src/option_key_fn.sml


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functor OptionKeyFn(K : ORD_KEY) : ORD_KEY = struct

type ord_key = K.ord_key option

val compare =
 fn (NONE, NONE) => EQUAL
  | (NONE, _) => LESS
  | (_, NONE) => GREATER
  | (SOME x, SOME y) => K.compare (x, y)

end