instance-impl

A GHC plugin that adds a special rust inspired syntax for declaring instances of GHC.Records.HasField, for use with -XOverloadedRecordDot (introduced in GHC 9.2).

See this blog post by Matt Parsons for what inspired this.

Example

With this plugin enabled (using -fplugin=InstanceImpl), this code:

data Pt = Pt { x, y :: !Double }

instance impl Pt where
  r :: Double
  r = sqrt $ sq self.x + sq self.y
    where sq v = v*v

will allow you to use r as a field with -XOverloadedRecordDot just as you would use x and y.

You can also use polymorphism, both in the type of the record and the type of the field:

data Pt a = Pt { x, y :: !a }

instance impl (Pt a) where
  x' :: Real a => Double
  x' = realToFrac self.x

  r :: Floating a => a
  r = sqrt $ sq self.x + sq self.y
    where sq v = v*v

  display :: (Show a, IsString str) => str
  display = fromString $ "<" ++ show self.x ++ "," ++ show self.y ++ ">"

Warning

This is still pretty experimental, so things may not work as expected and the error messages can sometimes be pretty bad. Feel free to create an issue if something goes wrong.