[nybble41]

Haskell has confined mutation with the State and ST types, which guarantees that whatever may be going on inside, the external interface to the function is pure. My main problem with F# (and other multi-paradigm / "functional-first" languages) is that they do not provide any such guarantees; purity becomes merely a matter of convention and convenience, not something ensured by the language. This misses the entire point of functional programming, IMHO, which is referential transparency, and greatly increases the complexity since there are things that matter in F#, like evaluation order, which purity renders completely irrelevant in Haskell.

(Yes, Haskell does have unsafePerformIO and the like for special cases, but the expectation is still that the external interface remains pure. If you deliberately circumvent the language's protections and fail to adhere to this rule then you get to keep both pieces when it inevitably breaks. The compiler is still going to operate under the assumptions that the result depends only on the explicit inputs and that evaluating the function has no side effects.)