@Scott Turner This actually works surprisingly well with Either showError :: (Result `Either` Error3 `Either` Error) -> String showError = `mapSum3` (result -> "no error" ,error3 -> "error3: " ++ show error3 ,error3 -> "error8: " ++ show error8 ) mapSum3 :: (x `Either` y `Either` z) -> (x->a,y->a,z->a) -> a But sum tuples would look cooler still :)
I'm a little puzzled by all of this, having learned ML before there was such a thing as Standard ML. Anonymous sums, with inl, inr, outl, outr and so on are one thing I was very happy to see the back of. This sudden enthusiasm for them strikes me as being like a sudden enthusiasm for punched cards, paper tape, or PL/I.
Either (and to some lesser extent Maybe) are an anonymous sum type and it is used all the time. I'm just saying it does not have the ideal syntax. P.S. The (a||) really conflicts with operators but (a;;) might actually still be possible.