| Copyright | (C) 2020 Csongor Kiss |
|---|---|
| License | BSD3 |
| Maintainer | Csongor Kiss <kiss.csongor.kiss@gmail.com> |
| Stability | experimental |
| Portability | non-portable |
| Safe Haskell | None |
| Language | Haskell2010 |
Data.Generics.Product.Fields
Contents
Description
Derive record field getters and setters generically.
Synopsis
- class HasField (field :: Symbol ) s t a b | s field -> a, t field -> b, s field b -> t, t field a -> s where
- class HasField' (field :: Symbol ) s a | s field -> a where
- class HasField_ (field :: Symbol ) s t a b where
- getField :: forall f a s. HasField' f s a => s -> a
- setField :: forall f s a. HasField' f s a => a -> s -> s
Lenses
Running example:
>>>:set -XTypeApplications>>>:set -XDataKinds>>>:set -XDeriveGeneric>>>:set -XGADTs>>>:set -XFlexibleContexts>>>import GHC.Generics>>>:m +Data.Generics.Internal.VL.Lens>>>:m +Data.Function>>>:{data Human a = Human { name :: String , age :: Int , address :: String , other :: a } | HumanNoAddress { name :: String , age :: Int , other :: a } deriving (Generic, Show) human :: Human Bool human = Human { name = "Tunyasz", age = 50, address = "London", other = False } :}
class HasField (field :: Symbol ) s t a b | s field -> a, t field -> b, s field b -> t, t field a -> s where Source #
Records that have a field with a given name.
Methods
field :: Lens s t a b Source #
A lens that focuses on a field with a given name. Compatible with the
lens package's
Lens
type.
>>>human ^. field @"age"50
Type changing
>>>:t humanhuman :: Human Bool
>>>:t human & field @"other" .~ (42 :: Int)human & field @"other" .~ (42 :: Int) :: Human Int
>>>human & field @"other" .~ 42Human {name = "Tunyasz", age = 50, address = "London", other = 42}
Type errors
>>>human & field @"weight" .~ 42... ... The type Human Bool does not contain a field named 'weight'. ...
>>>human & field @"address" .~ ""... ... Not all constructors of the type Human Bool ... contain a field named 'address'. ... The offending constructors are: ... HumanNoAddress ...
Instances
class HasField_ (field :: Symbol ) s t a b where Source #
Records that have a field with a given name.
This is meant to be more general than
HasField
, but that is not quite the
case due to the lack of functional dependencies.
The types
s
and
t
must be applications of the same type constructor.
In contrast,
HasField
also requires the parameters of that type constructor
to have representational roles.
One use case of
HasField_
over
HasField
is for records defined with
data instance
.