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#lang typed/racket/base
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(require racket/set racket/sequence)
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(require typed/sugar/define)
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(provide Lengthable)
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(define-type Lengthable (U (Listof Any) String Symbol Path (Vectorof Any) HashTableTop (Setof Any) (Sequenceof Any)))
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(define/typed+provide (len x)
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(Lengthable -> Nonnegative-Integer)
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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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[(path? x) (len (path->string x))]
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[(vector? x) (vector-length 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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[(and (sequence? x) (not (integer? x))) (len (sequence->list x))]
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[else (error "len: can’t calculate length of" x)]))
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