view package:rio

view is a synonym for (^.), generalised for MonadReader (we are able to use it instead of (^.) since functions are instances of the MonadReader class):

>>> view _1 (1, 2)
1

When you're using Reader for config and your config type has lenses generated for it, most of the time you'll be using view instead of asks:

doSomething :: (MonadReader Config m) => m Int
doSomething = do
thingy        <- view setting1  -- same as “asks (^. setting1)”
anotherThingy <- view setting2
...

Analyse the left end of a sequence.

Analyse the right end of a sequence.

View of the left end of a sequence.

View of the right end of a sequence.

preview is a synonym for (^?), generalised for MonadReader (just like view, which is a synonym for (^.)).

>>> preview each [1..5]
Just 1

O(log n). Retrieves the value associated with maximal key of the map, and the map stripped of that element, or Nothing if passed an empty map.

maxView (fromList [(5,"a"), (3,"b")]) == Just ("a", singleton 3 "b")
maxView empty == Nothing

O(log n). Retrieves the maximal (key,value) pair of the map, and the map stripped of that element, or Nothing if passed an empty map.

maxViewWithKey (fromList [(5,"a"), (3,"b")]) == Just ((5,"a"), singleton 3 "b")
maxViewWithKey empty == Nothing

O(log n). Retrieves the value associated with minimal key of the map, and the map stripped of that element, or Nothing if passed an empty map.

minView (fromList [(5,"a"), (3,"b")]) == Just ("b", singleton 5 "a")
minView empty == Nothing

O(log n). Retrieves the minimal (key,value) pair of the map, and the map stripped of that element, or Nothing if passed an empty map.

minViewWithKey (fromList [(5,"a"), (3,"b")]) == Just ((3,"b"), singleton 5 "a")
minViewWithKey empty == Nothing

O(log n). Retrieves the maximal key of the set, and the set stripped of that element, or Nothing if passed an empty set.

O(log n). Retrieves the minimal key of the set, and the set stripped of that element, or Nothing if passed an empty set.