packageFlagErr :: PackageFlag
-> [(PackageConfig, UnusablePackageReason)]
-> IO a
+
+-- for missing DPH package we emit a more helpful error message, because
+-- this may be the result of using -fdph-par or -fdph-seq.
+packageFlagErr (ExposePackage pkg) [] | is_dph_package pkg
+ = ghcError (CmdLineError (showSDoc $ dph_err))
+ where dph_err = text "the " <> text pkg <> text " package is not installed."
+ $$ text "To install it: \"cabal install dph\"."
+ is_dph_package pkg = "dph" `isPrefixOf` pkg
+
packageFlagErr flag reasons = ghcError (CmdLineError (showSDoc $ err))
where err = text "cannot satisfy " <> ppr_flag <>
(if null reasons then empty else text ": ") $$