The thing that's really undesirable about the clojure code is an unfortunate consequence of dynamic typing. Note that the bottom right corner of the expression

The thing that's really undesirable about the clojure code is an unfortunate consequence of dynamic typing. Note that the bottom right corner of the expression problem matrix isn't filled in. This means that newOp(newType) will fail at run time - you've simply replaced the difficult task of filling in the matrix with the easier task of partially filling it in and hoping for the best. Hope you've got a great test suite! I must say that I strongly prefer the Haskell/Scala/etc approach where that missing square is an explicit compile time warning/error.