// file:    pairs.rex
// author:  Robert Keller
// purpose: defining function 'pairs' which creates the list of all pairs from
//          two lists


// pairs(L, M), where L and M are lists, produces the list of all pairs,
// the first element of the pair being from L and the second from M

pairs(L, M) = mappend((X) => map((Y) => [X, Y], M), L);

// How it works: Consider X to be a typical element of the first list L.
// Then map((Y) => [X, Y], M)) creates a list of pairs, pairing each
// element Y of M with X.  The overall RHS then does this for each element
// of L and appends the resulting lists together into a single list.

test(pairs(["a", "b", "c", "d"], [1, 2, 3]), 

	   [["a", 1], ["a", 2], ["a", 3], 
	    ["b", 1], ["b", 2], ["b", 3], 
	    ["c", 1], ["c", 2], ["c", 3], 
	    ["d", 1], ["d", 2], ["d", 3]]);