Lens.Micro.Extras

This module provides functions and types that are inferior to their lens counterparts.

Documentation

view :: Getting a s a -> s -> a

view is a synonym for (^.):

>>> view _1 (1, 2)
1

The reason it's in this module is that view in lens has a more general signature:

view :: MonadReader s m => Getting a s a -> m a

So, you would be able to use this view with functions, but not in various reader monads. For most people this shouldn't be an issue.

preview :: Getting (First a) s a -> s -> Maybe a

preview is a synonym for (^?):

>>> preview _head [1,2,3]
Just 1

The reason it's in this module is that preview in lens has a more general signature:

preview :: MonadReader s m => Getting (First a) s a -> m (Maybe a)

So, just like with view, you would be able to use this preview with functions, but not in reader monads.

type Getter s a = forall r. Getting r s a

A Getter s a extracts a from s; so, it's the same thing as (s -> a), but you can use it in lens chains. For details, see Getting.

The reason it's in this module is that the actual Getter from lens is more general:

type Getter s a =
  forall f. (Contravariant f, Functor f) => (a -> f a) -> s -> f s

I'm not currently aware of any functions that take lens's Getter but won't accept this Getter, but you should try to avoid exporting Getters anyway to minimise confusion.

Lens users: you can convert fake getters to real getters by applying to . view to them.

type Fold s a = forall r. Applicative (Const r) => Getting r s a

A Fold s a extracts several as from s; so, it's pretty much the same thing as (s -> [a]), but you can use it with lens operators.

The reason it's in this module is that the actual Fold from lens is more general:

type Fold s a =
  forall f. (Contravariant f, Applicative f) => (a -> f a) -> s -> f s

I'm only aware of 2 functions that accept lens's Fold but won't accept this Fold – they are foldByOf and foldMapByOf. They aren't used often, but you should try to avoid exporting Folds anyway to minimise confusion (and to prevent people cursing you when they want to use foldByOf with your fold).

Lens users: you can convert fake folds to real folds by applying folded . toListOf to them.