Safe Haskell | Safe |
---|---|
Language | Haskell2010 |
When I use Data.Aeson library to decode a json string into a Haskell value,
I want to provide default values for Maybe
fields, which will be Nothing
when these fields are omitted in the json string. It's a hard work when there
are lots of Maybe
fields or deeply nested fields, it will also make your code
hard to read and to maintain. This module provides a solution by using
Higher-kinded data (HKD).
See this blog
for more information about HKD
.
For example, if you have a Config
type as follows,
data Config = Config { dbHost :: String , dbPort :: Int , dbName :: String ... }
and you want to read these configuration data from a json file when you start
you application, you instantiate Data.Aeson.FromJSON for the Config
,
data Config = Config { dbHost :: String , dbPort :: Int , dbName :: String ... } deriving Generic instance FromJSON Config
and you want dbPort
can be omitted in the json string, a default value will be
used when it is omitted, you change String
type to Maybe String
,
data Config = Config { dbHost :: String , dbPort :: Maybe Int , dbName :: String ... } deriving Generic instance FromJSON Config
and decode and use the configuration data in main function as below,
main = do config <- fromJust <$> decodeFileStrict "./config.json" let host = dbHost config port = fromMaybe defaultDBPort $ dbPort config ... dbConn <- connectDB host port ... ...
it is neither elegant nor easy to maintain when you have lots of configuration items.
By using HKD
and type family
, it becomes easier to maintain your code.
type family HKD f a where HKD Identity a = a HKD f a = f a data ConfigH f = Config { dbHost = String , dbPort = HKD f String ... } deriving Generic instance Default ConfigH instance FromJSON (ConfigH Maybe) type Config = Config Identity instance FromJSON Config where parseJSON v = applyDef def <$> parseJSON v where def = Config undefined 3306 ... main = do -- Enable RecordWildCards extension Config{..} <- fromJust <$> decodeFileStrict "./config.json" dbConn <- connectDB dbHost dbPort ...
More Examples
>>>
:set -XDeriveGeneric
>>>
:set -XFlexibleInstances
>>>
:set -XStandaloneDeriving
>>>
import Data.Functor.Identity
>>>
import GHC.Generics
>>>
import HKD.Default
>>>
:{
data Triple f = Triple String (f Int) (f Double) deriving Generic instance Default Triple deriving instance Show (Triple Identity) :}>>>
let def = Triple "hello" (Identity 123) pi :: Triple Identity
>>>
applyDef def $ Triple "world" (Just 456) Nothing
Triple "world" (Identity 456) (Identity 3.141592653589793)
>>>
:set -XDeriveGeneric
>>>
:set -XStandaloneDeriving
>>>
:set -XFlexibleInstances
>>>
:set -XOverloadedStrings
>>>
import Data.Aeson
>>>
import Data.Functor.Identity
>>>
import GHC.Generics
>>>
import HKD.Default
>>>
:{
data Name f = Name { first :: f String , last_ :: f String } deriving Generic instance Default Name deriving instance Show (Name Identity) instance FromJSON (Name Maybe) data Person f = Person { name :: Name f -- name is required , age :: f Int -- age is optional (can be omitted) } deriving Generic instance Default Person deriving instance Show (Person Identity) instance FromJSON (Person Maybe) instance FromJSON (Person Identity) where parseJSON v = applyDef def <$> parseJSON v where def = Person (Name (Identity "Jorah") (Identity "Gao")) (Identity 28) :}>>>
decode "{\"name\": {}}" :: Maybe (Person Identity)
Just (Person {name = Name {first = Identity "Jorah", last_ = Identity "Gao"}, age = Identity 28})>>>
decode "{}" :: Maybe (Person Identity)
Nothing
>>>
:set -XDeriveGeneric
>>>
:set -XFlexibleInstances
>>>
:set -XFlexibleContexts
>>>
:set -XStandaloneDeriving
>>>
:set -XTypeFamilies
>>>
import Data.Functor.Identity
>>>
import GHC.Generics
>>>
import HKD.Default
>>>
:{
type family HKD f a where HKD Identity a = a HKD f a = f a data Shape f = Square (HKD f Double) | Circle (HKD f Double) deriving Generic deriving instance Show (Shape Identity) instance Default Shape where defs = [("Square", Square 1.0), ("Circle", Circle 1.0)] data Container f = Container { base :: HKD f (Shape f) , height :: HKD f Double } deriving Generic deriving instance Show (Container Identity) instance Default Container :}>>>
let def = Container (Square 10.0) 10.0
>>>
applyDef def $ Container Nothing Nothing
Container {base = Square 10.0, height = 10.0}>>>
applyDef def $ Container (Just $ Square Nothing) Nothing
Container {base = Square 10.0, height = 10.0}>>>
applyDef def $ Container (Just $ Circle Nothing) Nothing
Container {base = Circle 1.0, height = 10.0}>>>
applyDefs $ Square Nothing
Square 1.0>>>
applyDefs $ Circle Nothing
Circle 1.0
Documentation
class Default (t :: (* -> *) -> *) where Source #
In most cases, use the default implementation for Generic
instance.
Nothing
defs :: [(String, t Identity)] Source #
Only used for datatypes with multiple data constructors,
default implementation is []
.
Since: 1.1.0
lookupDef :: String -> Maybe (t Identity) Source #
You should either provide lookupDef
or defs
, default implementation
is to look up by the given constructor name from defs
.
Since: 1.1.0
applyDef :: t Identity -> t Maybe -> t Identity Source #
Apply the given default value, and fallback to applyDefs
if the
default value's constructor does not match.
applyDef :: (Generic (t Identity), Generic (t Maybe), GConsName (Rep (t Maybe)), GDefault (Rep (t Identity)) (Rep (t Maybe))) => t Identity -> t Maybe -> t Identity Source #
Apply the given default value, and fallback to applyDefs
if the
default value's constructor does not match.
applyDefs :: t Maybe -> t Identity Source #
Look up the appropriate default value from defs and try to apply.
The default implementation will raise "Can't find default value" error when
the result of looking up from defs
is Nothing
.
The default implementation will raise "Mismatch Constructor" error when the default value's constructor does not match.
Since: 1.1.0
applyDefs :: (Generic (t Identity), Generic (t Maybe), GConsName (Rep (t Maybe)), GDefault (Rep (t Identity)) (Rep (t Maybe))) => t Maybe -> t Identity Source #
Look up the appropriate default value from defs and try to apply.
The default implementation will raise "Can't find default value" error when
the result of looking up from defs
is Nothing
.
The default implementation will raise "Mismatch Constructor" error when the default value's constructor does not match.
Since: 1.1.0