6.5.10. 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.