The generated shell.nix file accepts a string argument called "compiler" that
determines the package set used to instantiate the generated expression. For
example, running "nix-shell --argstr compiler ghc7102" would evaluate the build
inside of "pkgs.haskell.packages.ghc7102". Earlier versions of cabal2nix had the
current default compiler hard-coded in the expression, but after this change this
is no longer the case. When "compiler" remains unspecified, it defaults to
"default", and this value causes evaluation in "pkgs.haskellPackages", which is
the package set most people would like to use by default. That change has to
benefits:
1) Generated expression no longer contain any particular compiler version. The
choice of the default compiler depends on the version of Nixpkgs that's used
to build the expression.
2) When the default compiler is used, overrides configured for the default
package set apply, which was not the case in earlier versions.
This update greatly enhances the accuracy with which dependencies are expressed
in the generated Nix files. Previous versions distinguished dependencies for
building ("buildDepends") and testing ("testDepends"). This distinction didn't
apply to system packages or build tools, however: the fields "extraLibs" and
"buildTools" applied to the entire build. This meant that dependencies required
only for testing would be pulled in regardless of whether the test were
actually being run, etc.
These days, we distinguish dependencies for libraries, executables, and tests,
and for each of those types we distinguish dependencies on Haskell libraries,
system libraries, pkgconfig libraries, and build tools. This gives us a
whopping 12 new attributes
xxxHaskellDepends
xxxSystemDepends
xxxPkgconfigDepends
xxxToolDepends
where "xxx" is any of "library", "executable", or "test".
The old dependency attributes are no longer generated by cabal2nix. The generic
builder in Nixpkgs still accepts them, though, for the sake of backwards
compatibility. This means that you don't have to re-generate all your build
expressions with the new version, but you *should*.
