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

[Ashley Yakeley]

Ganesh Sittampalam wrote:
> Every single call to newIORef, across the whole world, returns a 
> different ref.

How do you know? How can you compare them, except in the same Haskell 
expression?

> The "same" one as a previous one can only be returned 
> once the old one has become unused (and GCed).

Perhaps, but internally the IORef is a pointer value, and those pointer 
values might be the same. From the same perspective, one could say that 
every single call to newUnique across the whole world returns a 
different value, but internally they are Integers that might repeat.

>> It's the scope in which values from newUnique are supposed to be 
>> different, and it would also be the scope in which top-level <- would 
>> be called at most once.
> 
> I don't really follow this. Do you mean the minimal such scope, or the 
> maximal such scope? The problem here is not about separate calls to 
> newIORef, it's about how many times an individual <- will be executed.

Two IO executions are in the same "global scope" if their resulting 
values can be used in the same expression. Top-level <- declarations 
must execute at most once in this scope.

-- 
Ashley Yakeley