deriving-th

A proof of concept GHC plugin for Template Haskell based deriving.

Inspired By https://twitter.com/TechnoEmpress/status/1492411553894871042

Usage

To use deriving-th with a class, all you have to do is create an instance of the DerivingTH.Class.DeriveTH class. For example:

class Foo a where
  foo :: a -> Bool

instance DeriveTH Foo where
  deriveTH name = [d| instance Foo $(pure $ ConT name) where foo _ = True |]

The plugin is then invoked with the via template deriving strategy. This is used just as normal -XDerivingVia but with the word template replacing the via type.

data Bar = Bar
  deriving Foo via template

Or in combination with -XStandaloneDeriving:

data Baz = Baz

deriving via template instance Foo Baz

You can see these examples in use in the test/ folder.

TH Caveats

All of the normal Template Haskell caveats apply:

• Cross-compilation issues
• DeriveTH instances must be defined in a separate module from where they are used
• Instances must be derived further in the module than the type that they are for and before where they are used

Note that splices for standard deriving clauses are inserted directly after the data type definition they are a part of in the same order that they appear in the source. If greater flexibility in the location of splices is needed -XStandaloneDeriving can be used.

Implied Extensions

This plugin automatically enables the following extensions.

• -XDerivingVia for syntax
• -XTemplateHaskell for splices
• -XTemplateHaskellQuotes for name quotes

Extensions that are normally implied by these are not enabled automatically.

Advanced Usage

The DeriveTH class is much more flexible than the standard use case requires. The deriveTH method gives an arbitrary Q [Dec] so you can (ab)use it to generate more than one instance or even declarations other than instances. Additionally, the parameter of DeriveTH is fully polymorphic so you can use it with classes with any kind, partially applied classes, or even non-classes. This adaptability may be useful if you have mutually dependent instances or are wrapping some already existing declaration generating functions.

Supported GHC Versions

At the moment GHC versions 8.10.1 though 9.2.1 are supported. If you need another version, let me know.