Re: [Haskell] Re: Global Variables and IO initializers
(indexing with TypeRep)
This is yet another incidence where Robert Will's ByMaps would be very useful
In fact GHC at least *already* generates a unique integer for each TypeRep. A good idea, since it means comparisons can be done in unit time. Thus indexing can be done trivially using this integer as a hash function.
On Monday 29 November 2004 11:35, George Russell wrote:
(indexing with TypeRep)
This is yet another incidence where Robert Will's ByMaps would be very useful
In fact GHC at least *already* generates a unique integer for each TypeRep. A good idea, since it means comparisons can be done in unit time. Thus indexing can be done trivially using this integer as a hash function.
Yes, I have seen this in the code, too. The Ord and Typeable instances should be trivial. [off topic:] There was a recent discussion about allowing to derive an instance from anywhere at the top-level, and not only in the type definition. This is one more example where such a feature would be very useful. Another related example is the class Typeable itself. It has been noted by others that the current interface is not type safe, since mkTyCon gets an arbitrary string as argument. (Unfortunately this means that GlobalVariables.hs and ExecutionContext.hs aren't really type safe either). Typeable would be completely safe if the only way to declare instances would be to derive them, but this is only practical if it can be done from anywhere outside the data type definition. Can anyone think of a situation where adding a derived instance to an abstract data type breaks one of its invariants? Ben
On Mon, Nov 29, 2004 at 11:57:31AM +0100, Benjamin Franksen wrote:
Can anyone think of a situation where adding a derived instance to an abstract data type breaks one of its invariants?
Yes, I was thinking of this the other day, newtype LessThan5 = LessThen5 Int new x | x < 5 = LessThen5 x | otherwise = error "not less than five" if someone were allowed to do a derive (Enum LessThan5) in another module, then they could break the invarient with toEnum 6 for instance. For safety, one should only be able to remotely derive if all the constructors of the type are in scope as well as the type. However, this is too strong of a constraint for deriving Typeable which does not care about the constructors. It is not clear what the correct thing to do is, perhaps have 2 types of derivable classes, ones which need the constructors and ones which don't? Hmm.. I am sort of of the practically motivated opinion that Typable should be a built-in that everything is automatically an instance of, but I don't know if that is really the right thing to do or just a convinient hack. John -- John Meacham - ⑆repetae.net⑆john⑈
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Benjamin Franksen -
George Russell -
John Meacham