BSD-3-Clause licensed by Peter Trško
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Used by 1 package in lts-9.21(full list with versions):

Overloaded Records

Hackage Hackage Dependencies Haskell Programming Language BSD3 License



Implementation of Overloaded Record Fields based on current GHC proposal. It is built on top of functionality that is included in GHC 8.0.1, but it works on older GHC versions as well. Most importantly, this library provides Template Haskell functions for automatic deriving of instancess for HasField and ModifyField type classes. With these instances overloaded fields can be used directly as getters and lenses.

import Data.Default (Default(def))
import Data.OverloadedRecords.TH (overloadedRecord)

newtype Bar a = Bar {_bar :: a}

overloadedRecord def ''Bar

On GHC 8.0.1 it is possible to just write:

{-# LANGUAGE OverloadedLabels #-}

import Control.Lens ((+~))

add :: Int -> Bar Int -> Bar Int
add n = #bar +~ n

For older GHC versions there is a family of Template Haskell functions that will derive overloaded labels in form of standard haskell definitions:

import Control.Lens ((+~))
import Data.OverloadedLabels.TH (label)

label "bar"

add :: Int -> Bar Int -> Bar Int
add n = bar +~ n

More about the current status of OverloadedRecordFields language extension can be found on GHC Wiki: OverloadedRecordFields.

Usage Example

Following is a more complex usage example that demonstrates some of the possibilities of Overloaded Labels provided by this library.

-- Basic set of language extensions required when defining instances for
-- classes and type families from "Data.OverloadedRecords".
{-# LANGUAGE DataKinds #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE TemplateHaskell #-}
{-# LANGUAGE TypeFamilies #-}

-- Following language extensions are required by code like this:

{-# LANGUAGE ConstraintKinds #-}
    -- Codomain of type family 'R' is a 'Constraint' kind.

{-# LANGUAGE FlexibleContexts #-}
    -- Required in example when field type (second argument of ':::') is a
    -- specific type instead of a polymorphic type.

{-# LANGUAGE TypeOperators #-}
    -- Required due to usage of ':::' type alias.

-- Following language extensions are available only in GHC >=8:

{-# LANGUAGE OverloadedLabels #-}
    -- Enables #label syntactic sugar.

module Example

import Data.Default (Default(def))
    -- Provided by one of these packages:
    -- * data-default
    -- * data-default-extra

import Data.OverloadedRecords
import Data.OverloadedRecords.TH (overloadedRecord)

data V3 a = V3
    { v3x :: !a
    , v3y :: !a
    , v3z :: !a
  deriving Show

-- Following line derives instances for various type classes and type
-- families that are provided by the overloaded-records library.
-- However with def (default settings) this is done only for fields that
-- start with type name, data constructor name, or underscore. Prefix is
-- stripped. In example "v3x" is transformed in to "x" and so would be
-- "_x".
overloadedRecord def ''V3

data V4 a = V4
    { v4x :: !a
    , v4y :: !a
    , v4z :: !a
    , v4t :: !a
  deriving Show

overloadedRecord def ''V4

    :: (Num a, R '["x" ::: a, "y" ::: a, "z" ::: a] r)
    => r -> r
zeroV3 = set' #x 0 . set' #y 0 . set' #z 0

The following type signatures for zeroV3 are equivalent:

    :: (Num a, R '["x" ::: a, "y" ::: a, "z" ::: a] r)
    => r -> r
    ::  ( Num a
        , ModifyField' "x" r a
        , ModifyField' "y" r a
        , ModifyField' "z" r a
    => r -> r

One of the biggest features of Overloaded Records is the possibility to define functions that do not depend on concrete data types, but on the “fields” they provide. In example function zeroV3 can be applied to anything that has fields "x", "y", and "z" that reference values of some Num type:

λ> zeroV3 (V3 1 1 1 :: V3 Int)
V3 {_x = 0, _y = 0, _z = 0}
λ> zeroV3 (V4 1 1 1 1 :: V4 Int)
V4 {_x = 0, _y = 0, _z = 0, _t = 1}

Function zeroV3 can be also defined using operators from lens library:

import Control.Lens ((.~), simple)

    :: (Num a, R '["x" ::: a, "y" ::: a, "z" ::: a] r)
    => r -> r
zeroV3 r = r
    & #x . simple .~ 0
    & #y . simple .~ 0
    & #z . simple .~ 0


The BSD 3-Clause License, see LICENSE file for details. This implementation is based on original prototype, which is under MIT License.


Contributions, pull requests and bug reports are welcome! Please don’t be afraid to contact author using GitHub or by e-mail.

Related Work

  • ruin is a DSL for working with record types that also leverages OverloadedLabels language extension.
  • vinyl provides extensible records for Haskell with lenses using modern GHC features.


ChangeLog / ReleaseNotes


  • Rec instances for Eq1, Ord1, Show1, Eq, Ord, and Show. Instances for Eq1, Ord1 and Show1 are available only with transformers ==0.5.* or with base >=4.9 (i.e. GHC >=8). (new)
  • Introducing WrappedLensLike data type along with a simplified type WrappedLensLike'. For each of these we have corresponding lns and lns' operation for unpacking WrappedLensLike. (new)
    • lns :: WrappedLensLike f s t a b -> (a -> f b) -> s -> f t
    • lns' :: WrappedLensLike' f s a -> (a -> f a) -> s -> f s
  • Allowing fields/accessors to occur in multiple data constructors by not creating duplicit instances, thanks to Andy Morris. See pull request #2. (change)
  • Data type Label (l :: Symbol) that can be used to pass label value around. (new)
  • Instances for fst, snd, thd and curry fields for 11-tuples, 12-tuples up to 15-tuples. (new)
  • Instances for head and tail fields for NonEmpty list. (new)
  • Instances for HasField and ModifyField Rec context r where r is monomorphic. This is just a poor man’s instance, since its usability is very limited, but helpful in those special cases. Hopefully we will be able to find a way how to define overloaded records machinery for Rec context r in general. (new)



  • Renamed SetField type class to ModifyField, it now contains following methods (breaking change):
    • modifyField :: Proxy# l -> (a -> b) -> s -> t
    • setField :: Proxy# l -> s -> b -> t
    • fieldLens :: Functor f => Proxy# l -> (a -> f b) -> s -> f t
  • Instances for tuples (i.e. (a, b), (a, b, c), …) and lists (i.e. [a]). (new)
  • Definitions from Data.OverloadedRecords.TH were moved to Data.OverloadedRecords.TH.Internal, so that API can be split in to stable and unstable (internal) API. (change)
  • Aliases HasField' and ModifyField' that enforce s = t and a = b. This is similar to definitions like Lens'. Simplified versions of methods and functions are included using the same naming convention. (new)
  • Setter and Setter' changed to type aliases for Modifier type. (breaking change)
  • Introducing Setting type alias for Modifier along with setting, which is an alternative to set operation. (new)
    • setting :: Setting a s t b -> Proxy a -> b -> s -> t
  • Changed order of arguments of functions set and set' (breaking change):
    • set :: Setter s t b -> b -> s -> t
    • set' :: Setter' s a -> a -> s -> s
  • Introduced type family R that can be used to define more compact type signatures when mentioning multiple record fields in it. (new)
  • Uploaded to Hackage:


  • Fixed defaultMakeFieldName, which actually behaved correctly only in very few cases. (bug fix)
  • Added missing HAVE_OVERLOADED_LABELS macro, that actually allows us to use GHC’s IsLabel on GHC >8. (bug fix)
  • Exposing previously hidden FromArrow and IsFieldAccessor. (change)
  • Few simple unit tests. (new)
  • Uploaded to Hackage:


  • Function overloadedRecords renamed to overloadedRecord. There is also new overloadedRecords function, that behaves as overloadedRecord, but for multiple types at once. (breaking change)
  • It is now possible to customize overloadedRecord* family of functions with custom getter and setter implementation. (new)
  • Types and functions follow, hopefully, better naming conventions. (change)
  • More low-level template haskell functions for those cases when it is necessary to build your own higher-level ones, or when you need much more control over the result. (new)
  • Uploaded to Hackage: