Meta Language, aka Meta Language, is an actively used programming language created in 1973.

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Example code from Wikipedia:

structure Rational : ARITH =
struct
        datatype t = Rat of int * int;
        val zero = Rat(0,1);
        fun succ(Rat(a,b)) = Rat( a+b , b  );
        fun sum (Rat(a,b),  Rat(c,d)) = Rat(a*d+ c*b  , b*d) : t ;
end
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Last updated February 11th, 2019