Re: Why is NoExt inhabited?

On Jun 29, 2019, at 11:48 AM, Carter Schonwald wrote:

Phrased differently is a magic stand in for 0 ary sum and products ?

Or do you mean how n-ary sum and product are the same structure when n=1?

No -- I just mean that 0 (that is, Void) is the identity for sums and 1 (that is, Unit) is the identity for products. Sorry if this wasn't clear! Richard

Forgot to cc list. Pardon the double send :)

On Mon, Jun 24, 2019 at 11:22 PM Richard Eisenberg mailto:rae@richarde.dev> wrote: 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.

Thanks, Richard

...

On Jun 24, 2019, at 4:57 PM, Ben Gamari mailto:ben@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 https://gitlab.haskell.org/ghc/ghc/issues/16863 _______________________________________________ ghc-devs mailing list ghc-devs@haskell.org mailto:ghc-devs@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs

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