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doc: Add explanation of --fixed-output-derivations arg for dhall-nixpkgs to Dhall section

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(cdep)illabout 2021-10-29 17:17:19 +09:00
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doc/languages-frameworks/dhall.section.md
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@ -342,6 +342,37 @@ $ dhall-to-nixpkgs directory ~/proj/dhall-semver
}
```
### Remote imports as fixed-output derivations {#ssec-dhall-remote-imports-as-fod}
`dhall-to-nixpkgs` has the ability to fetch and build remote imports as
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:
```bash
$ dhall-to-nixpkgs directory --fixed-output-derivations ~/proj/dhall-semver
{ buildDhallDirectoryPackage, buildDhallUrl }:
buildDhallDirectoryPackage {
name = "proj";
src = /Users/gabriel/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=";
dhall-hash = "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