[Haskell-cafe] Pointer equality for nullary constructors

[Gregory Collins]

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)
>

You mean like this?
https://github.com/gregorycollins/hashtables/blob/master/src/Data/HashTable/Internal/UnsafeTricks.hs#L72

My experience is that this works, except when doing coverage, where the
compiler adds instrumentation code that breaks the technique. I #ifdef'd my
way around the problem, swapping out a slower impl when I was doing code
coverage.

Greg
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