Strictness annotation on unlifted type [GHC-55666]

Flag: -Wredundant-strictness-flags

A strictness annotation (also called a bang: !) can be used to denote that a value should not be evaluated lazily. In some cases this can lead to faster code because fewer heap allocations are required. Here it is used to mark a that a field in a datatype should not be evaluated lazily.

However, values of unlifted types like Int# are strict by definition because they represent an actual value, not a pointer to a potentially unevaluated value (thunk).

Therefore, adding strictness annotations to unlifted types or fields of such types is redundant. They should be omitted to avoid confusion.

Examples

src/MyLib.hs:16:5: warning: [GHC-55666] [-Wredundant-strictness-flags]
    * Strictness flag has no effect on unlifted type `Int#'
    * In the definition of data constructor `MkT'
      In the data type declaration for `T'
   |
   |   = MkT !Int#
   |     ^^^^^^^

Bang_on_unlifted_type.hs

{-# LANGUAGE MagicHash #-}

module Bang_on_unlifted_type where

import GHC.Base (Int#)

data T = MkT !Int#

{-# LANGUAGE MagicHash #-}

module Bang_on_unlifted_type where

import GHC.Base (Int#)

data T = MkT Int#