ekmett/lens — pull request #487 by neongreen (2014-10-07 13:51:23+00:00)
Title: Rewrite the “usage” example for the greater good

Since this came up on #haskell today:

(03:35:07 PM) danilo2: Hello! I just dioscovered that lenses are PURE
EVIL. I spent hours to track a bug caused by how lenses are stupid. I
would love to ask you If uis there **any way** to fix the following
behaviour. I've got a simple code: http://lpaste.net/112230 After
refactoring I changed some names and everythiong compiled and worked even
if the name was not found in the datatype. Instead of reporting error,
lenses used the monoid to insert empty v

(03:35:31 PM) danilo2: Is there any way, to make lenses throwing error
when field name is not matched instead of just hiding this from me and
using monoid there?