# Deferred Derivation

## justifiably lazy orphans

(alternative subtitle: “I used the TemplateHaskell to destroy the TemplateHaskell”)

At the day job, we use the aeson-typescript library to generate TypeScript types from our Haskell types. One problem is that the library uses TemplateHaskell to do this, which means we have TemplateHaskell in almost all of our datatype-defining modules. Due to the TemplateHaskell recompilation avoidance bug (any module that uses TemplateHaskell must always be recompiled if any transitive dependency is changed), this means we spend a lot of time recompiling a lot of modules that don’t need to change. Freeing a module from TemplateHaskell speeds up our build tremendously - not because TemplateHaskell is slow (it’s very fast) but because compiling at all is slow. The best way to speed something up is to spend 0 time doing it - don’t do it at all!

Anyway, I investigated switching the library to a Generics based approach, but it’s complicated and looked tricky, so I decided not to pursue that. The library defines a type class HasJSONOptions, which lets you re-use the same JSON options for both aeson’s JSON encoding classes and the TypeScript class.

I had recently done some work with discoverEntities, a TemplateHaskell function which gathers the PersistEntity instances in scope and collects their entityDef (Proxy :: Proxy a). I started wondering - can I use this trick to defer the derivation and omit the TemplateHaskell? In preparation, I wrote discover-instances, which generalized the above pattern (and was a fun exercise in typed TemplateHaskell quotes).

Now, we might have had an instance like this:

data X = X { ... }

$(deriveJSON defaultOptions ''X)$(deriveTypeScript defaultOptions ''X)


We can excise the TemplateHaskell entirely by using Generics-based derivation for the JSON classes, and specify an instance for the HasJSONOptions class.

data X = X { ... }
deriving stock Generic

instance HasJSONOptions X where
getJSONOptions _ =
defaultOptions

instance ToJSON X where
toJSON =
genericToJSON (getJSONOptions (Proxy :: Proxy X))

instance FromJSON X where
parseJSON =
genericFromJSON (getJSONOptions (Proxy :: Proxy X))


But we don’t yet have that TypeScript instance.

Let’s look at the module that actually generates our TypeScript code.

module MakeTypeScript where

import Model.X
import Model.Y
import Model.Z

writeTypeScript :: IO ()
writeTypeScript = do
writeFile frontendTypes $concat [ makeTypeScriptFor (Proxy :: Proxy X) , makeTypeScriptFor (Proxy :: Proxy Y) , makeTypeScriptFor (Proxy :: Proxy Z) ]  This is the only place those instances are ever used. With the above change (eg deleting the deriveTypeScript stuff), there’s no longer an instance present. But I can fix that by deriving the instance in this file. module MakeTypeScript where import Model.X import Model.Y import Model.Z$(deriveTypeScript (getJSONOptions (Proxy :: Proxy X)) ''X)
$(deriveTypeScript (getJSONOptions (Proxy :: Proxy Y)) ''Y)$(deriveTypeScript (getJSONOptions (Proxy :: Proxy Z)) ''Z)

writeTypeScript :: IO ()
writeTypeScript = do
writeFile frontendTypes $concat [ makeTypeScriptFor (Proxy :: Proxy X) , makeTypeScriptFor (Proxy :: Proxy Y) , makeTypeScriptFor (Proxy :: Proxy Z) ]  This is deeply unsatisfying to me. We have three points of repetition: 1. importing a type 2. deriveing an instance for it 3. Mentioning the type in writeTypeScript. Let’s improve this. First, we want to use discoverInstances to splice in all the HasJSONOptions instances that are visible. Second, we’ll iterate over each instance, and derive the code there. module MakeTypeScript where import Model.X import Model.Y import Model.Z$(do
decs <-
forInstances
$$(discoverInstances @HasJSONOptions)  \proxy@(Proxy :: Proxy a) -> do deriveTypeScript (getJSONOptions proxy) (nameForType proxy) pure (concat decs) ) writeTypeScript :: IO () writeTypeScript = do writeFile frontendTypes  concat [ makeTypeScriptFor (Proxy :: Proxy X) , makeTypeScriptFor (Proxy :: Proxy Y) , makeTypeScriptFor (Proxy :: Proxy Z) ]  forInstances is a convenience function that lets you operate on the proxy with the constraint in scope. This totally works. We’ve derived all of the instances of TypeScript, and now we’re using them quite happily. Now we’re down to two points of repetition - importing a type and writing it specifically in makeTypeScriptFor. We can’t get rid of imports, so let’s look at the writeFile list. This is pretty easy to get rid of using our SomeDict TypeScript. module MakeTypeScript where import Model.X import Model.Y import Model.Z (do decs <- forInstances$$(discoverInstances @HasJSONOptions)
$\proxy@(Proxy :: Proxy a) -> do deriveTypeScript (getJSONOptions proxy) (nameForType proxy) pure (concat decs) ) writeTypeScript :: IO () writeTypeScript = do writeFile frontendTypes$ concat $withInstances$$(discoverInstances @TypeScript)$ \proxy -> makeTypeScriptFor proxy


And we’re done!

# Wins:

1. No more TemplateHaskell requirement in a module that defines an API type!
2. No more boring repetition!
3. We got to use fancy types!

# Losses:

1. Orphan instances :(
2. There’s still TemplateHaskell even if it’s localized
3. Generic-based deriving is slower, so a clean build will be worse