->i applies contract projections multiple times?

This question comes from someone else on Stack Overflow, which they
asked here: http://stackoverflow.com/q/41144374/465378

I think it’s likely that the people who can answer this are probably
only on the mailing list, though, so I figured I’d ask it here.

Basically, the ->i contract applies contract projections more than
once, despite the fact that it seems unnecessary. Here’s the example
provided in the question:

  (define/contract (foo x y z)
    (->i ([x       (λ (x) (begin (displayln 0) #t))]
          [y (x)   (λ (y) (begin (displayln 1) #t))]
          [z (x y) (λ (z) (begin (displayln 2) #t))])
         any)
    #t)

  (foo 1 2 3)

The above example prints the following when run:

  0
  0
  1
  1
  2
  #t

This indicates that values used dependently for determining other
contracts cause their contracts to be applied more than once. The
above example only uses flat contracts, but I tested with a
side-effectful impersonator contract and got the same result.

After some very simple pondering, I couldn’t come up with a situation
in which applying the contracts more than once would be necessary,
and this could obviously have a performance penalty given that ->i
is already a fairly slow contract. Is this a bug in ->i, or is it
necessary or intentional behavior? If the latter, why is it necessary?

Alexis

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