I just watched the video by Barbara Liskov. He references this to show how even ...

Peaker · on March 21, 2011

I think saying that FP is bad for "manipulating state" is misleading.

FP is great at manipulating state. A pure Haskell function (State -> State) manipulates state.

What's being discussed is whether or not the state is manipulated destructively. And personally I don't see any real benefits to the destructive method, except performance issues in some cases, and arguably stylistic ones.

Returning new values instead of in-place updates may seem cumbersome before you get to know the abstractions that make it easy to use.

As an example, in Haskell, if you want to update a value deep inside multiple nested records, you do something like:

    movePlayer :: (Pos -> Pos) -> GameState -> GameState
    movePlayer f gs = gs { player = oldPlayer { position = f oldPos } }
      where
        oldPlayer = player gs
        oldPos = position oldPlayer

That looks horrible, compared to something like:

    gs.player.position = f(gs.player.position);

But then, if you discover the Semantic Editor Combinators abstraction (http://conal.net/blog/posts/semantic-editor-combinators/), then you can write that as:

    movePlayer :: (Pos -> Pos) -> GameState -> GameState
    movePlayer f = (atPlayer . atPosition) f

or just:

    movePlayer = atPlayer . atPosition

Then you can use:

    movePlayer (+delta)

to add delta to the position.

Now one nice thing about the return-new-value approach is that it is more composable than the destructive update approach. When you compose state manipulation functions, you get transactional state changes.

So paradoxically, I believe functional languages to be better at manipulating state!