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@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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