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#lang racket/base
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(require racket/contract racket/set)
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(provide len)
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(define/contract (has-length? x)
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(any/c . -> . boolean?)
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(ormap (λ(proc) (proc x)) (list list? string? symbol? vector? hash? set?)))
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;; general way of asking for length
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(define/contract (len x)
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(any/c . -> . (or/c integer? #f))
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(cond
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[(list? x) (length x)]
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[(string? x) (string-length x)]
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[(symbol? x) (len (symbol->string x))]
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[(vector? x) (len (vector->list x))]
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[(hash? x) (len (hash-keys x))]
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[(set? x) (len (set->list x))]
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[(integer? x) (len (number->string x))]
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[else (error "len: can’t calculate length of" x)]))
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