[Haskell-cafe] Pointer equality for nullary constructors

[David Feuer]

Sure, but that's okay, because I never need to expose the type to anyone.
I'm just nervous because I haven't seen documentation saying those values
are type-specific.

On Feb 11, 2018 10:37 PM, "Edward Kmett" <ekmett at gmail.com> wrote:

> Keep in mind a newtype of Foo will have a different TypeRep, but will
> compare as equal under reallyUnsafePtrEquality#.
>
> -Edward
>
> On Sun, Feb 11, 2018 at 5:14 AM, David Feuer <david.feuer at gmail.com>
> wrote:
>
>> Can I use reallyUnsafePtrEquality# reliably to identify whether a value
>> is a nullary constructor of a particular type? For example, if I have
>>
>> data Foo = Foo
>>
>> Can I write
>>
>> isFoo :: a -> Bool
>> isFoo !a = isTrue# (reallyUnsafePtrEquality# a Foo)
>>
>> instead of
>>
>> isFoo :: forall a. Typeable a => a -> Bool
>> isFoo a
>>   | Just Refl <- eqTypeRep (typeRep @a) (typeRep @Foo)
>>   , Foo <- a
>>   = True
>>   | otherwise = False
>>
>> The reason I'm asking is because this would let me (potentially) raiseIO#
>> a nullary constructor and then catch# it and see if it was what I was
>> looking for rather than having to open a SomeException to get to an
>> Exception dictionary, open that to get a TypeRep, and then peer inside that
>> to check a Fingerprint. That is, I'd get lighter-weight exceptions that
>> only carry the information I actually need.
>>
>> Thanks,
>> David
>>
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>
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