lix/src/libexpr/fetchurl.nix

{
  system ? "", # obsolete
  url,
  hash ? "", # an SRI hash

  # Legacy hash specification
  md5 ? "",
  sha1 ? "",
  sha256 ? "",
  sha512 ? "",
  outputHash ?
    if hash != "" then
      hash
    else if sha512 != "" then
      sha512
    else if sha1 != "" then
      sha1
    else if md5 != "" then
      md5
    else
      sha256,
  outputHashAlgo ?
    if hash != "" then
      ""
    else if sha512 != "" then
      "sha512"
    else if sha1 != "" then
      "sha1"
    else if md5 != "" then
      "md5"
    else
      "sha256",

  executable ? false,
  unpack ? false,
  name ? baseNameOf (toString url),
  impure ? false,
}:

derivation (
  {
    builder = "builtin:fetchurl";

    # New-style output content requirements.
    outputHashMode = if unpack || executable then "recursive" else "flat";

    inherit
      name
      url
      executable
      unpack
      ;

    system = "builtin";

    # No need to double the amount of network traffic
    preferLocalBuild = true;

    impureEnvVars = [
      # We borrow these environment variables from the caller to allow
      # easy proxy configuration.  This is impure, but a fixed-output
      # derivation like fetchurl is allowed to do so since its result is
      # by definition pure.
      "http_proxy"
      "https_proxy"
      "ftp_proxy"
      "all_proxy"
      "no_proxy"
    ];

    # To make "nix-prefetch-url" work.
    urls = [ url ];
  }
  // (if impure then { __impure = true; } else { inherit outputHashAlgo outputHash; })
)