[pcwalton]

Rust used to have a "pure" annotation, but it's gone, partially because it was a pain to have to write the annotation everywhere, partially because it's not needed for memory safety anymore, partially because nobody can agree on what "pure" means.

[fusiongyro]

I wouldn't have anticipated that, but now that you say it I could see why that would lead to a debate. How would you deal with mutable data structures, for instance? What if it accesses the environment, but in a fashion you could somehow guarantee were safe? In Haskell the programmer can circumvent the system with unsafePerformIO if they know something they can't convince Haskell otherwise, but it almost seems like you'd need a "pure-but-not-really" annotation to do this kind of thing in an imperative language that actually enforced purity.

[laureny]

C++ walked in these very same footsteps. First by not having const, then by having it, then by allowing exceptions to constness, then by introducing const_cast and finally by allowing temporarily mutable const objects.

[epsylon]

C++ const is defective because it's a shallow const. You can modify an object through a const pointer.

The D language "fixes" this by making const transitive (and also adding an immutable annotation, which means the object is truely read-only, as in "read-only memory").

[lclarkmichalek]

"pure nothrow @safe" is what you get to prepend to your functions in D. I prefer Haskell's approach.