[Haskell-cafe] Re: [Haskell] Top Level <-

Lennart Augustsson lennart at augustsson.net
Thu Aug 28 13:31:19 EDT 2008

I don't think anyone has claimed that any interface can be implemented
without globals.
Of course some can't (just pick an interface that is the specification
of a global variable).

What I (and others) claims is that such interfaces are bad.  Using a
global variable makes an assumption that there's only ever going to be
one of something, and that's just an inflexible assumption to make.

You think global variables are essential, I think they are a sign of
bad design.  So we have different opinions and neither one of us is
going to convince the other.

I think a lot of things related to the IO monad in Haskell is bad
design; influenced by imperative thinking.

For instance, I think the main function should have a type like
  main :: (IOMonad io) => io a
where IOMonad contains some basic functionality like calling C.
Then you could do things like implement runInsandboxIO which traces all C calls.

  -- Lennart

On Thu, Aug 28, 2008 at 5:26 PM, Adrian Hey <ahey at iee.org> wrote:
> Jonathan Cast wrote:
>> On Thu, 2008-08-28 at 10:00 +0100, Adrian Hey wrote:
>>> Lennart Augustsson wrote:
>>>  > I don't don't think global variables should be banned, I just think
>>>  > they should be severly discouraged.
>>> If you're saying a language should not provide a sound way to do
>>> this (as I believe you are), then AFAICT for all practical purposes
>>> you *are* saying you think global variables should be banned.
>>> Where are we going to be if the unsafePerformIO hack ever becomes
>>> *really* unsafe?
>>> and..
>>>> I'm certain you can write a kernel in Haskell where the only use of
>>>> global variables is those that hardware interfacing forces you to use.
>>> But what you haven't explained is why this is even desirable? I don't
>>> doubt it's true in an academic sense if you don't mind sacrificing
>>> safety
>> What `safety' is being sacrificed?
>>> and modularity.
>> What modularity?
> As I've pointed out several times already you can find simple examples
> in the standard haskell libs. So far nobody has accepted my challenge to
> re-implement any of these "competantly" (I.E. avoiding the use of global
> variables).
> Why don't you try it with Data.Unique and find out :-)
> Regards
> --
> Adrian Hey
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