One way to get Lenses to construct non-POD containers is to specify a lens.Step object as a step (i.e. key) when constructing a Lens. Construction of a lens.Step requires three Functions:
- A function to retrieve the Maybe value on the "down" side of the step: it receives the container and must return
{}if the value is not present or{just: value}if the value is present (as described in The Maybe Monad). - A function to create and return a clone of the container (first argument) with a specified change (second argument) given as a Maybe, either:
{just: newValue}indicating anewValueto give this slot in the clone, or{}indicating that the slot should not be present in the clone.
- A function to construct the container in an empty state (no arguments given)
While passing null for one or more of these won't cause an error immediately, it will cause errors if the Lens is used for for certain operations:
lens.Step Constructor Argument |
Missing Function | Failing Operations |
|---|---|---|
| first | get_maybe |
Retrieving or transforming any value, which also prevents any additional steps in the Lens |
| second | updatedClone |
Setting any item (even in a container that doesn't exist) |
| third | construct |
Setting an item in a container that doesn't exist in the input; xformInClone with addMissing and xformInClone_maybe if the container is missing from the subject |