@code{#:python} parameter.
@end defvr
+@defvr {Scheme Variable} haskell-build-system
+This variable is exported by @code{(guix build-system haskell)}. It
+implements the Cabal build procedure used by Haskell packages, which
+involves running @code{runhaskell Setup.hs configure
+--prefix=/gnu/store/@dots{}} and @code{runhaskell Setup.hs build}.
+Instead of installing the package by running @code{runhaskell Setup.hs
+install}, to avoid trying to register libraries in the read-only
+compiler store directory, the build system uses @code{runhaskell
+Setup.hs copy}, followed by @code{runhaskell Setup.hs register}. In
+addition, the build system generates the package documentation by
+running @code{runhaskell Setup.hs haddock}, unless @code{#:haddock? #f}
+is passed. Optional Haddock parameters can be passed with the help of
+the @code{#:haddock-flags} parameter. If the file @code{Setup.hs} is
+not found, the build system looks for @code{Setup.lhs} instead.
+
+Which Haskell compiler is used can be specified with the @code{#:haskell}
+parameter which defaults to @code{ghc}.
+@end defvr
+
Lastly, for packages that do not need anything as sophisticated, a
``trivial'' build system is provided. It is trivial in the sense that
it provides basically no support: it does not pull any implicit inputs,