Add shellHook to clang.nix

etc/nix/clang.nix

@@ -5,5 +5,10 @@
     clang
     llvmPackages.openmp
   ];
+
+  shellHook = ''
+    export OMPI_CC=clang
+    export OMPI_CXX=clang++
+  '';
 }
 

shell.nix

@@ -1,9 +1,12 @@
 { pkgs ? import <nixpkgs> {} , with-clang ? false }:
 
 let
+
+  clang = import ./etc/nix/clang.nix { inherit pkgs; };
+
   compiler-configuration
     = if with-clang
-      then (import ./etc/nix/clang.nix { inherit pkgs; }).buildInputs
+      then clang.buildInputs
       else [ pkgs.gcc ];
 
 in
@@ -30,6 +33,8 @@ pkgs.mkShell {
     export OPENBLAS_PATH=${pkgs.openblas}
     export SCALAPACK_PATH=${pkgs.scalapack}
     export LD_LIBRARY_PATH=${pkgs.scalapack}/lib:$LD_LIBRARY_PATH
-  '';
+  ''
+  + (if with-clang then clang.shellHook else "")
+  ;
 
 }