I agree completely that this example is not fully type safe---there is an implicit, partial schema where either Python exceptions or Haskell Nothings imply vali

I agree completely that this example is not fully type safe---there is an implicit, partial schema where either Python exceptions or Haskell Nothings imply validation failure, but that's beside the point. The example written has a lot of type resolution done implicitly by the typeclass mechanism. It ensures that only valid lens-like combinators are applied to the traversal chain... which is only an issue due to how far we can flex the lens library to do other things. A better example is the somewhat more explicit version of this code it ^? _JSON . _Array . nth 0 . _Object . key "someObject" . _Object . key "version" . _Array . nth 1 Where the _JSON, _Array, nth, _Object, and key declarations must match otherwise its a type error. Furthermore, the `nth` and `key` calls are both just type-restricted aliases for `ix` which help to convey via types what kinds of very general tools should be applied here. We could also, for instance, throw another _JSON Prism into the chain to do a nested, schema-based validation for any types which have ToJSON/FromJSON instances.