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*To*: srfi-85@xxxxxxxxxxxxxxxxx*Subject*: Re: building other EQUIV?-like predicates*From*: William D Clinger <will@xxxxxxxxxxx>*Date*: Fri, 10 Mar 2006 21:55:16 -0500*Delivered-to*: srfi-85@xxxxxxxxxxxxxxxxx

Sebastian Egner raised an excellent point: > In many cases I need an application-specific > equivalence which is not among the predefined > ones. For example, it should deal with cyclic > structures the way EQUIV? does, but floats are > compared only up to a certain tolerance (not > that this results in an actual 'equivalence,' > but it is often needed as a predicate anyhow). > Another example is the presence of user-defined > record types; equivalence predicates that are > meaningful for the application can ignore some > fields while recursing only on others. > > In these situations, I would still end up implementing a > lot of code similar to the reference implementation you > give but with little modifications in the recursion, or > for the atomic types. It would not be hard to provide parameterized versions, say equivalent? and dag-equivalent?, that take a predicate as their first argument and use that predicate when comparing leaf nodes (defined as anything that isn't a pair or vector). Then (equiv? x y) would be like (equivalent? eqv? x y), and (dag-equiv? x y) like (dag-equivalent? eqv? x y). Your example might be something like (define myequiv? (let* ((epsilon 1e-14) (leaf-equal? (lambda (x y) (if (and (complex? x) (complex? y)) (< (magnitude (- x y)) (* epsilon (max 1.0 (magnitude x) (magnitude y)))) (eqv? x y))))) (lambda (x y) (equivalent? leaf-equal? x y)))) It might be better to define make-equiv-predicate and make-dag-equiv-predicate, which would take a total predicate as their lone argument and return a recursive binary predicate as their result. Will

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