It goes in `Cabal-syntax/src/Language/Haskell/Extensions.hs`, in `data KnownExtension`. The parse table is built from the constructor names, and `No` prefixes are handled automatically. Note that you'll need to also bump the `cabal-version` (file format version) and add an entry to the file format documentation (`doc/file-format-changelog.rst`).

On Sun, Mar 15, 2026 at 1:18 PM Simon Peyton Jones via ghc-devs <ghc-devs@haskell.org> wrote:

Friends

As part of the reinstallable base project I'm adding an extension "ImplicitKnownKeyNames" to GHC. But if I put "NoImplicitKnownKeyNames" in base.cabal.in I get the error

Warning: [unknown-extension] Unknown extensions: NoImplicitKnownKeyNames

when building the base library. I think this comes from cabal.

How can I tell cabal about the new extension?

Thanks

Simon
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