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[question] Can't build with cabal using the JS backend
I'm trying to build a project using the JS backend in GHC 9.6 (or 9.7.20230406). I'm getting GHC from nixpkgs (`pkgsCross.ghcjs.haskell.compiler.ghc96`) and running this command: ``` cabal --with-compiler=javascript-unknown-ghcjs-ghc --with-ghc-pkg=javascript-unknown-ghcjs-ghc-pkg build ``` This eventually fails to build `lens`, because `ghcjs_HOST_OS` is not set, and [this file](https://github.com/ekmett/lens/blob/master/src/Control/Lens/Empty.hs#L84-L86) relies on it. Setting `--ghc-option='-Dghcjs_HOST_OS'` doesn't change anything. So I tried this instead: ``` cabal --ghcjs --with-ghcjs=javascript-unknown-ghcjs-ghc --with-ghcjs-pkg=javascript-unknown-ghcjs-ghc-pkg build ``` Which now complains that it can't determine the version of `javascript-unknown-ghcjs-ghc`. And indeed, Cabal is trying to run `javascript-unknown-ghcjs-ghc --numeric-ghcjs-version`, which is not a recognised flag. How do I use the new JS backend with Cabal?
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