[Haskell-cafe] Top-level state debate on the wiki

Jules Bean jules at jellybean.co.uk
Wed Dec 1 16:00:55 EST 2004

On 1 Dec 2004, at 18:29, Ben Rudiak-Gould wrote:
> Here's the page:
>    http://haskell.org/hawiki/GlobalMutableState

Nice summary.

What I think is missing is an explanation of when you would want this 
feature (and when you wouldn't, more importantly).  Here is the kind of 
platonic dialogue that summarises my limited understanding:

A: I want a global variable x which IO actions f and g can alter. Why 
can't I say x <- newIORef 0::Int at the top level?

B: Why would you? Put the newIORef command at the beginning of your 
main action and pass the reference to f and g. This is much better 
programming practice: you can invoke f and g again on different 
references whenever you want to, useful for debugging and testing.

A: Ah, but this isn't scaleable, is it? What if I have y and z and q 
and r and..... I really don't want f and g to have *that* many 

B: That's easy, just wrap them up in a single type. Call it GEnv, if 
you like, for Global Environment. Now all your IO actions just need one 
extra parameter.

A: But the bodies of f and g call many other actions, most of which 
need access to some part of this environment. It's a real pain 
threading the global environment parameter to all of these actions! It 
would be so much simpler to just make them top-level globals!

B: Problem threading parameters? Sounds to me like you want a state 
monad. You know you can use StateT GEnv IO to layer that over IO, 
right? In fact, in many programs you will not even need IORefs any 
more, since the StateT gives you modifiability for free.

[A wanders off muttering...]

[...some time later...]

A: Ah! I've got you now! I'm implementing a module which exports a Map 
interface. It's a pure interface, no IO monads in sight. But, I want to 
implement it with constant-time lookup, so I need a hashing algorithm 
and a mutable hashtable, right?

B: No problem, you should be using the ST monad and STUArray, probably.

....which is to say, that I don't have a very clear idea of when you do 
need it, although I think I have some ideas of when you don't need it. 
The example that I haven't addressed is the 'OS-in-haskell' family of 
examples, which I don't understand clearly enough to summarise.


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