Add hamming exercise

[mbertheau]

No description provided.

[lexi-lambda]

Instead of returning #f on invalid input, this should throw an exception. Also, the loop could be built more elegantly using for/sum. Finally, you should use eqv? with characters, not eq?.

(define (hamming-distance dna1 dna2)
  (unless (= (string-length dna1) (string-length dna2))
    (raise-argument-error 'hamming-distance "strings of equal length" 1 dna1 dna2))
  (for/sum ([c1 (in-string dna1)]
            [c2 (in-string dna2)])
    (if (eqv? c1 c2) 0 1)))

You can add tests for the bad cases using check-exn.

(check-exn #rx"strings of equal length"
           (thunk (hamming-distance "AATG" "AAA"))
           "disallow first strand longer")

[mbertheau]

Hmm, @lexi-lambda what do you think about this suggestion by zeodary? I think it expresses the idea of counting differences more directly.