[wz1000]

Linear types support is coming along nicely, there is a fork of GHC with a prototype implementation.

https://github.com/tweag/ghc/tree/linear-types

However, to allow the compiler to optimise pure operations to impure ones, I don't think linear types are sufficient. You need something like uniqueness types.

[btcindivist]

I think C++ compiles down to a pure functional language that is then optimized into an efficient beast. So I'm not sure that pure code is hard to optimize.

[seanmcdirmid]

SSA is quite different from a pure FP; for example it doesn’t do anything about effects, it only gets rid of local imperative assignments that don’t really interfere with purity anyways.

[tathougies]

Most imperative languages have an intermediate language in which local variables are immutable. However, purity is about more than immutability. For example, in C++

    x = doSomething(x) + 1

Can be written to not overwrite x

    int x2 = doSomething(x) + 1

This is equivalent in some ways to Haskell

    let x' = doSomething x + 1

However, I know that in Haskell, evaluating 'doSomething x' will not turn off the computer, display anything to the user, or launch missiles. I have no idea what evaluating 'doSomething(x)' does in C++. It may add things to caches, exit the program, etc.