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schemegeneric-programming

Idiomatic scheme and generic programming, why only on numbers?

In Scheme, procedures like +, -, *, / works on different types of numbers, but we don't much see any other generic procedures.

For example, length works only on list so that vector-length and string-length are needed.

I guess it comes from the fact that the language doesn't really offer any mechanism for defining generic procedure (except cond of course) like "type classes" in Haskell or a standardized object system.

Is there an idiomatic scheme way to handle generic procedures that I'm not aware of ?

Solution

  • Finally, I found out a very neat solution in PLT Scheme :

    (require (rename-in scheme [length list-length]))

(define length
 (λ (x)
  ((cond [(list? x) list-length]
         [(string? x) string-length]
         [(vector? x) vector-length]
         [else (error "whatever")]) x)))

(length '(a b c))
(length "abc")
(length #(1 2 3))