Add fetch.py, for auto-updating to the newest version

A small python script, which nix-prefetch-url’s the latest
dhall-haskell release and puts it in a json file.

fetch.py

@@ -0,0 +1,47 @@
+#!/usr/bin/env nix-shell
+#!nix-shell -i python3 -p python3 curl nix
+
+import json
+import re
+import subprocess as sub
+import sys
+
+# fetch the release info from releases/latest
+def curl_latest_release():
+    url = "https://api.github.com/repos/dhall-lang/dhall-haskell/releases/latest"
+    return json.loads(sub.check_output(["curl", url]))
+
+# call nix-prefetch-url on each asset to get their hashes
+def prefetch_binaries(release):
+    res = []
+    for a in release['assets']:
+        if "linux" in a['name'] or "macos" in a['name']:
+            print(a['name'], file=sys.stderr)
+            hash = sub.check_output(["nix-prefetch-url", a['browser_download_url']]).strip().decode()
+            res += [{
+                'name': a['name'],
+                'url': a['browser_download_url'],
+                'hash': hash
+            }]
+    return res
+
+# convert the list of binaries to an object we can address
+def postprocess(fetched):
+    obj = {}
+    for i in fetched:
+        # split on the first digit ("dhall-foo-bar-1.2.3")
+        name = re.split(r'-\d', i['name'])[0]
+        post = "-linux" if "linux" in i['name'] else "-darwin"
+        obj[name + post] = i
+    return(obj)
+
+
+if __name__ == "__main__":
+    release = curl_latest_release()
+    version = release['tag_name']
+    print("updating to release {}".format(version), file=sys.stderr)
+    fetched = prefetch_binaries(release)
+    res = postprocess(fetched)
+    print("writing ./release.json", file=sys.stderr)
+    with open("./release.json", mode='w') as f:
+        json.dump(res, f, indent=2)