 1 John C Rimson has defined...

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1. John C. Rimson has defined reverse-if as follows: The problem with this implementation is the (predicate) – which (because it is enclosed in parentheses) is a call to the (no-arguments) procedure predicate . So, one can write an if expression that will evaluate to the string “Consequence” as follows: However, if one tries to perform a similar operation using John’s reverse-if (with the alternative and consequence inverted): an error is returned. In an if expression, the first argument (the predicate) can be any expression which evaluates to a value (even if that value is a procedure): But expressions which do not evaluate to anything, will cause an error: However, John’s reverse-if requires that predicate be a no-arguments procedure, making it far less versatile than the standard if . This problem can be fixed by using the following version of reverse-if : Now, for our earlier example, reverse-if behaves as expected: But suppose I define a procedure PrintLineAndEvaluate as follows: This procedure takes a single argument, which it prints and returns as its value: Using this method as part of an if expression yield the expected results: But, a reverse-if expression does not have equivalent results:

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If is a special form which does not evaluate in the same way as a standard Scheme expression. Normally, a procedure is evaluated by first evaluating each of its arguments (in some arbitrary order) and binding these values to the process’s variables (a process being an evaluation of a procedure for a specific set of arguments). However, an if is evaluated by first evaluating the predicate. If the
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