diff --git a/pkgs/development/tools/parsing/happy/1.19.3.nix b/pkgs/development/tools/parsing/happy/1.19.3.nix
index b1a03fa3b9f8..c43806cd2df2 100644
--- a/pkgs/development/tools/parsing/happy/1.19.3.nix
+++ b/pkgs/development/tools/parsing/happy/1.19.3.nix
@@ -8,6 +8,10 @@ cabal.mkDerivation (self: {
   isExecutable = true;
   buildDepends = [ mtl ];
   buildTools = [ perl ];
+  preConfigure = self.stdenv.lib.optional (self.stdenv.lib.versionOlder "7.9" self.ghc.version) ''
+    sed -i -e 's|extensions: |extensions: FlexibleContexts, |' happy.cabal
+    sed -i -e 's|> import System.Exit|> import System.Exit ( exitWith, ExitCode(..) )|' src/Main.lhs
+  '';
   meta = {
     homepage = "http://www.haskell.org/happy/";
     description = "Happy is a parser generator for Haskell";