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