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The children of a type are the types of its fields. The Children type family maps a type a to its set of children.

This type family is parameterized by a symbol ch (that can be declared as an empty data type). The symbol ChGeneric provides a default definition. You can create new symbols to override the set of children of abstract, non-generic types.

The following example declares a Custom symbol to redefine Children for some abstract types from the time library.

data Custom
type instance Children Custom a = ChildrenCustom a

type family ChildrenCustom (a :: Type) where
  ChildrenCustom DiffTime        = '[]
  ChildrenCustom NominalDiffTime = '[]
  -- Add more custom mappings here.

  ChildrenCustom a = Children ChGeneric a

To use this definition, replace types with typesUsing @Custom.

The default definition of Children. Primitive types from core libraries have no children, and other types are assumed to be Generic.

Since: 1.2.0.0

By adding instances to this class, we can override the default behaviour in an ad-hoc manner. For example:

instance HasTypesCustom Custom Opaque Opaque String String where
  typesCustom f (Opaque str) = Opaque $ f str

Since: 1.2.0.0