Data.OverloadedRecords

Magic classes for OverloadedRecordFields.

Implementation is based on: https://github.com/adamgundry/records-prototype/blob/master/CoherentPrototype.hs by Adam Gundry under MIT License.

Oveloaded Labels

module Data.OverloadedLabels

Overloaded Record Fields

Getter

type family FieldType l s :: *

When accessing field named l :: Symbol of a record s :: *, then the type of the value in that field is FieldType l s.

Instances

type FieldType "fieldDerivation" DeriveOverloadedRecordsParams = FieldDerivation

class HasField l s a | l s -> a where

Definition of this class is based on: https://phabricator.haskell.org/D1687

Methods

getField :: Proxy# l -> s -> a

Get value of a field.

Instances

HasField "fieldDerivation" DeriveOverloadedRecordsParams FieldDerivation

Setter

type family UpdateType l s a :: *

If field l :: Symbol of a record s :: * is set to new value which has type a :: *, then the modified record will have type UpdateType l s a.

Instances

type UpdateType "fieldDerivation" DeriveOverloadedRecordsParams FieldDerivation = DeriveOverloadedRecordsParams

class (HasField l s b, FieldType l s ~ b) => SetField l s b where

Methods

setField :: Proxy# l -> s -> b -> UpdateType l s b

Set value of a field.

Instances

SetField "fieldDerivation" DeriveOverloadedRecordsParams FieldDerivation

data Setter s t b

Wrapper for a set function, lens naming convention is used for type variables. Its instance for IsLabel forces overloaded label to behave as a setter.

Instances

(SetField l s b, (~) * (UpdateType l s b) t) => IsLabel l (Setter s t b)

set :: Setter s t b -> s -> b -> t

Extract set function from Setter. Using Setter instance for IsLabel forces overloaded label to behave as a setter.

Usage example:

newtype Bar a = Bar {_bar :: a}
  deriving Show

overloadedRecord ''Bar

>>> set bar (Bar (Just False)) Nothing
Bar {_bar = Nothing}

IsLabel For Getter and Lens

type family FromArrow a :: Bool

Returns True if type a is a function.

Equations

FromArrow (x -> y) = True

FromArrow t = False

class (z ~ FromArrow x) => IsFieldAccessor l x y z | l y -> x where

Distinguish between getter and lens.

Methods

fieldAccessor :: Proxy# l -> x -> y

Instances

(HasField l s a, (~) * (FieldType l s) a, (~) Bool (FromArrow s) False) => IsFieldAccessor l s a False

Overloaded getter:

Proxy# l -> r -> a

(Functor f, HasField l s a, SetField l s b, (~) * (FieldType l s) a, (~) * (UpdateType l s b) t) => IsFieldAccessor l (a -> f b) (s -> f t) True

Overloaded lens:

Functor f => Proxy# l -> (a -> f b) -> s -> f t