%ifdef environment
# Get the value of an environment variable.
# Returns `default` if the variable is not set.
# @category System
def environment.get(~default="", s) =
  env = environment()
  if
    not list.assoc.mem(s, env)
  then
    default
  else
    list.assoc(default=default, s, env)
  end
end
%endif

# Split the arguments of an url of the form `arg=bar&arg2=bar2` into
# `[("arg","bar"),("arg2","bar2")]`. The returned strings are decoded (see
# `url.decode`).
# @category String
# @param args Argument string to split.
def url.split_args(args) =
  def f(x) =
    ret = r/=/.split(x)
    arg = url.decode(list.nth(default="", ret, 0))
    val = url.decode(list.nth(default="", ret, 1))
    (arg, val)
  end

  l = r/&/.split(args)
  list.map(f, l)
end

# Split an url of the form `foo?arg=bar&arg2=bar2` into
# `("foo",[("arg","bar"),("arg2","bar2")])`. The returned strings are decoded
# (see `url.decode`).
# @category String
# @param uri Url to split.
def url.split(uri) =
  ret = r/(?<uri>[^\?]*)\?(?<args>.*)/.exec(uri).groups
  args = ret["args"]
  if
    args != ""
  then
    (url.decode(ret["uri"]), url.split_args(args))
  else
    (url.decode(uri), [])
  end
end

# Memoize the result of a function, making sure it is only executed once.
# @category Programming
def memoize(fn) =
  cached_result = ref([])
  fun () ->
    begin
      if
        cached_result() != []
      then
        list.hd(cached_result())
      else
        result = fn()
        cached_result := [result]
        result
      end
    end
end