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Comment by asQuirreL | Hacker News Reader

There doesn't need to be an overhead to this extra type disciplining of side-effectful computing. It is perfectly possible for the compiler to take the pure monadic code and implement it with actual side effects (which I believe GHC does do).

What I do miss though (dearly) is that there is a certain cost as to what can be encapsulated. For instance, in Haskell, it is no longer so easy to hide side effects behind a pure interface.

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>For instance, in Haskell, it is no longer so easy to hide side effects behind a pure interface.

This is not true. There are a number of ways to do this in Haskell.

Also this is tantamount to, "my programming language won't let me lie", and is often associated with an infacility with Haskell. I mean no offense, just speaking from experience.

You don't need to hide effects behind a pure interface once you're comfortable with the abstractions provided.

Edit: tel's comment is quite good as-is, but I'd like to add that part of what makes ST nice isn't just having an eliminator, but also so you have a more precise delineation of what "effects" could happen. IO is kind of a big type. Still very valuable.

So the example I have in mind is that of a Randomised Binary Search Tree (or Treap, if you want). Its interface is pure, however it requires access to an RNG for insertions. Because of this, it cannot be treated as a pure collection: you need to tell the type system, hang on, this RBST carts around some state which changes when you insert to it, despite the fact that the state doesn't change the extensional behaviour of the structure. This is a provable property, but maybe not provable by Haskell's type system.

Now I know that you can hide unsafePerformIO in the interface, and in this case I expect it is fine, but this is not easy, it feels like a hack.

Another example that I have personally watched play itself out in disbelief was in a fairly large Haskell project: There was need of a lookup table, with some 10000 elements. In such cases, I would have stored the lookup in a database, or a separate file at least, but because they wanted to maintain purity so badly, the solution was to write a 10000 line long Haskell file. But wouldn't it be nice if you could load a file and tell Haskell: don't worry, this won't change, so treat reads to this purely.

I never said such things were impossible, merely not easy, and I don't believe this is by accident. I am almost certain that such things are made difficult to prevent abusing such features, but that doesn't stop me from missing them :) .

To clarify coolsunglasses's note that it is actually possible to hide side effects behind a pure interface (when they can be proven pure) here's the "counter" example in the paper

    import Control.Monad.ST 
    import Data.STRef

    data Counter s = Counter { get :: ST s a, inc :: ST s () }

    mkCounter :: STRef s Int -> Counter s
    mkCounter v = Counter (readSTRef v) (incRef v) where
      incRef ref = do
        i <- readSTRef v
        writeSTRef v (i + 1)

A little noisier since we note that reading the ref is an impure action and we track a phantom `s` parameter around in order to convince Haskell that certain uses of counters are pure. Now

    comp :: ST s Int
    comp = do
      v <- newSTRef 0
      let c = mkCounter v

      inc c
      inc c
      inc c
      get c

This is technically a pure computation despite using an impure mechanism internally. We can get Haskell to prove this for us

    pureComp :: Int
    pureComp = runST comp

Here `runST :: (forall s . ST s a) -> a` completes the proof that if the `s` variables in a computation are completely undetermined then that computation must be pure.

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