I've used a similar construct in dealing with some JSON data recently, using Either String a: (?) :: Maybe a -> String -> Either String a Nothing ? s =

I've used a similar construct in dealing with some JSON data recently, using Either String a: (?) :: Maybe a -> String -> Either String a Nothing ? s = Left s Just a ? _ = Right a foo :: Value -> Either String Int foo val = do nested <- val ^? key "foo" . key "bar" ? "Could not find foo.bar" mid <- nested ^? nth 2 . _Integral ? "Second element of foo.bar was not an Integral value" return mid which has really helped deal with the stringly typed nature of JSON. I should also mention I stole the idea (and operator name) from one of the Haskell parser combinator libraries which allowed you to name a whole parser expression, so errors would say "could not find ident" instead of "Expected on of a,b,c,d,e,d,f,g,h...., found X"