title: How do I reference / import a derivation from a file in Nix?
body: <p>I'm trying to use this project: <a href="https://github.com/mpickering/old-ghc-nix" rel="nofollow noreferrer">https://github.com/mpickering/old-ghc-nix</a></p>
  
  <p>I'm assuming I'm meant to 'import' the <code>ghc.nix</code> (which generates a derivation?):</p>
  
  <p><strong>shell.nix</strong>:</p>
  
  <pre><code>with (import &lt;nixpkgs&gt; {});
  # let ghc = nur.repos.mpickering.ghc.ghc841;
  let ghc = import /home/chris/temp/old-ghc-nix/ghc.nix;
  in haskell.lib.buildStackProject {
    inherit ghc;
    name = "myEnv";
    buildInputs = [ pcre ];
  }
  </code></pre>
  
  <p>However this gives me an error of:</p>
  
  <pre><code>  ...
    error: cannot coerce a function to a string, at /home/chris/fromLaptopt/usbflash/Haskell/CeriumYankeeAvocado/shell.nix:6:3
    ...
  </code></pre>
  
  <p>How do I "execute" the function / build the derivation? </p>
  
  <hr>
  
  <p><strong>ghc.nix</strong>:</p>
  
  <pre><code>{ stdenv
  , fetchurl, perl, gcc, llvm_39
  , ncurses5, gmp, glibc, libiconv
  }: { version, url, hash, ...}:
  
  # Prebuilt only does native
  assert stdenv.targetPlatform == stdenv.hostPlatform;
  
  let
    libPath = stdenv.lib.makeLibraryPath ([
      ncurses5 gmp
    ] ++ stdenv.lib.optional (stdenv.hostPlatform.isDarwin) libiconv);
  
    libEnvVar = stdenv.lib.optionalString stdenv.hostPlatform.isDarwin "DY"
      + "LD_LIBRARY_PATH";
  
    glibcDynLinker = assert stdenv.isLinux;
      if stdenv.hostPlatform.libc == "glibc" then
         # Could be stdenv.cc.bintools.dynamicLinker, keeping as-is to avoid rebuild.
         ''"$(cat $NIX_CC/nix-support/dynamic-linker)"''
      else
        "${stdenv.lib.getLib glibc}/lib/ld-linux*";
  
  in
  
  stdenv.mkDerivation rec {
  
    name = "ghc-${version}-binary";
  
    src = fetchurl {
        url = url;
        sha256 = hash;
      };
  
    nativeBuildInputs = [ perl ];
    buildInputs = stdenv.lib.optionals (stdenv.targetPlatform.isAarch32 || stdenv.targetPlatform.isAarch64) [ llvm_39 ];
  
    # Cannot patchelf beforehand due to relative RPATHs that anticipate
    # the final install location/
    ${libEnvVar} = libPath;
  
    postUnpack =
      # GHC has dtrace probes, which causes ld to try to open /usr/lib/libdtrace.dylib
      # during linking
      stdenv.lib.optionalString stdenv.isDarwin ''
        export NIX_LDFLAGS+=" -no_dtrace_dof"
        # not enough room in the object files for the full path to libiconv :(
        for exe in $(find . -type f -executable); do
          isScript $exe &amp;&amp; continue
          ln -fs ${libiconv}/lib/libiconv.dylib $(dirname $exe)/libiconv.dylib
          install_name_tool -change /usr/lib/libiconv.2.dylib @executable_path/libiconv.dylib -change /usr/local/lib/gcc/6/libgcc_s.1.dylib ${gcc.cc.lib}/lib/libgcc_s.1.dylib $exe
        done
      '' +
  
      # Some scripts used during the build need to have their shebangs patched
      ''
        patchShebangs ghc-${version}/utils/
        patchShebangs ghc-${version}/configure
      '' +
  
      # Strip is harmful, see also below. It's important that this happens
      # first. The GHC Cabal build system makes use of strip by default and
      # has hardcoded paths to /usr/bin/strip in many places. We replace
      # those below, making them point to our dummy script.
      ''
        mkdir "$TMP/bin"
        for i in strip; do
          echo '#! ${stdenv.shell}' &gt; "$TMP/bin/$i"
          chmod +x "$TMP/bin/$i"
        done
        PATH="$TMP/bin:$PATH"
      '' +
      # We have to patch the GMP paths for the integer-gmp package.
      ''
        find . -name integer-gmp.buildinfo \
            -exec sed -i "s@extra-lib-dirs: @extra-lib-dirs: ${gmp.out}/lib@" {} \;
      '' + stdenv.lib.optionalString stdenv.isDarwin ''
        find . -name base.buildinfo \
            -exec sed -i "s@extra-lib-dirs: @extra-lib-dirs: ${libiconv}/lib@" {} \;
      '' +
      # Rename needed libraries and binaries, fix interpreter
      stdenv.lib.optionalString stdenv.isLinux ''
        find . -type f -perm -0100 -exec patchelf \
            --replace-needed libncurses${stdenv.lib.optionalString stdenv.is64bit "w"}.so.5 libncurses.so \
            --replace-needed libtinfo.so libtinfo.so.5 \
            --interpreter ${glibcDynLinker} {} \;
  
        sed -i "s|/usr/bin/perl|perl\x00        |" ghc-${version}/ghc/stage2/build/tmp/ghc-stage2
        sed -i "s|/usr/bin/gcc|gcc\x00        |" ghc-${version}/ghc/stage2/build/tmp/ghc-stage2
      '';
  
    configurePlatforms = [ ];
    configureFlags = [
      "--with-gmp-libraries=${stdenv.lib.getLib gmp}/lib"
      "--with-gmp-includes=${stdenv.lib.getDev gmp}/include"
    ] ++ stdenv.lib.optional stdenv.isDarwin "--with-gcc=${./gcc-clang-wrapper.sh}"
      ++ stdenv.lib.optional stdenv.hostPlatform.isMusl "--disable-ld-override";
  
    # Stripping combined with patchelf breaks the executables (they die
    # with a segfault or the kernel even refuses the execve). (NIXPKGS-85)
    dontStrip = true;
  
    # No building is necessary, but calling make without flags ironically
    # calls install-strip ...
    dontBuild = true;
  
    # On Linux, use patchelf to modify the executables so that they can
    # find editline/gmp.
    preFixup = stdenv.lib.optionalString stdenv.isLinux ''
      for p in $(find "$out" -type f -executable); do
        if isELF "$p"; then
          echo "Patchelfing $p"
          patchelf --set-rpath "${libPath}:$(patchelf --print-rpath $p)" $p
        fi
      done
    '' + stdenv.lib.optionalString stdenv.isDarwin ''
      # not enough room in the object files for the full path to libiconv :(
      for exe in $(find "$out" -type f -executable); do
        isScript $exe &amp;&amp; continue
        ln -fs ${libiconv}/lib/libiconv.dylib $(dirname $exe)/libiconv.dylib
        install_name_tool -change /usr/lib/libiconv.2.dylib @executable_path/libiconv.dylib -change /usr/local/lib/gcc/6/libgcc_s.1.dylib ${gcc.cc.lib}/lib/libgcc_s.1.dylib $exe
      done
  
      for file in $(find "$out" -name setup-config); do
        substituteInPlace $file --replace /usr/bin/ranlib "$(type -P ranlib)"
      done
    '';
  
    doInstallCheck = true;
    installCheckPhase = ''
      unset ${libEnvVar}
      # Sanity check, can ghc create executables?
      cd $TMP
      mkdir test-ghc; cd test-ghc
      cat &gt; main.hs &lt;&lt; EOF
        {-# LANGUAGE TemplateHaskell #-}
        module Main where
        main = putStrLn \$([|"yes"|])
      EOF
      $out/bin/ghc --make main.hs || exit 1
      echo compilation ok
      [ $(./main) == "yes" ]
    '';
  
    passthru = {
      targetPrefix = "";
      enableShared = true;
    };
  
    meta.license = stdenv.lib.licenses.bsd3;
    meta.platforms = ["x86_64-linux" "i686-linux" "x86_64-darwin"];
  }
  </code></pre>