6.5.5. Duplicate record fields

DuplicateRecordFields

Implies:

DisambiguateRecordFields

Since:

8.0.1

Allow definition of record types with identically-named fields.

Going beyond DisambiguateRecordFields (see Record field disambiguation), the DuplicateRecordFields extension allows multiple datatypes to be declared using the same field names in a single module. For example, it allows this:

module M where
  data S = MkS { x :: Int }
  data T = MkT { x :: Bool }

Uses of fields that are always unambiguous because they mention the constructor, including construction and pattern-matching, may freely use duplicated field names. For example, the following are permitted (just as with DisambiguateRecordFields):

s = MkS { x = 3 }

f (MkT { x = b }) = b

While DuplicateRecordFields permits multiple fields with the same name in a single module, it does not permit a field and a normal value binding to have the same name. For that, use NoFieldSelectors.

As of GHC 9.4.1, selector names have to be entirely unambiguous (under the usual name resolution rules), while for record updates, there must be at most one datatype that has all the field names being updated.

module M
  ( S(x)
  , T(..)
  ) where

data S = MkS { x :: Int }

data T = MkT { x :: Bool }

module M (x) where ...

import M (S(x))        -- imports the type S and the 'x' field of S (but not the field of T)
import M (x)           -- imports both 'x' fields
import M hiding (S(x)) -- imports everything except the type S and its 'x' field
import M hiding (x)    -- imports everything except the two 'x' fields