Re: Deprecate Foldable for Either

On Fri, Mar 3, 2017 at 10:48 AM, Richard Eisenberg wrote:

On Mar 3, 2017, at 12:45 PM, Kris Nuttycombe wrote:

This is an interesting assertion; I don't believe that these goals work against one another at all. In fact, I find that (b) leads to "if it compiles, it works" much more often than not. John De Goes goes into depth on this principle here: http://degoes.net/articles/insufficiently- polymorphic where he points out "Monomorphic code is much more likely to be incorrect than polymorphic code, because for every type signature, there are many more possible implementations.".

Ah -- very good point. This is true for the *implementation* of a polymorphic function, where a polymorphic type signature beautifully restricts what the function can do. But it is not true for monomorphic *uses* of a polymorphic function, where the generality can lead to an unexpected instance selection and thus runtime behavior.

I guess that, in the limit, this is why my code has been progressively mutating to a style in which I only define data structures for the purpose of being able to easily generate classy lenses/prisms into their components/inhabitants. I now generally find concrete types more restrictive and error-prone than I care for.