Default Value Lens

[iamnoah]

Lens that calculates a default value if the value is undefined. Also can apply to whole objects. Setting a value sets the "real" value such that the default will no longer apply.

[iamnoah]

Lens Laws:

 * 1. You get back what you put in.
    l.get(l.set(o, v)) === v

Passes

 * 2. Putting back what you got doesn't change anything:        
    l.set(o, l.get(o)) === o

Violates this one. o will be changed as the value will now be set.

 * 3. Setting twice is the same as setting once:
    l.set(l.set(o, v) v2) === l.set(o, v2)

Passes.

[iclanzan]

Putting back what you got doesn't change anything
Violates this one. o will be changed as the value will now be set.

Could be solved by having a setter that sets the value to undefined if the default is being set, right?