On Tue, Sep 10, 2013 at 1:31 AM, Charlie Paul
I've been looking through Edward Kmett's lens library, and I'm a bit befuddled about Getters. In my own code, why would I want to have something be a Getter instead of a plain function? As far as I can see, a plain function is simpler to use, and can be converted to a Getter with "to" if we want to use it as a Fold. Is there a situation where writing a Getter is superior than writing a function, then lifting it as needed?
As I understand it, you'd be better off declaring a function and then
using "to".
Getters are a midpoint between a Lens and a Fold. With a lens, you can read
and write a single value in a larger structure. With a fold, you can read a
sequence of values in a larger structure.
The need for getters comes from to. "myLens . to f" cannot be a lens,
because modifying the value it points to would require an inverse for "f",
but if making it a fold would lose the property that it points to exactly
one value. So a getter lets you read a single value in a larger structure.
That being said, "view (myLens . to f)" is isomorphic to "f . view myLens".
--
Dave Menendez