types package:generic-lens

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.

Derive traversals of a given type in a product.

This function should never be used directly, only to override the default traversal behaviour. To actually use the custom traversal strategy, see typesUsing. This is because typesUsing does additional optimisations, like ensuring that nodes with no relevant members will not be traversed at runtime.

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