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52 lines
1.5 KiB
52 lines
1.5 KiB
2 years ago
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Exercise 2.81: Louis Reasoner has noticed that
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apply-generic may try to coerce the arguments to each other’s type even
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if they already have the same type. Therefore, he reasons, we need to put
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procedures in the coercion table to
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coerce arguments of each type to
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their own type. For example, in addition to the
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scheme-number->complex coercion shown above, he would do:
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(define (scheme-number->scheme-number n) n)
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(define (complex->complex z) z)
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(put-coercion 'scheme-number 'scheme-number
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scheme-number->scheme-number)
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(put-coercion 'complex 'complex
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complex->complex)
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With Louis’s coercion procedures installed, what happens if
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apply-generic is called with two arguments of type scheme-number
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or two arguments of type complex for an operation that is not found in
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the table for those types? For example, assume that we’ve defined a generic
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exponentiation operation:
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(define (exp x y)
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(apply-generic 'exp x y))
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and have put a procedure for exponentiation in the Scheme-number
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package but not in any other package:
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;; following added to Scheme-number package
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(put 'exp
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'(scheme-number scheme-number)
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(lambda (x y)
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(tag (expt x y))))
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; using primitive expt
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What happens if we call exp with two complex numbers as arguments?
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Is Louis correct that something had to be done about coercion with arguments of
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the same type, or does apply-generic work correctly as is?
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Modify apply-generic so that it doesn’t try coercion if the two
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arguments have the same type.
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