fixed-output derivations by using their Dhall integrity check. This is
sometimes easier than manually packaging all remote imports.

This can be used like the following:

```ShellSession
$ dhall-to-nixpkgs directory --fixed-output-derivations ~/proj/dhall-semver
{ buildDhallDirectoryPackage, buildDhallUrl }:
  buildDhallDirectoryPackage {
    name = "proj";
    src = ~/proj/dhall-semver;
    file = "package.dhall";
    source = false;
    document = false;
    dependencies = [
      (buildDhallUrl {
        url = "https://prelude.dhall-lang.org/v17.0.0/package.dhall";
        hash = "sha256-ENs8kZwl6QRoM9+Jeo/+JwHcOQ+giT2VjDQwUkvlpD4=";
        dhallHash = "sha256:10db3c919c25e9046833df897a8ffe2701dc390fa0893d958c3430524be5a43e";
        })
      ];
    }
```

Here, `dhall-semver`'s `Prelude` dependency is fetched and built with the
`buildDhallUrl` helper function, instead of being passed in as a function
argument.

## Overriding dependency versions {#ssec-dhall-overriding-dependency-versions}

Suppose that we change our `true.dhall` example expression to depend on an older
version of the Prelude (19.0.0):

```dhall
-- ./true.dhall

let Prelude =
      https://prelude.dhall-lang.org/v19.0.0/package.dhall
        sha256:eb693342eb769f782174157eba9b5924cf8ac6793897fc36a31ccbd6f56dafe2

in  Prelude.Bool.not False
```

If we try to rebuild that expression the build will fail:

```ShellSession
$ nix build --file ./example.nix dhallPackages.true
builder for '/nix/store/0f1hla7ff1wiaqyk1r2ky4wnhnw114fi-true.drv' failed with exit code 1; last 10 log lines:

  Dhall was compiled without the 'with-http' flag.

  The requested URL was: https://prelude.dhall-lang.org/v19.0.0/package.dhall


  4│       https://prelude.dhall-lang.org/v19.0.0/package.dhall
  5│         sha256:eb693342eb769f782174157eba9b5924cf8ac6793897fc36a31ccbd6f56dafe2

  /nix/store/rsab4y99h14912h4zplqx2iizr5n4rc2-true.dhall:4:7
[1 built (1 failed), 0.0 MiB DL]
error: build of '/nix/store/0f1hla7ff1wiaqyk1r2ky4wnhnw114fi-true.drv' failed
```

… because the default Prelude selected by Nixpkgs revision
`94b2848559b12a8ed1fe433084686b2a81123c99` is version 20.1.0, which doesn't
have the same integrity check as version 19.0.0. This means that version
19.0.0 is not cached, and the interpreter is not allowed to fall back to
importing the URL.

However, we can override the default Prelude version by using `dhall-to-nixpkgs`
to create a Dhall package for our desired Prelude:

```ShellSession
$ dhall-to-nixpkgs github https://github.com/dhall-lang/dhall-lang.git \
    --name Prelude \
    --directory Prelude \
    --rev v19.0.0 \
    > Prelude.nix
```

… and then referencing that package in our Dhall overlay, by either overriding
the Prelude globally for all packages, like this:

```nix
{
  dhallOverrides = self: super: {
    true = self.callPackage ./true.nix { };

    Prelude = self.callPackage ./Prelude.nix { };
  };
}
```

… or selectively overriding the Prelude dependency for just the `true` package,
like this:

```nix
{
  dhallOverrides = self: super: {
    true = self.callPackage ./true.nix {
      Prelude = self.callPackage ./Prelude.nix { };
    };
  };
}
```

## Overrides {#ssec-dhall-overrides}

You can override any of the arguments to `buildDhallGitHubPackage` or
`buildDhallDirectoryPackage` using the `overridePackage` attribute of a package.
For example, suppose we wanted to selectively enable `source = true` just for the Prelude.  We can do that like this:

```nix
{
  dhallOverrides = self: super: {
    Prelude = super.Prelude.overridePackage { source = true; };

    # ...
  };
}
```