1 files changed, 23 insertions, 0 deletions

diff --git a/preinstall.js b/preinstall.js
new file mode 100644
index 0000000..1b859fc
--- /dev/null
+++ b/preinstall.js
@@ -0,0 +1,23 @@
+"use strict";
+
+const { https } = require("follow-redirects");
+const tar = require("tar");
+const unbz2 = require("unbzip2-stream");
+
+const pkg = require("./package.json");
+
+const trim = str => str && String(str).trim();
+
+const dhallVersion = trim(pkg["dhall-version"] || process.env.DHALL_VERSION);
+if (!dhallVersion) throw new Error("Missing DHALL_VERSION environment variable.");
+
+const dhallJsonVersion = trim(pkg["dhall-json-version"] || process.env.DHALL_JSON_VERSION);
+if (!dhallJsonVersion) throw new Error("Missing DHALL_JSON_VERSION environment variable.");
+
+const release = `https://github.com/dhall-lang/dhall-haskell/releases/download/${dhallVersion}/dhall-json-${dhallJsonVersion}`;
+
+const url = `${release}-x86_64-linux.tar.bz2`;
+https.get(url, res => {
+  if (res.statusCode >= 400) throw new Error(`Error fetching ${url}: ${res.statusMessage}`);
+  res.pipe(unbz2()).pipe(tar.x({ C: __dirname }));
+});