Data.ProtoLens.Field

A simple interface supporting overloaded record fields.

An example instance:

data Foo = Foo { _bar :: Bool }
type instance Field "bar" Foo = Bool

instance HasField "bar" Foo Foo where
    field _ = lens _bar (\f x -> f { _bar = x })

bar :: HasField "bar" a a => Lens a b Bool Bool
bar = field (ProxySym :: ProxySym "bar")

The proto compiler defines separate instances of HasField for each field of each record. It defines the accessors (e.g., the lens bar above) once per unique field name, shared between different message types within the same module.

Overloaded fields

type family Field (s :: Symbol) a

The type of the field s in a message of type a.

class HasField s a b | a -> b, b -> a where

A class for overloaded fields.

• s is a Symbol representing the field name.
• a is the message type for getting this field
• b is the message type after setting this field.

Currently a and b are the same in all generated instances. In the future, when we support type-checking of required fields we'll need different a and b; for more details see go/proto-lens.

Minimal complete definition

field

Methods

field :: ProxySym s -> Lens a b (Field s a) (Field s b)

data ProxySym s

A "proxy" value for a type-level string (i.e., a type of kind Symbol).

Constructors

ProxySym

Helper lenses

maybeLens :: b -> Lens' (Maybe b) b

A helper lens for accessing optional fields. This is used as part of code generation, and should generally not be needed explicitly.

Note that maybeLens does not satisfy the lens laws, which expect that set l (view l x) == x. For example,

set (maybeLens 'a') (view (maybeLens 'a') Nothing) == Just 'a'

However, this is the behavior generally expected by users, and only matters if we're explicitly checking whether a field is set.