Comparing StableNames of different type

Manuel M T Chakravarty chak at
Sun Aug 26 08:03:44 CEST 2012

Emil Axelsson <emax at>:
> 2012-08-24 11:08, Simon Marlow skrev:
>> On 24/08/2012 07:39, Emil Axelsson wrote:
>>> 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:

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

You may be able to use our entire scheme. (We have started to write it up as a paper, but the write up isn't in a coherent state yet.)


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