Hi all, I think I've found a bug in Typed Racket's predicates, but investigating it raised a lot of questions, so I was hoping someone wouldn't mind explaining what's going on here.
A couple weeks ago I wrote a TR program using version 5.1 (like a bonehead, my shortcut to DrRacket was running it out of the wrong directory so I wasn't using the version that I thought I was.) When I recently converted it to 5.1.3, I started noticing intermittent pattern matching failures. I managed to isolate it to the following standalone: #lang typed/racket (define-type Float-Or-Integer (U Float Integer)) (define-predicate Float-Or-Integer? Float-Or-Integer) (define-predicate Float? Float) (define (weird-predicate-behavior) (: integer->float (Integer -> Float)) (define (integer->float int) (exact->inexact int)) (define x (integer->float 0)) (printf "eq? 0.0 ~A ~%" (eq? 0.0 x)) (printf "eqv? 0.0 ~A ~%" (eqv? 0.0 x)) (printf "Float? ~A ~%" (Float? x)) (printf "Float-Or-Integer? ~A" (Float-Or-Integer? x))) Running this produces: eq? 0.0 #f eqv? 0.0 #t Float? #t Float-Or-Integer? #f The part of TR that generates the compound contracts for unions containing Float types is using eq? instead of eqv? for the 0.0 and -0.0 case (which started in version 5.1.1), which explains why the predicate fails. Somehow, in the repro above, a 0.0 is being produced that doesn't eq? with the literal 0.0. I tried doing this with, say, (eq? 0.0 (- (+ 3.0 1.0) 4.0)) or (eq? (exact->inexact 0) 0.0), but those both evaluate to #t, so either the compiler is doing some very clever constant folding here, or eq? is supposed to do value equality for floats. I'm assuming the former, but especially for the exact->inexact case that seems pretty darn clever. Anyhow, I was hoping someone could confirm my suspicion that this bug was so hard to reproduce because of crazy compiler magic (also, assuming this is right and it's simply an eq? vs. eqv? issue, I've sent along a patch). Thanks, Luke
0001-Fixed-a-bug-where-predicates-would-sometimes-fail-to.patch
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