I don't know too much about JS or python (I'm mostly C/C++/Java/Clojure/Matlab) but there's nothing to stop you from accessing C, in Haskell either, nor from using unsafePerformIO, and while in Java you can't prevent access to random numbers etc. statically, but you can certainly do it at runtime (though, in Java, too, you can call C and then all bets are off).

> Haskell is unique among any languages I've ever used in production that allows deterministic testability as a static, enforced-by-the-compiler guarantee.

Oh, that's true. It's just that if your tests are guaranteed to be deterministic, I don't see why the fact that the property is enforced statically is so important.

Personally, I think Haskell sacrifices a lot for the sake of static guarantees (including the ability to reason about programs not before they run, but during and after). Also, its reliance on Curry-Howard for verification is limited and narrow-sighted, and its insistence on referential transparency everywhere is interesting yet misguided. I'm now preparing a talk for the Curry On/ECOOP conference where I'll show -- among other things -- how all monads can be easily translated to imperative constructs (I won't go into details here, but what you need is continuations and continuation-global variables) in a way that makes them more understandable and compose more naturally than in PFP (i.e. there's no need for monad transformers). Even though this model does not equate the subroutine with the mathematical function -- as PFP does -- it is no less verifiable than the PFP approach. But I digress...