28 Sep 2021

Using lens to set a value based on another

I started writing a small tool for work that consumes YAML files and combines the data into a single YAML file. To be specific it consumes YAML files containing snippets of service specification for Docker Compose and it produces a YAML file for use with docker-compose. Besides being useful to me, I thought it'd also be a good way to get some experience with lens.

The first transformation I wanted to write was one that puts in the correct image name. So, only slightly simplified, it is transforming

panda:
    x-image: panda
goat:
    x-image: goat
tapir:
    image: incorrent
    x-image: tapir

into

panda:
    image: panda:latest
    x-image: panda
goat:
    image: goat:latest
    x-image: goat
tapir:
    image: tapir:latest
    x-image: tapir

That is, it creates a new key/value pair in each object based on the value of x-image in the same object.

First approach

The first approach I came up with was to traverse the sub-objects and apply a function that adds the image key.

setImage :: Value -> Value
setImage y = y & members %~ setImg
  where
    setImg o =
        o
            & _Object . at "image"
            ?~ String (o ^. key "x-image" . _String <> ":latest")

It did make me wonder if this kind of problem, setting a value based on another value, isn't so common that there's a nicer solution to it. Perhaps coded up in a combinator that isn't mentioned in Optics By Example (or mabye I've forgot it was mentioned). That lead me to ask around a bit, which leads to approach two.

Second approach

Arguably there isn't much difference, it's still traversing the sub-objects and applying a function. The function makes use of view being run in a monad and ASetter being defined with Identity (a monad).

setImage' :: Value -> Value
setImage' y =
    y
        & members . _Object
        %~ (set (at "image") . (_Just . _String %~ (<> ":latest")) =<< view (at "x-image"))

I haven't made up my mind on whether I like this better than the first. It's disappointingly similar to the first one.

Third approach

Then I it might be nice to split the fetching of x-image values from the addition of image key/value pairs. By extracting with an index it's possible to keep track of what sub-object each x-image value comes from. Then two steps can be combined using foldl.

setImage'' :: Value -> Value
setImage'' y = foldl setOne y vals
  where
    vals = y ^@.. members <. key "x-image" . _String
    setOne y' (objKey, value) =
        y'
            & key objKey . _Object . at "image"
            ?~ String (value <> ":latest")

I'm not convinced though. I guess I'm still holding out for a brilliant combinator that fits my problem perfectly.

Please point me to "the perfect solution" if you have one, or if you just have some general tips on optics that would make my code clearer, or shorter, or more elegant, or maybe just more lens-y.