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Editorial Team
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Asked: 2026-05-17T20:46:01+00:00

Cabal is giving me mixed messages. When I say: Extensions: DeriveFunctor It says: Warning:

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Cabal is giving me mixed messages. When I say:

Extensions: DeriveFunctor

It says:

Warning: Unknown extensions: DeriveFunctor

But when I say:

GHC-Options: -XDeriveFunctor

It says:

Warning: Instead of 'ghc-options: -XDeriveFunctor' use 'extensions:
DeriveFunctor'

For now I’m just going to use the {-# LANGUAGE DeriveFunctor #-} pragma.

$ cabal --version
cabal-install version 0.8.2
using version 1.8.0.6 of the Cabal library 
$ ghc --version
The Glorious Glasgow Haskell Compilation System, version 6.12.3
$ basename $(mdfind haskell-platform | grep .dmg)
haskell-platform-2010.2.0.0.i386.dmg
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1 Answer

  1. Editorial Team

    According to the Hackage documentation, as of Cabal-1.8.0.6 DeriveFunctor is not recognized. It’s a relatively new addition to GHC and it doesn’t seem to have wide use, so I’m not surprised it would have been overlooked for Cabal. This should probably be filed as a bug (feature request?) against Cabal.

    @Tom Lokhorst is right that a LANGUAGE pragma is the best option. I don’t like using Cabal’s Extensions field because then all extensions are active for all modules, which I often don’t want.

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