Version of cabal.nix that can be retrieved directly from the gist

cabal.nix

{ basePath ? ./.}:
with builtins; rec {
  cabalProjects = listToAttrs (if pathExists (basePath + "/cabal.project")
                  then projectParse
                  else [ { name = baseNameOf basePath; value = basePath; } ] );
  projectParse = let
    contents = readFile (basePath + "/cabal.project");
    trimmed = replaceStrings ["packages:" " "] ["" ""] contents;
    packages = filter (x: isString x && x != "") (split "\n" trimmed);
    package = p: substring 0 (stringLength p - 1) p;
    paths = map (p: let p' = package p; in { name = p'; value = toPath (basePath + "/${p'}"); } ) packages;
  in paths;
}.cabalProjects

default.nix

let
# ...
  # Parse the list of targets to build from cabal.project, or from <basename>.cabal if no project file exists
  targets = import (builtins.fetchurl {
    url = "https://gist.githubusercontent.com/codebje/6bb4d9887cca0fd34c359d033f0c80d8/raw/ddda82c94dff6bfe010bae6b22491c4991a43373/cabal.nix";
    sha256 = "0zackipwxw4qnxap89srl3gmlgd9nqrjrh6fghxsf6ib96fif71n";
  }) { basePath = ./.; };
# ...
in {}