Why is NoExt inhabited?

Richard Eisenberg rae at richarde.dev
Tue Jun 25 03:21:57 UTC 2019

I forget if I'm the contributor Ben mentions below, but regardless: NoExt is used in two separate ways within GHC. It's used both as the extension of a sum (new constructor) and the extension of a product (new field in a constructor). But the identity for sum is different than the identity for products. To me, it makes good sense for the NoExt used as a product extension (new field) to have the 1 trivial inhabitant (thus forming the identity for products), but the NoExt used as a sum extension (new constructor) to have 0 inhabitants (thus forming the identity for sums). Clearly, one of these will need a new name.


> On Jun 24, 2019, at 4:57 PM, Ben Gamari <ben at smart-cactus.org> wrote:
> Hi Shayan,
> During code review a contributor asked about a TTG extension point.
> While answering his question I realized I didn't have a good explanation
> for why NoExt is inhabited; as far as I can tell it should rather be
> unhabited.
> If there is a reason for this then can you:
> 1. describe why this is so in #16863 [1]
> 2. open an MR documenting the reason in Note [Trees that Grow].
> If not, perhaps we should consider changing it; the current state of
> things is a bit awkward.
> Cheers,
> - Ben
> [1] https://gitlab.haskell.org/ghc/ghc/issues/16863
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