Moduletext-display-0.0.5.2Haskell2010

Data.Text.Display.Generic

Generic machinery for automatically deriving display instances for record types

2 types
2 classes
1 value

Packagetext-display-0.0.5.2
Exports6
LanguageHaskell2010
LicenceMIT
SourceGeneric.hs

classclass GDisplay1 (f :: Type -> Type) where

Generic typeclass machinery for inducting on the structure of the type, such that we can thread Display instances through the structure of the type. The primary use case is for implementing RecordInstance, which does this "threading" for record fields. This machinery does, crucially, depend on child types (i.e. the type of a record field) having a Display instance.

Methods

gdisplayBuilder1 :: f p -> Builder

Instances8GDisplay1, …

GDisplay1 U1Defined in text-display-0.0.5.2 · Data.Text.Display.Generic
GDisplay1 V1Defined in text-display-0.0.5.2 · Data.Text.Display.Generic
Display c => GDisplay1 (K1 i c)Defined in text-display-0.0.5.2 · Data.Text.Display.Generic
This is the most important instance, it can be considered as the "base case". It requires a non-generic Display instance. All this generic machinery can be conceptualized as distributing these displayBuilder calls across a product type.
(GDisplay1 a, GDisplay1 b) => GDisplay1 (a :*: b)Defined in text-display-0.0.5.2 · Data.Text.Display.Generic
(GDisplay1 a, GDisplay1 b) => GDisplay1 (a :+: b)Defined in text-display-0.0.5.2 · Data.Text.Display.Generic
GDisplay1 f => GDisplay1 (M1 D s f)Defined in text-display-0.0.5.2 · Data.Text.Display.Generic
(Constructor c, GDisplay1 f) => GDisplay1 (M1 C c f)Defined in text-display-0.0.5.2 · Data.Text.Display.Generic
(Selector s, GDisplay1 f) => GDisplay1 (M1 S s f)Defined in text-display-0.0.5.2 · Data.Text.Display.Generic

valuegdisplayBuilderDefault :: (Generic a, GDisplay1 (Rep a)) => a -> Builder

newtypenewtype RecordInstance a

This wrapper allows you to create an Display instance for a record, so long as all the record fields have a Display instance as well.

Example

data Password = Password
 deriving Display
   via (OpaqueInstance "[REDACTED]" Password)

data MyRecord =
   MyRecord
     { fieldA :: String
     , fieldB :: Maybe String
     , fieldC :: Int
     , pword :: Password
     }
     deriving stock (Generic)
     deriving (Display) via (RecordInstance MyRecord)

putStrLn . Data.Text.unpack . display $ MyRecord "hello" (Just "world") 22 Password

MyRecord
  { fieldA = hello
  , fieldB = Just world
  , fieldC = 22
  , pword = [REDACTED]
  }

Constructors

RecordInstance
- unDisplayProduct :: a

Instances3Generic, Display, Rep

Generic a => Generic (RecordInstance a)Defined in text-display-0.0.5.2 · Data.Text.Display.Generic
(AssertNoSumRecordInstance Display a, Generic a, GDisplay1 (Rep a)) => Display (RecordInstance a)Defined in text-display-0.0.5.2 · Data.Text.Display.Generic
We leverage the AssertNoSum type family to prevent consumers from deriving instances for sum types. Sum types should use a manual instance or derive one via ShowInstance.
type Rep (RecordInstance a) = Rep aDefined in text-display-0.0.5.2 · Data.Text.Display.Generic

familytype family HasSum (f :: Type -> Type) :: Bool where

This type family is lifted from generic-data. We use it to prevent the user from deriving a RecordInstance for sum types

Equations

HasSum V1 = 'False
HasSum U1 = 'False
HasSum (K1 i c) = 'False
HasSum (M1 i c f) = HasSum f
HasSum (f :*: g) = HasSum f || HasSum g
HasSum (f :+: g) = 'True

classclass Assert (pred :: Bool) (msg :: ErrorMessage)

Instances2Assert

TypeError msg ~ '() => Assert 'False msgDefined in text-display-0.0.5.2 · Data.Text.Display.Generic
Assert 'True msgDefined in text-display-0.0.5.2 · Data.Text.Display.Generic

type

type AssertNoSumRecordInstance (constraint :: Type -> Constraint) a = Assert (Not (HasSum (Rep a))) ((((('Text "\128683 Cannot derive " ':<>: 'ShowType constraint) ':<>: 'Text " instance for ") ':<>: 'ShowType a) ':<>: 'Text " via RecordInstance due to sum type") ':$$: 'Text "\128161 Sum types should use a manual instance or derive one via ShowInstance.")

Constraint to prevent misuse of RecordInstance deriving via mechanism.

Example

data MySum = A | B | C deriving stock (Generic) deriving (Display) via (RecordInstance MySum)

   • 🚫 Cannot derive Display instance for MySum via RecordInstance due to sum type
     💡 Sum types should use a manual instance or derive one via ShowInstance.
   • When deriving the instance for (Display MySum)