I also blog frequently on the Yesod Web Framework blog, as well as the FP Complete blog.

Playing with lens-aeson

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At LambdaConf last week, Tony Morris convinced me I should take another stab at getting more comfortable with lens, and after chatting with a few other people (including at least Chris Allen), I decided that the lens-aeson/JSON parsing use case would be a good at forcing me to play with more of the lens ecosystem than I have previously.

This is not a normal blog post for me. I'm not an expert (or even competent) on the topic of lens. In fact, odds are no one should read this blog post. Really consider it me thinking out loud, and obnoxiously doing so on my blog. I'll excuse the weird nature of this by saying I'm running on little sleep, and I'm bored in an airport and on an airplane.


Let's start off with a simple JSON file containing color names and values that looks like this:

[
    {
        "color": "red",
        "value": "#f00"
    },
    {
        "color": "black",
        "value": "#000"
    }
]

This is a relatively simple file format, with an array of individual objects, and each object having the same keys. We want to get the names of all the colors from this, ignoring the values. Let's start off by implementing such a program using an explicit FromJSON instance, which is probably the most obvious thing to do based on the lens documentation.

#!/usr/bin/env stack
-- stack --resolver lts-8.12 script
{-# LANGUAGE OverloadedStrings #-}
import Data.Aeson
import Data.Text (Text)
import qualified Data.ByteString as B

data Color = Color { colorName :: !Text }

instance FromJSON Color where
  parseJSON = withObject "Color" $ \o -> Color <$> o .: "color"

main :: IO ()
main = do
  bs <- B.readFile "colors.json"
  case eitherDecodeStrict' bs of
    Left e -> error e
    Right colors -> print $ map colorName colors

This is pretty straightforward: we define a data type Color, which contains the fields we care about (here, just the name of the color). Then we declare a FromJSON instance which parses out the color key. In our main function, we read the raw bytes, and use eitherDecodeStrict' to parse the JSON into a Value and then use our FromJSON instance to convert that Value into a list of Color values. We then apply colorName to each value in that list to extract the name, and print the list.

That works, but it's far from inspiring. We're declaring a Color datatype simply for the purpose of writing a typeclass instance. But it feels pretty heavyweight to have to declare a data type and make a typeclass instance for just one use site. Let's try what I'd consider the next most obvious approach: work directly on the Value data type's constructors:

#!/usr/bin/env stack
-- stack --resolver lts-8.12 script
{-# LANGUAGE OverloadedStrings #-}
import Data.Aeson
import Data.Text (Text)
import qualified Data.ByteString as B
import qualified Data.Vector as V
import qualified Data.HashMap.Strict as HashMap

main :: IO ()
main = do
  bs <- B.readFile "colors.json"
  case eitherDecodeStrict' bs of
    Left e -> error e
    Right (Array array) -> do
      colors <- V.forM array $ \v ->
        case v of
          Object o ->
            case HashMap.lookup "color" o of
              Nothing -> error "Didn't find color key"
              Just (String c) -> return c
              Just v' -> error $ "Expected a String, got: " ++ show v'
          _ -> error $ "Expected an object, got: " ++ show v
      print colors
    Right v -> error $ "Unexpected top level type: " ++ show v

This works, but is thoroughly unappetizing. We need to take into account a lot of corner cases and explicitly handle looping over the Vector. It's unpleasant, and for a non-toy example, would be downright tedious.

Let's try to avoid the tedium, and if you read my intro paragraph, you won't be surprised to hear that the answer I'm proposing is lens-aeson.

#!/usr/bin/env stack
-- stack --resolver lts-8.12 script
{-# LANGUAGE OverloadedStrings #-}
import Control.Lens
import Data.Aeson.Lens
import qualified Data.ByteString as B

main :: IO ()
main = do
  bs <- B.readFile "colors.json"
  print $ bs^..values.key "color"._String

This code looks almost too short to work, but it produces exactly the same output as before for our colors.json file. To see how it works:

  • We don't need to do any explicit parsing of our ByteString value. lens-aeson contains a number of typeclasses for matching JSON values, and provides instances for ByteString, Text, and String that will perform an initial parse to a Value for you automatically.
  • The ^.. operator comes from the lens package, which is a synonym for toListOf. As you might imagine, it converts something into a list. Our ^.. operator will take the value on the left hand side (bs here) and apply the Fold on the right to it, collecting the results into a list.
  • Now we need to understand how we construct our Fold. We start off with values, which will match a JSON array and provide all of the values inside of it.
  • Next we compose with the key "color" Fold, which takes a Value, checks that it is an Object, and looks up the given key, in this case "color".
  • Finally, we use the _String Fold to check that we have a string value (as opposed to something like a number or a boolean) and returns it.

The behavior of this isn't exactly identical to our previous versions. In particular, if there are values in our array that don't match our requirements, they'll simply be dropped instead of producing an error. Whether this is acceptable for your case is up to you. And I'm hoping that someone reading this post will provide a good example of how to do the error-checking version with lens-aeson.

Not just a Fold

Above, I mentioned the term Fold many times. A Fold is one kind of optic from the lens package, which "allows you to extract multiple results from a container." However, if you're familiar with lens, you may know that optics form a hierarchy.

NOTE An optic is a more general term that encompasses a lot of the types in the lens package, like lenses, foldables, prisms, traversables, isos, getters, etc. Because of how optics are structured, they compose together nicely. And because of how the typeclasses are structured, optics have a nice subtyping system, which I'm hinting at here.

For example, a Traversal is a generalization of a Fold which also allows us to "traverse over a structure and change out its contents with monadic side-effects." Our values Fold isn't just a Fold. It allows us to also update all of the values inside the array, making it a valid Traversal. Let's see how we can use that:

#!/usr/bin/env stack
-- stack --resolver lts-8.12 script
{-# LANGUAGE OverloadedStrings #-}
import Control.Lens
import Data.Aeson.Lens
import qualified Data.ByteString as B

main :: IO ()
main = do
  let bs = "[1,2,3]" :: B.ByteString
  print $ bs & values._Number %~ (+ 1)

Instead of reading our ByteString from a file, we're now defining our bs value in our Haskell code, giving it the JSON representation of the array of numbers 1, 2, and 3.

We then take our ByteString and use the & operator, which is reverse function application. This means that we will apply whatever's on the right hand side of & to our ByteString on the left. Let's look at that function:

values._Number %~ (+ 1)

The %~ operator will apply some modification function using a Setter. And guess what: a Traversal is a generalization of a Setter, so we can use a Traversal. As we said, values is a Traversal. _Number is also a Traversal, so their composition makes a Traversal. And then we apply our + 1 function inside of it.

So to sum up, our bs & values._Number %~ (+ 1) expression will do the following:

  • Parse the raw bytestring value in bs into a JSON Value
  • Inspect that value and see if it's an array
  • For each element in that array, check if it's a number
  • If it's a number, add 1 to it
  • Finally, take the newly created Value and render it back into a bytestring value

That's quite the power-to-weight ratio. I recommend writing the same thing without lens for comparison.

Not just a Traversal

The same way a Traversal is a generalization of a Fold, a Prism is a generalization of a Traversal. While a Traversal represents the ability to look inside a value, find 0 or more values of a given type, and either get them (the Fold power) or modify them (the Traversal power), a Prism specificies that it will have exactly 0 or 1 values, and that, given one value of the target type, you create the original type.

Did that sound confusing? I certainly think so. So let's say it another way: a Prism is an optic version of a data constructor. When you have a sum type Either a b, you can always get exactly 0 or 1 a values (0 if the value is Right, 1 if the value is Left). And, given an a value, you can always construct a value of type Either a b.

#!/usr/bin/env stack
-- stack --resolver lts-8.12 script
import Control.Lens
import Test.Hspec

main :: IO ()
main = hspec $ do
  it "constructs with _Left" $
    (1 ^. re _Left) `shouldBe`
    (Left 1 :: Either Int String)
  it "constructs with _Right" $
    ("hello" ^. re _Right) `shouldBe`
    (Right "hello" :: Either Int String)
  it "traverses with _Left" $
    (Left 1 & _Left %~ (+ 1)) `shouldBe`
    (Left 2 :: Either Int String)
  it "traverse can do nothing" $
    (Right "hello" & _Left %~ (+ 1)) `shouldBe`
    (Right "hello" :: Either Int String)
  it "folds with _Left" $
    (Left 1 ^.. _Left) `shouldBe`
    [1 :: Int]
  it "folds with _Right" $
    (Left 1 ^.. _Right) `shouldBe`
    ([] :: [()])

So apparently, if you're totally bought in on the lens ecosystem, you're free to never use your data constructors again and just use re. But anyway, we were dealing with JSON data; can we construct a simple JSON value like this? Sure.

#!/usr/bin/env stack
-- stack --resolver lts-8.12 script
{-# LANGUAGE OverloadedStrings #-}
import Control.Lens
import Data.Aeson.Lens
import qualified Data.ByteString as B
import qualified Data.Vector as V

main :: IO ()
main = putStrLn $ 1 ^. re _Number.to (V.replicate 5).re _Array

The to function converts a normal functions from a to b into an optic that does the same thing, a Getter a b. More idiomatically (I think), we'd actually use the type variables s and a and get to :: (s -> a) -> Getter s a.

This was actually more detailed on lens itself than I intended to get here, but since this blog post is just a forcing function for me to explore things and not actually useful for anyone else in the world, I guess that's OK.

More random fun

Alright, can I upper case all of the color names? Sure:

#!/usr/bin/env stack
-- stack --resolver lts-8.12 script
{-# LANGUAGE OverloadedStrings #-}
import Control.Lens
import Data.Aeson.Lens
import qualified Data.ByteString as B
import qualified Data.Text as T
import qualified Data.Vector as V

main :: IO ()
main = do
  bs <- B.readFile "colors.json"
  print $ bs & values.key "color"._String %~ T.toUpper

Now let's get a bit trickier: can I create an additional field color-upper with this upper cased version? I have no idea if this is idiomatic lens code, but it certainly works:

#!/usr/bin/env stack
-- stack --resolver lts-8.12 script
{-# LANGUAGE OverloadedStrings #-}
import Control.Lens
import Data.Aeson.Lens
import qualified Data.ByteString as B
import qualified Data.Text as T
import qualified Data.Vector as V

main :: IO ()
main = do
  bs <- B.readFile "colors.json"
  print $ bs & values._Object %~
    (\hm -> hm & at "color-upper" .~
      (hm^?at "color".folded._String.to T.toUpper.re _String))

That's a lot to unpack for me. First, I'm using bs & values._Object %~ ... to say "look inside the bytestring, treat it as JSON, look for an array, and find every object in that array and treat it as a HashMap Text Value, and modify each hashmap using the ..." It's the ... that I find confusing.

Next, we do hm & at "color-upper" .~ ..., which says "I want to set the value in the hashmap at the key color-upper to the Maybe Value value I'm giving you. Finally, we get our Maybe Value value with the rest of that expression, which reads:

hm^?at "color".folded._String.to T.toUpper.re _String

This reads to me as:

  • Take hm
  • Give me the first value that succeeds (^?), or Nothing if no value gets grabbed
  • Look up the "color" key
  • Flatten out that Maybe Value into just a Value
  • Check that it's a string
  • Convert it to upper case
  • Wrap it back in a String constructor using re _String

By way of contrast, I can write the same functionality the non-lens way with:

\hm ->
  case HashMap.lookup "color" hm of
    Just (String color) -> HashMap.insert
      "color-upper"
      (String (T.toUpper color))
      hm
    _ -> hm

For me personally, I find this version easier to read, but I'm also a lens usage novice. Maybe I just need to force myself to write airplane-powered rambling lens blog posts more often (or maybe write some real code).

Going for something much simpler, let's just delete all of the value keys:

#!/usr/bin/env stack
-- stack --resolver lts-8.12 script
{-# LANGUAGE OverloadedStrings #-}
import Control.Lens
import Data.Aeson.Lens
import qualified Data.ByteString as B

main :: IO ()
main = do
  bs <- B.readFile "colors.json"
  print $ bs & values._Object %~ sans "value"

Indexed

I wanted to play with indexed optics a bit. My goal had been to modify the following code:

#!/usr/bin/env stack
-- stack --resolver lts-8.12 script
{-# LANGUAGE OverloadedStrings #-}
import Control.Lens
import Data.Aeson.Lens
import qualified Data.ByteString as B

main :: IO ()
main = do
  bs <- B.readFile "colors.json"
  print $ bs ^.. values.key "color"._String

So that it printed a pair of the index in the array that the color appears at, and the color itself. Unfortunately, I couldn't figure out how to make that work. One thing I got was:

main = do
  bs <- B.readFile "colors.json"
  print $ bs ^@.. values

But this just keeps the entire object, not the string inside the color key like I wanted. The following is a bit closer, but (1) it keeps Nothing values in the result instead of just removing them (like a mapMaybe would) and (2) doesn't feel idiomatic:

main = do
  bs <- B.readFile "colors.json"
  print $ (bs ^@.. values) & each._2 %~ (^? key "color"._String)

Then I discovered the pre function, which let me do the following with identical output to the former:

main = do
  bs <- B.readFile "colors.json"
  print $ bs ^@.. values.pre (key "color"._String)

It does seem like I'm likely missing something obvious to remove drop the Nothing values and remove the Maybe wrapping entirely, but unfortunately I couldn't figure it out.

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