plugin-constraint is a GHC source plugin which demonstates two common tasks which need to be achieved when writing a source plugin.

1. How to interact with the constraint solver.
2. How to generate HsExpr GhcTc.

Both of these are not obvious!

We also demonstrate four different ways in order to build an HsExpr GhcTc.

1. mkNewExprTc - Directly constructing an HsExpr GhcTc.
2. mkNewExprRn - Building from HsExpr GhcRn
3. mkNewExprPs - Building from HsExpr GhcPs
4. mkNewExprTh - Using a template haskell quasiquote

These methods are ordered from the least convient to the most convenient but different approaches might work better for different tasks.

All the plugin actually does is insert a single new binding.

myBinding = print ()

We first find the Show instance for (). This is done by the generateDictionary function which returns the variable which the dictionary will be bound to and the dictionary itself. We then demonstrate in mkNewExprTc how to directly use this dictionary by using it to create the new binding. A type checked term uses HsWrap constructors to apply type arguments, dictionaries and to bind dictionaries. Thus, we insert an HsWrap constructor around the print function which instantiates the type variable a to (), passes the dictionary variable and then wraps the function with the evidence.

By using mkNewExprRn, it isn't necessary to directly apply the HsWrap ourselves as it is something the compiler inserts during type checking. In this function it is necessary to lookup the name you want to use by using functions from RnEnv. Performing actions usually performed by the renamer.

In mkNewExprPs, we make code as would be produced by the parser. It's necessary to be careful when making OccNames so that they have the right namespace.

In mkNewExprTh, we use a TH quote in order to generate a HsExpr GhcPs. This is very convenient because you can write actual Haskell source code