Fix unexpected behavior of Lens.withDefault #18
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This tracks #17
The issue is that
Lens.withDefaultwith unexpectedly use anundefinedvalue in place of a value passed into aLens.setwhen it is equal to the default value provided.It makes sense, as it is more lawful (in lens laws terms) to remove a value when set to default. Although technically, the current implementation is not lawful either, because
{ a: 5, b: undefined }is not strictly equal to{ a: 5 }.I see these options:
Lens.withDefaultto actually modify the data structure even when set to a default value.Lens.withDefaultas it is and implement a separate lens with this new behavior.Bonus option: if we decide to keep current
Lens.withDefault, fix it to be truly lawful.