Re: Comparing StableNames of different type

Emil Axelsson :

2012-08-26 08:03, Manuel M T Chakravarty skrev:

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Emil Axelsson :

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2012-08-24 11:08, Simon Marlow skrev:

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On 24/08/2012 07:39, Emil Axelsson wrote:

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Hi!

Are there any dangers in comparing two StableNames of different type?

stEq :: StableName a -> StableName b -> Bool stEq a b = a == (unsafeCoerce b)

I could guard the coercion by first comparing the type representations, but that would give me a `Typeable` constraint that would spread throughout the code. It should be safe even if the types are different, but I presume you expect the types to be the same, since otherwise the comparison would be guaranteed to return False, right?

No, I want to do observable sharing of heterogeneously typed expressions which means I will be comparing expressions of different type.

You may like to have a look at how we have done this in the Accelerate EDSL:

https://github.com/AccelerateHS/accelerate/blob/master/Data/Array/Accelerate...

In particular, the Eq instance for stable names of AST nodes is at

https://github.com/AccelerateHS/accelerate/blob/master/Data/Array/Accelerate...

Hm, doesn't this impose a `Typeable` constraint on all AST nodes (which is what I'm trying to avoid)?

Yes, you need Typeable, but arguably that is cleaner than unsafeCoerce. In any case, as I am not aware of any other system than Accelerate that implements observable sharing on typed ASTs, you may find the Accelerate code to be a helpful source of inspiration. Manuel