SML Mutable Ref
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fun get_value_at_reference(reference : 'a ref) : 'a = !reference fun set_value_at_reference(reference : 'a ref, next_value : 'a) : 'a = let val previous_value = !reference val _ = reference := next_value in previous_value end (* alternate version fun set_value_at_reference(reference : 'a ref, next_value : 'a) : 'a = let val previous_value = !reference in ( reference := next_value ; previous_value ) end *)