maintainers/scripts/haskell/regenerate-hackage-packages.sh: init

Introduces a script that can be used to update the Nix expressions for

the Haskell package set. In service of that, also

- introduces cabal2nix-latest, which pins the hackage2nix version used

- changes all-cabal-hashes to use fetchFromGitHub

- adds update-hackage.sh & update-cabal2nix-latest.sh & update-stackage.sh maintainer scripts

maintainers/scripts/haskell/update-hackage.sh

@@ -0,0 +1,21 @@
+#! /usr/bin/env nix-shell
+#! nix-shell -i bash -p nix curl jq nix-prefetch-github -I nixpkgs=.
+
+# See regenerate-hackage-packages.sh for details on the purpose of this script.
+
+set -euo pipefail
+
+pin_file=pkgs/data/misc/hackage/pin.json
+current_commit="$(jq -r .commit $pin_file)"
+head_commit="$(curl -H "Accept: application/vnd.github.v3+json" https://api.github.com/repos/commercialhaskell/all-cabal-hashes/branches/hackage | jq -r .commit.sha)"
+
+if [ "$current_commit" != "$head_commit" ]; then
+   url="https://github.com/commercialhaskell/all-cabal-hashes/archive/$head_commit.tar.gz"
+   hash="$(nix-prefetch-url "$url")"
+   jq -n \
+     --arg commit "$head_commit" \
+     --arg hash "$hash" \
+     --arg url "$url" \
+     '{commit: $commit, url: $url, sha256: $hash}' \
+     > $pin_file
+fi