The type for keys whose lookup value is of type 'a.

The type for heterogeneous value maps.

empty is the empty map.

is_empty m is true iff m is empty.

mem k m is true iff k is bound in m.

add k v m is m with k bound to v.

singleton k v is add k v empty.

rem k m is m with k unbound.

find k m is the value of k's binding in m, if any.

find_exn k m is the value of k's binding find_exn m.

• raises Invalid_argument

  if k is not bound in m.

sexp_of_t t converts the request t to an s-expression

pp_hum formats the request t as a standard HTTP request