demonstrate how to fetch URL imports in Dhall by turning the integrity checks into Nix fixed-output-derivations

fetch-dhall-as-binary.nix

with import <nixpkgs> {};

let
  packageSet = fetchurl {
    url = "https://github.com/purescript/package-sets/releases/download/psc-0.14.4-20210905/packages.dhall";
    # The hash below is a base64-encoded version of the base-16 hash that is
    # specified in our local packages.dhall file:
    # ```
    # $ cat packages.dhall
    # let upstream =
    #       https://github.com/purescript/package-sets/releases/download/psc-0.14.4-20210905/packages.dhall sha256:140f3630801f2b02d5f3a405d4872e0af317e4ef187016a6b00f97d59d6275c6
    # in  upstream
    # ```
    #
    # So sha256-FA82MIAfKwLV86QF1IcuCvMX5O8YcBamsA+X1Z1idcY= is the base64-encoded version of
    # 140f3630801f2b02d5f3a405d4872e0af317e4ef187016a6b00f97d59d6275c6 (which is base16-encoded).
    # This proves that we can turn URL references in dhall into a FOD in
    # Nix (as long as the URL references has an integrity check).
    hash = "sha256-FA82MIAfKwLV86QF1IcuCvMX5O8YcBamsA+X1Z1idcY=";
    downloadToTemp = true;
    postFetch = ''
      ${dhall}/bin/dhall encode --file "$downloadedFile" > $out
    '';
  };

  decodedPackageSet = runCommand "decodedPackageSet" {} ''
    ${dhall}/bin/dhall decode --file "${packageSet}" > $out
  '';
 
in

dhallToNix (builtins.readFile decodedPackageSet)

This Nix file demonstrates how Dhall URLs with integrity checks can be turned into Nix fixed-output derivations (FODs).

This Nix file can be evaluated like the following:

$ nix eval --file ./fetch-dhall-as-binary.nix
{ abides = { dependencies = [ "enums" "foldable-traversable" ]; repo = "https://github.com/athanclark/purescript-abides.git"; version = "v0.0.1"; }; ace = { dependencies = [ "arrays" "effect" "foreign" "nullable" "prelude" "web-html" "web-uievents" ]; repo = "https://github.com/purescript-contrib/purescript-ace.git"; version = "v8.0.0"; }; aff = { dependencies = [ "datetime" "effect" "exceptions" "functions" "parallel" "transformers" "unsafe-coerce" ]; repo = "https://github.com/purescript-contrib/purescript-aff.git"; version = "v6.0.0"; }; ...

<nixpkgs> is currently pointing to 21.05.2549.a1007637cea.