feat(users/Profpatsch): add importDhall r/3876

Makes it possible to import a dhall file as a nix expression (at IfD
time), embedding dhall into nix.

There’s some setup for adding dhall dependencies as well, but it
hasn’t been really battle-tested yet.

Change-Id: I3e5670f93c612f2eb530d7c65d6bb4b1bf7bd8bd
Reviewed-on: https://cl.tvl.fyi/c/depot/+/5333
Reviewed-by: Profpatsch <mail@profpatsch.de>
Tested-by: BuildkiteCI

1 files changed, 78 insertions, 0 deletions

diff --git a/users/Profpatsch/importDhall.nix b/users/Profpatsch/importDhall.nix
new file mode 100644
index 0000000000..9713b2cfb8
--- /dev/null
+++ b/users/Profpatsch/importDhall.nix
@@ -0,0 +1,78 @@
+{ pkgs, depot, lib, ... }:
+let
+
+  # import the dhall file as nix expression via dhall-nix.
+  # Converts the normalized dhall expression to a nix file,
+  # puts it in the store and imports it.
+  # Types are erased, functions are converted to nix functions,
+  # unions values are nix functions that take a record of match
+  # functions for their alternatives.
+  # TODO: document better
+  importDhall =
+    {
+      # Root path of the dhall file tree to import (will be filtered by files)
+      root
+    , # A list of files which should be taken from `root` (relative paths).
+      # This is for minimizing the amount of things that have to be copied to the store.
+      # TODO: can you have directory prefixes?
+      files
+    , # The path of the dhall file which should be evaluated, relative to `root`, has to be in `files`
+      main
+    , # List of dependencies (TODO: what is a dependency?)
+      deps
+    , # dhall type of `main`, or `null` if anything should be possible.
+      type ? null
+    }:
+    let
+      src =
+        depot.users.Profpatsch.exactSource
+          root
+          # exactSource wants nix paths, but I think relative paths
+          # as strings are more intuitive.
+          (
+            let abs = path: toString root + "/" + path;
+            in ([ (abs main) ] ++ (map abs files))
+          );
+
+      cache = ".cache";
+      cacheDhall = "${cache}/dhall";
+
+      typeAnnot = if type == null then "" else ": ${type}";
+
+      convert = pkgs.runCommandLocal "dhall-to-nix" { inherit deps; } ''
+        mkdir -p ${cacheDhall}
+        for dep in $deps; do
+          ${pkgs.xorg.lndir}/bin/lndir -silent $dep/${cacheDhall} ${cacheDhall}
+        done
+
+        export XDG_CACHE_HOME=$(pwd)/${cache}
+        # go into the source directory, so that the type can import files.
+        # TODO: This is a bit of a hack hrm.
+        cd "${src}"
+        printf '%s' ${lib.escapeShellArg "${src}/${main} ${typeAnnot}"} \
+          | ${pkgs.dhall-nix}/bin/dhall-to-nix \
+          > $out
+      '';
+    in
+    import convert;
+
+
+  # read dhall file in as JSON, then import as nix expression.
+  # The dhall file must not try to import from non-local URLs!
+  readDhallFileAsJson = dhallType: file:
+    let
+      convert = pkgs.runCommandLocal "dhall-to-json" { } ''
+        printf '%s' ${lib.escapeShellArg "${file} : ${dhallType}"} \
+          | ${pkgs.dhall-json}/bin/dhall-to-json \
+          > $out
+      '';
+    in
+    builtins.fromJSON (builtins.readFile convert);
+
+in
+{
+  inherit
+    importDhall
+    readDhallFileAsJson
+    ;
+}