Unused Record Wildcard [GHC-83475]

Flag: -Wunused-record-wildcards
Enabled by: -Wall

If RecordWildCards is enabled we can automatically bind fields of a record by using patterns like MyRecord{..}. Not using any of the record fields that were bound using the record wildcard syntax will result in a warning.

Examples

Pattern contains unused record wildcard

None of the fields bound by Foo{..} are used in f, so the pattern may be safely removed.

Warning

GHC-83475/example1/before/Example1.hs:7:7: warning: [-Wunused-record-wildcards]
    No variables bound in the record wildcard match are used
      Possible fix: omit the ‘..’
  |
7 | f Foo{..} = "Hello"
  |       ^^

Example1.hs

Before

{-# OPTIONS_GHC -fwarn-unused-record-wildcards #-}
{-# LANGUAGE RecordWildCards #-}
module Example1 where

data Foo = Foo { x :: Int, y :: Int, name :: String }

f :: Foo -> String
f Foo{..} = "Hello"

After

{-# OPTIONS_GHC -fwarn-unused-record-wildcards #-}
{-# LANGUAGE RecordWildCards #-}
module Example1 where

data Foo = Foo { x :: Int, y :: Int, name :: String }

f :: Foo -> String
f Foo{} = "Hello"