Traverse all types in the given structure.

For example, to update all Strings in a WTree (Maybe String) String, we can write

>>> myTree = WithWeight (Fork (Leaf (Just "hello")) (Leaf Nothing)) "world"
>>> over (types @String) (++ "!") myTree
WithWeight (Fork (Leaf (Just "hello!")) (Leaf Nothing)) "world!"

The traversal is deep, which means that not just the immediate children are visited, but all nested values too.

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: generic-lens-core-1.2.0.0

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: generic-lens-core-1.2.0.0