Final script from "Using a development version of GHC with nix"

head-overlay.nix

let                                                                                
  nixpkgs = import <nixpkgs> {};                                                   
  patchRepo =                                                                      
    nixpkgs.fetchFromGitHub {                                                      
      owner = "mpickering";                                                        
      repo = "head.hackage";                                                       
      rev = "767dcf11b367ccff4a9fcd55df9c2432cd485fbe";                            
      sha256 = "1cdrcw7grpc2dyxnb7a5bg9mi1h7qnblcibi91s348034zf7a0vj";};           
                                                                                   
  patchDir = "${patchRepo}/patches";                                               
  patchScript = "${patchRepo}/scripts/overrides.nix";                              
in                                                                                 
self: super:                                                                       
{                                                                                  
  patches = super.callPackage patchScript                                          
              { patches = patchDir; };                                             
                                                                                   
  new-ghcHEAD =                                                                    
    let ghcPackageOverrides = super.callPackage self.patches {};                   
        localOverrides      =                                                      
          sel: sup: { haskell-src-exts                                             
                        = sel.callHackage "haskell-src-exts" "1.20.1" {};          
                      mkDerivation                                                 
                        = drv: sup.mkDerivation (drv //                            
                             { jailbreak = true; doHaddock = false;});             
                     };                                                            
                                                                                   
    in super.haskell.packages.ghcHEAD.extend                                       
         (self.lib.composeExtensions localOverrides ghcPackageOverrides); 
}