Hi folks,
I'm writing a stats library (I'm sure there is already one out there;
this is a learning exercise), and I'm currently implementing Kendall's
Tau-a. I've got my head wrapped part of the way around contracts, but
not all the way. I'd like to be able to express the following:
(tau l m)
- l is a non-empty list of numbers, all greater than 0
- m is a non-empty list of numbers, all greater than 0
- l and m are of the same length
Here's a stub version that does the first two checks:
(define/contract (tau l m)
(-> (listof (and/c integer? (>/c 0)))
(listof (and/c integer? (>/c 0)))
number?)
300)
(tau '(8) '(9)) ;; should succeed and does
(tau '(8) '(9.0)) ;; should fail (element is not integer) => SUCCEEDS??
;;
;; not implemented
;; (tau '(8) '(9 10)) ;; should fail -- different length lists
I figured I'd let the 'integer?' failure go for a minute and focus on
the missing criteria (lists are of equal length). First, I wanted to
make sure that I had understood the ->i syntax, so I did this:
(define/contract (tau l m)
(->i ([list1 (listof (and/c integer? (>/c 0)))]
[list2 (list1) (and/c
(listof (and/c integer? (>/c 0))))])
[result (list1 list2) number?])
300)
Okay, good, that has the same results as the one above. Now let's try
to get the 'equal length' criteria working.
(define/contract (tau l m)
(->i ([list1 (listof (and/c integer? (>/c 0)))]
[list2 (list1) (and/c
(listof (and/c integer? (>/c 0)))
(equal? (length list1) (length list2)))])
[result (list1 list2) number?])
(begin
(println "I was called")
300))
Result =>
t.rkt:21:79: list2: unbound identifier in module
in: list2
Nope. Unsurprisingly, 'arg2' is not available inside its own binding.
At this point I faffed around with implementing the check as a
precondition or in the result function, and everything else I could
think of, all to no avail.
How do I do this? It feels like it shouldn't be this hard.
On a separate question: How do I contribute to the docs? The
contract docs are nearly silent on using preconditions and
postconditions, Section 7.4, "A Thorough Example of Contracts" is
anything but thorough, and the docs could stand to include more
examples in general. I'd love to help future Racket students
understand what's going on.
Dave
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