6.5.9. Overloaded record dot

OverloadedRecordDot

Since:9.2.0

Provides record ‘.’ syntax e.g. x.foo

When OverloadedRecordDot is enabled one can write a.b to mean the b field of the a record expression.

Example:

{-# LANGUAGE OverloadedRecordDot #-}
{-# LANGUAGE DuplicateRecordFields #-}

data Person = Person { name :: String }
data Company = Company { name :: String, owner :: Person }

main = do
  let c = Company { name = "Acme Corp."
                  , owner = Person { name = "Wile E. Coyote" } }
  print $ c.name ++ " is run by " ++ c.owner.name

You may also write (.b) to mean a function that “projects the b field from its argument”. For example, (.b) a means the same thing as a.b).

OverloadedRecordDot is normally implemented by desugaring record . expressions to GHC.Records.getField expressions. By enabling OverloadedRecordDot and RebindableSyntax together it is possible to desugar . expressions into your own getField implementations.

When considering a.b, the b field that is meant is determined by solving HasField constraints. See Solving HasField constraints.