{ lib, stdenv, fetchurl, fetchzip }@args:

let
  lib = import ../extra-lib.nix {
    inherit (args) lib;
  };

  inherit (lib)
    attrNames
    fakeSha256
    filter
    findFirst
    head
    isAttrs
    isPath
    isString
    last
    length
    optionalAttrs
    pathExists
    pred
    sort
    switch
    switch-if
    versionAtLeast
    versions
    ;

  inherit (lib.strings) match split;

  default-fetcher = {domain ? "github.com", owner ? "", repo, rev, name ? "source", sha256 ? null, artifact ? null, ...}@args:
    let kind = switch-if [
          { cond = artifact != null; out = {ext = "tbz"; fmt = "tbz"; fetchfun = fetchurl; }; }
          { cond = args?sha256 ; out = {ext = "zip"; fmt = "zip"; fetchfun = fetchzip; }; }
        ] {ext = "tar.gz"; fmt = "tarball"; fetchfun = builtins.fetchTarball; }; in
    with kind; let
        pr  = match "^#(.*)$" rev;
        url = switch-if [
          { cond = pr == null && (match "^github.*" domain) != null && artifact != null;
            out = "https://github.com/${owner}/${repo}/releases/download/${rev}/${artifact}"; }
          { cond = pr == null && (match "^github.*" domain) != null;
            out = "https://${domain}/${owner}/${repo}/archive/${rev}.${ext}"; }
          { cond = pr != null && (match "^github.*" domain) != null;
            out = "https://api.${domain}/repos/${owner}/${repo}/${fmt}/pull/${head pr}/head"; }
          { cond = pr == null && (match "^gitlab.*" domain) != null;
            out = "https://${domain}/${owner}/${repo}/-/archive/${rev}/${repo}-${rev}.${ext}"; }
          { cond = (match "(www.)?mpi-sws.org" domain) != null;
            out = "https://www.mpi-sws.org/~${owner}/${repo}/download/${repo}-${rev}.${ext}";}
        ] (throw "meta-fetch: no fetcher found for domain ${domain} on ${rev}");
        fetch = x: fetchfun (if args?sha256 then (x // { inherit sha256; }) else x);
    in fetch { inherit url ; };
in
{
  fetcher ? default-fetcher,
  location,
  release ? {},
  releaseRev ? (v: v),
  releaseArtifact ? (v: null)
}:
let
    isVersion      = x: isString x && match "^/.*" x == null && release?${x};
    shortVersion   = x: if (isString x && match "^/.*" x == null)
      then findFirst (v: versions.majorMinor v == x) null
        (sort versionAtLeast (attrNames release))
      else null;
    isShortVersion = x: shortVersion x != null;
    isPathString   = x: isString x && match "^/.*" x != null && pathExists x; in
arg:
switch arg [
  { case = isNull;       out = { version = "broken"; src = ""; broken = true; }; }
  { case = isPathString; out = { version = "dev"; src = arg; }; }
  { case = pred.union isVersion isShortVersion;
    out = let
      v = if isVersion arg then arg else shortVersion arg;
      given-sha256 = release.${v}.sha256 or "";
      sha256 = if given-sha256 == "" then fakeSha256 else given-sha256;
      rv = release.${v} // { inherit sha256; };
    in
      {
        version = rv.version or v;
        src = rv.src or fetcher (location // { rev = releaseRev v; artifact = releaseArtifact v; } // rv);
      };
    }
  { case = isString;
    out = let
        splitted  = filter isString (split ":" arg);
        rev       = last splitted;
        has-owner = length splitted > 1;
        version   = "dev"; in {
      inherit version;
      src = fetcher (location // { inherit rev; } //
        (optionalAttrs has-owner { owner = head splitted; }));
    }; }
  { case = isAttrs;
    out = {
      version = arg.version or "dev";
      src = (arg.fetcher or fetcher) (location // (arg.location or {})); }; }
  { case = isPath;
    out = {
      version = "dev" ;
      src = builtins.path {path = arg; name = location.name or "source";}; }; }
] (throw "not a valid source description")