Replacing Nothing values with Just values in a nested structure

This is what I expect will become the first of a series of posts where I detail some of the solutions I've arrived at, based on the test programs that I've written to work through the problem.

The situation I was running into was this:

I was writing tests for some parsing code. The output of the parse was a nested data structure. Some (well, many) parts of the data structure were optional. One in particular was giving me a hard time, though—it was a default being set from a dynamic value being passed into the parser. Obviously I had to get that same dynamic value into the test data as well, in order for them to match.

(I will certainly admit that there might be better ways to structure all this, but this is what I have for the moment.)

Anyway, at the time I run a test, I have the test case (which has both the inputs and the expected outputs), and I have the dynamic value, and I needed to insert that dynamic value into the expected outputs before comparing with the actual outputs.

Oh, and did I mention that there was a list of possible outputs, all of which needed to be massaged?

So, I created the following fake data structure that, nonetheless, mirrors the attributes of my test data structure that I care about.

Anyway, the lens library makes these sorts of traversal-with-replacement bits of code quite concise. In this example code, we use over to apply the function that is its second argument (which will either simply pass along an existing Just value, or return a constant value, and wrap either in another Just) to all of the entities that are described by its second argument—that is the containerItem contained in each entry in the collectionItems list.

There is a part of me that suspects that there's an even easier way to do this—it seems like there should be some way for me to narrow the traversal to only pick up items with the value of Nothing (using the _Nothing prism), and then simply setting those items to the default value. But the obvious way of constructing that (adding _Nothing to the traversal, and dropping the fromMaybe in favor of a simple Just value) did not typecheck.

{-# LANGUAGE OverloadedStrings, TemplateHaskell #-}module Main whereimport Control.Lensimport Data.Maybeimport Data.Textdata Item = Item {
  _itemName :: Text} deriving (Show)
makeLenses ''Item
makePrisms ''Item

data Container = Container {
  _containerName :: Text,
  _containerItem :: Maybe Item} deriving (Show)
makeLenses ''Container
makePrisms ''Container

data Collection = Collection {
  _collectionName :: Text,
  _collectionItems :: ![Container]
} deriving (Show)
makeLenses ''Collection
makePrisms ''Collection

collection :: Collectioncollection = Collection "Outermost" [Container "A" (Just (Item "A")),
                                     Container "B" (Nothing),
                                     Container "C" (Just (Item "C")),
                                     Container "D" (Nothing),
                                     Container "E" (Just (Item "E")),
                                     Container "F" (Just (Item "F")),
                                     Container "G" (Just (Item "G"))]

makeDefault :: Collection -> CollectionmakeDefault =  over (collectionItems.each.containerItem) (Just . fromMaybe (Item "Bar"))