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> In each case return is singleton (it is not known if there exists a monad on lists with a different return). The "diagonal" monad join [[a, b, c, ...], [1, 2, 3, ...], [x, y, z, ...], ...] = [a, 2, z, ...] has return a = [a, a, a, ...], though I guess it's hard to define it in a way that's well-behaved for finite lists, so you might not consider it "a monad on lists". |