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 ''BarOn GHC 8.0.1 it is possible to just write:
{-# LANGUAGE OverloadedLabels #-}
import Control.Lens ((+~))
add :: Int -> Bar Int -> Bar Int
add n = #bar +~ nFor 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 +~ nMore about the current status of OverloadedRecordFields language extension can be found on GHC Wiki: OverloadedRecordFields.
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
where
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
zeroV3
:: (Num a, R '["x" ::: a, "y" ::: a, "z" ::: a] r)
=> r -> r
zeroV3 = set' #x 0 . set' #y 0 . set' #z 0The following type signatures for zeroV3 are equivalent:
zeroV3
:: (Num a, R '["x" ::: a, "y" ::: a, "z" ::: a] r)
=> r -> rzeroV3
:: ( Num a
, ModifyField' "x" r a
, ModifyField' "y" r a
, ModifyField' "z" r a
)
=> r -> rOne 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)
zeroV3
:: (Num a, R '["x" ::: a, "y" ::: a, "z" ::: a] r)
=> r -> r
zeroV3 r = r
& #x . simple .~ 0
& #y . simple .~ 0
& #z . simple .~ 0The 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.