1 files changed, 83 insertions, 0 deletions
diff --git a/pkgs/development/interpreters/dhall/build-dhall-package.nix b/pkgs/development/interpreters/dhall/build-dhall-package.nix
new file mode 100644
index 00000000000..739dc9b3d52
--- /dev/null
+++ b/pkgs/development/interpreters/dhall/build-dhall-package.nix
@@ -0,0 +1,83 @@
+{ haskell, haskellPackages, lib, lndir, runCommand, writeText }:
+
+{ name
+
+  # Expressions to add to the cache before interpreting the code
+, dependencies ? []
+
+  # A Dhall expression
+  #
+  # Carefully note that the following expression must be devoid of uncached HTTP
+  # imports.  This is because the expression will be evaluated using an
+  # interpreter with HTTP support disabled, so all HTTP imports have to be
+  # protected by an integrity check that can be satisfied via cached
+  # dependencies.
+  #
+  # You can add a dependency to the cache using the preceding `dependencies`
+  # option
+, code
+
+  # `buildDhallPackage` can include both a "source distribution" in
+  # `source.dhall` and a "binary distribution" in `binary.dhall`:
+  #
+  # * `source.dhall` is a dependency-free αβ-normalized Dhall expression
+  #
+  # * `binary.dhall` is an expression of the form: `missing sha256:${HASH}`
+  #
+  #   This expression requires you to install the cache product located at
+  #   `.cache/dhall/1220${HASH}` to successfully resolve
+  #
+  # By default, `buildDhallPackage` only includes "binary.dhall" to conserve
+  # space within the Nix store, but if you set the following `source` option to
+  # `true` then the package will also include `source.dhall`.
+, source ? false
+}:
+
+let
+  # `buildDhallPackage` requires version 1.25.0 or newer of the Haskell
+  # interpreter for Dhall.  Given that the default version is 1.24.0 we choose
+  # the latest available version instead until the default is upgraded.
+  #
+  # HTTP support is disabled in order to force that HTTP dependencies are built
+  # using Nix instead of using Dhall's support for HTTP imports.
+  dhall =
+    haskell.lib.justStaticExecutables
+      (haskell.lib.appendConfigureFlag
+        haskellPackages.dhall_1_29_0
+        "-f-with-http"
+      );
+
+  file = writeText "${name}.dhall" code;
+
+  cache = ".cache";
+
+  cacheDhall = "${cache}/dhall";
+
+  sourceFile = "source.dhall";
+
+in
+  runCommand name { inherit dependencies; } ''
+    set -eu
+
+    mkdir -p ${cacheDhall}
+
+    for dependency in $dependencies; do
+      ${lndir}/bin/lndir -silent $dependency/${cacheDhall} ${cacheDhall}
+    done
+
+    export XDG_CACHE_HOME=$PWD/${cache}
+
+    mkdir -p $out/${cacheDhall}
+
+    ${dhall}/bin/dhall --alpha --file '${file}' > $out/${sourceFile}
+
+    SHA_HASH=$(${dhall}/bin/dhall hash <<< $out/${sourceFile})
+
+    HASH_FILE="''${SHA_HASH/sha256:/1220}"
+
+    ${dhall}/bin/dhall encode --file $out/${sourceFile} > $out/${cacheDhall}/$HASH_FILE
+
+    echo "missing $SHA_HASH" > $out/binary.dhall
+
+    ${lib.optionalString (!source) "rm $out/${sourceFile}"}
+  ''