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32 lines
679 B
32 lines
679 B
2 years ago
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Exercise 4.52: Implement a new construct called
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if-fail that permits the user to catch the failure of an expression.
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If-fail takes two expressions. It evaluates the first expression as
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usual and returns as usual if the evaluation succeeds. If the evaluation
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fails, however, the value of the second expression is returned, as in the
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following example:
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;;; Amb-Eval input:
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(if-fail
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(let ((x (an-element-of '(1 3 5))))
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(require (even? x))
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x)
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'all-odd)
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;;; Starting a new problem
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;;; Amb-Eval value:
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all-odd
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;;; Amb-Eval input:
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(if-fail
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(let ((x (an-element-of '(1 3 5 8))))
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(require (even? x))
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x)
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'all-odd)
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;;; Starting a new problem
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;;; Amb-Eval value:
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8
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