The Problem.

In the project I’m working on there is some logic which assigns a UUID to a field in a bunch of values which make up an AST structure representing a user’s code. For testing purposes there are a series of functions which take a value in that structure and wipe out the unique ID so that we can use an equality check against it. Those are manually written functions which we have to update as the structure changes, leading to the odd “Wait, why has that still got a unique ID?” moment when we make changes. I had an inkling that if this was written in Haskell (the code I’m referring to is written in TypeScript) this manually written code, along with the maintenance cost, would be unnecessary.

First Steps.

I’ll be using nix-shell to get myself a REPL to work with the code, using this command:

nix-shell
  -p "haskellPackages.ghcWithPackages (p: with p; [ghc uniplate pretty-simple])"
  --command ghci

Turn on an option that we’ll be needing for an easy life:

GHCi> :set -XDeriveDataTypeable

Add an import related to that option, don’t worry about these for now:

GHCi> import Data.Data

Next we’ll use the multi-line input support in GHCi to input this (put :{ on an empty line first and on an empty line after write :} to finish the multi-line editing):

newtype UniqueID = UniqueID { uid :: String }
                   deriving (Eq, Show, Data)

data JSObject = JSObject { objectParts :: [(String, JSValue)] }
                deriving (Eq, Show, Data)

data JSArray = JSArray { arrayParts :: [JSValue] }
               deriving (Eq, Show, Data)

data JSString = JSString { stringValue :: String }
                deriving (Eq, Show, Data)

data JSBool = JSBool { boolValue :: Bool }
              deriving (Eq, Show, Data)

data JSNumber = JSNumber { numberValue :: Double }
                deriving (Eq, Show, Data)

data JSNull = JSNull deriving (Eq, Show, Data)

data JSUndefined = JSUndefined deriving (Eq, Show, Data)

data JSCode = JSCode { code :: String, codeUniqueID :: UniqueID }
              deriving (Eq, Show, Data)

data JSValue = JSValueObject JSObject
             | JSValueArray JSArray
             | JSValueString JSString
             | JSValueBool JSBool
             | JSValueNumber JSNumber
             | JSValueNull JSNull
             | JSValueUndefined JSUndefined
             | JSValueCode JSCode
             deriving (Eq, Show, Data)

What we have above is something that approximates the definition of JSON in most libraries but with the addition of the JSCode type which represents a chunk of arbitrary JavaScript and is where we hold our UniqueID value. As we can have one of those buried 9 layers down, we need to walk the tree to update those wherever we might find them.

Building The Parts.

We’ll start with a function to clear unique ID values, which just throws away the original value and gives us an empty one (use the multi-line support again for this):

clearUniqueID :: UniqueID -> UniqueID
clearUniqueID _ = UniqueID ""

Now build up a value to play with:

GHCi> exampleCode = JSCode "5 + 10" (UniqueID "ABC")
GHCi> exampleArray = JSArray [JSValueCode exampleCode]
GHCi> exampleString = JSString "Good News"
GHCi> exampleFirstValue = ("first", JSValueString exampleString)
GHCi> exampleSecondValue = ("second", JSValueArray exampleArray)
GHCi> exampleObject = JSValueObject (JSObject [exampleFirstValue, exampleSecondValue])

We can use the pretty-simple package to see the structure more clearly:

GHCi> import Text.Pretty.Simple
GHCi> pPrintNoColor exampleObject
JSValueObject
    ( JSObject
        { objectParts =
            [
                ( "first"
                , JSValueString
                    ( JSString { stringValue = "Good News" } )
                )
            ,
                ( "second"
                , JSValueArray
                    ( JSArray
                        { arrayParts =
                            [ JSValueCode
                                ( JSCode
                                    { code = "5 + 10"
                                    , codeUniqueID = UniqueID { uid = "ABC" }
                                    }
                                )
                            ]
                        }
                    )
                )
            ]
        }
    )

The Good Stuff.

With the Uniplate library we just need a couple of imports:

GHCi> import Data.Generics.Uniplate.Data
GHCi> import Data.Generics.SYB

Then with the everywhere function we apply our transformation from earlier:

GHCi> updatedExample = everywhere clearUniqueID exampleObject

Lets see the result.

GHCi> pPrintNoColor updatedExample
JSValueObject
    ( JSObject
        { objectParts =
            [
                ( "first"
                , JSValueString
                    ( JSString { stringValue = "Good News" } )
                )
            ,
                ( "second"
                , JSValueArray
                    ( JSArray
                        { arrayParts =
                            [ JSValueCode
                                ( JSCode
                                    { code = "5 + 10"
                                    , codeUniqueID = UniqueID { uid = "" }
                                    }
                                )
                            ]
                        }
                    )
                )
            ]
        }
    )

…that’s it! All done, see you next time!

A Deeper Look.

First off, lets have a look at the type of that everywhere function, because that was the most mysterious part.

everywhere :: Biplate b a => (a -> a) -> b -> b

So our clearUniqueID function slotted into the first parameter and then we get a function that transforms from JSValue to JSValue. But this is only if we have an instance of Biplate JSValue UniqueID.

Looking up Biplate we want a Biplate JSValue UniqueID, there’s an instance for (Data a, Data b, Uniplate b) => Biplate a b. So a Data JSValue, a Data UniqueID and a Uniplate UniqueID gives us that Biplate JSValue UniqueID.

Following that chain along to Uniplate, if we look at the instances for it we can see there’s this one: Data a => Uniplate a. So Data a gives us a Uniplate a for every Data a that exists.

Since everything ends up with Data JSValue and Data UniqueID, which GHC has graciously derived automatically for us, we don’t have to write all of that code for walking the various types.

So What Does This Give Us?

The benefits of this are as follows:

• No manually written pile of code that needs regular maintenance.
• Reflection or similar runtime introspection is avoided, which might drill into the wrong thing and possibly throw an exception.
• The types guide the behaviour, so we can guarantee that it’ll be applied to everything it should be and nothing else.

That last point is part of a bigger pattern, not using types as validation but as a building block for behaviour. Having the compiler do work which is repetitive and/or error prone and to the first point which then needs keeping up to date after it is first implemented.

Bonus Round.

There are other wonders to be found in the uniplate library too, like childrenBi which will get all the values matching a particular type from the hierarchy.

GHCi> childrenBi exampleObject :: [UniqueID]
[UniqueID "ABC"]