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#lang racket/base
( require racket/contract racket/set )
( provide len )
( define/contract ( has-length? x )
( any/c . -> . boolean? )
( ormap ( λ ( proc ) ( proc x ) ) ( list list? string? symbol? vector? hash? set? ) ) )
;; general way of asking for length
( define/contract ( len x )
( any/c . -> . ( or/c integer? #f ) )
( cond
[ ( list? x ) ( length x ) ]
[ ( string? x ) ( string-length x ) ]
[ ( symbol? x ) ( len ( symbol->string x ) ) ]
[ ( vector? x ) ( len ( vector->list x ) ) ]
[ ( hash? x ) ( len ( hash-keys x ) ) ]
[ ( set? x ) ( len ( set->list x ) ) ]
[ ( integer? x ) ( len ( number->string x ) ) ]
[ else ( error " len: can’ " x ) ] ) )