Elm: automate packaging with elm2nix

- rewrite documentation
- automate whole process into update.sh
- remove all legacy snippets

pkgs/development/compilers/elm/default.nix

@@ -2,54 +2,6 @@
 , haskell, nodejs
 , fetchurl, fetchpatch, makeWrapper, git }:
 
-# To update:
-
-# 1) Modify ./update.sh and run it
-
-# 2) to generate versions.dat:
-# 2.1) git clone https://github.com/elm/compiler.git
-# 2.2) cd compiler
-# 2.3) cabal2nix --shell . | sed 's/"default",/"ghc822",/' > shell.nix
-# 2.4) nix-shell
-# 2.5) mkdir .elm
-# 2.6) export ELM_HOME=$(pwd)/.elm
-# 2.7) cabal build
-# 2.8) cp .elm/0.19.0/package/versions.dat ...
-
-# 3) generate a template for elm-elm.nix with:
-# (
-#   echo "{";
-#   jq '.dependencies | .direct, .indirect | to_entries | .[] | { (.key) : { version : .value, sha256:  "" } } ' \
-#   < ui/browser/elm.json \
-#   | sed 's/:/ =/' \
-#   | sed 's/^[{}]//' \
-#   | sed -E 's/(["}]),?$/\1;/' \
-#   | sed -E 's/"(version|sha256)"/\1/' \
-#   | grep -v '^$';
-#   echo "}"
-# )
-#
-# ... then fill in the sha256s
-
-# Notes:
-
-# the elm binary embeds a piece of pre-compiled elm code, used by 'elm
-# reactor'. this means that the build process for 'elm' effectively
-# executes 'elm make'. that in turn expects to retrieve the elm
-# dependencies of that code (elm/core, etc.) from
-# package.elm-lang.org, as well as a cached bit of metadata
-# (versions.dat).
-
-# the makeDotElm function lets us retrieve these dependencies in the
-# standard nix way. we have to copy them in (rather than symlink) and
-# make them writable because the elm compiler writes other .dat files
-# alongside the source code. versions.dat was produced during an
-# impure build of this same code; the build complains that it can't
-# update this cache, but continues past that warning.
-
-# finally, we set ELM_HOME to point to these pre-fetched artifacts so
-# that the default of ~/.elm isn't used.
-
 let
   fetchElmDeps = import ./fetchElmDeps.nix { inherit stdenv lib fetchurl; };
   hsPkgs = haskell.packages.ghc822.override {
@@ -58,8 +10,8 @@ let
             elm = overrideCabal (self.callPackage ./packages/elm.nix { }) (drv: {
               # sadly with parallelism most of the time breaks compilation
               enableParallelBuilding = false;
-              preConfigure = fetchElmDeps {
-                elmPackages = (import ./packages/elm-elm.nix);
+              preConfigure = self.fetchElmDeps {
+                elmPackages = (import ./packages/elm-srcs.nix);
                 versionsDat = ./versions.dat;
               };
               buildTools = drv.buildTools or [] ++ [ makeWrapper ];
@@ -75,8 +27,6 @@ let
               '';
             });
 
-
-
             /*
             The elm-format expression is updated via a script in the https://github.com/avh4/elm-format repo:
             `pacakge/nix/build.sh`