Storable instance of () is broken

Sven Panne svenpanne at
Wed Jan 5 12:12:32 UTC 2022

Am Mi., 5. Jan. 2022 um 12:42 Uhr schrieb David Feuer <david.feuer at

> No. Consider a type like this:
> data Foo a = Foo !Int !a
> instance Storable a => Storable (Foo a) where ...
> Now if a happens to be (), we pay only one word per Foo. [...]

This is exactly the kind of breakage I had in mind: With the proposed
change, the storage layout would change, and the compiler wouldn't warn you
about that at all. Note that I'm not arguing about memory efficiency, it's
all about a subtle semantic change for the sake of a single library,
wanting to change something which was in place for 10-20 years. Seems like
an extremely bad move from the POV of the Haskell ecosystem: It's exactly
this kind of ad hoc changes which annoys people.
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