^?

Perform a safe head of a Fold or Traversal or retrieve Just the result from a Getter or Lens. When using a Traversal as a partial Lens, or a Fold as a partial Getter this can be a convenient way to extract the optional value. Note: if you get stack overflows due to this, you may want to use firstOf instead, which can deal more gracefully with heavily left-biased trees. This is because ^? works by using the First monoid, which can occasionally cause space leaks.

>>> Left 4 ^?_Left
Just 4

>>> Right 4 ^?_Left
Nothing

>>> "world" ^? ix 3
Just 'l'

>>> "world" ^? ix 20
Nothing

This operator works as an infix version of preview.

(^?) ≡ flip preview

It may be helpful to think of ^? as having one of the following more specialized types:

(^?) :: s -> Getter s a     -> Maybe a
(^?) :: s -> Fold s a       -> Maybe a
(^?) :: s -> Lens' s a      -> Maybe a
(^?) :: s -> Iso' s a       -> Maybe a
(^?) :: s -> Traversal' s a -> Maybe a

s ^? t returns the 1st element t returns, or Nothing if t doesn't return anything. It's trivially implemented by passing the First monoid to the getter. Safe head:

>>> [] ^? each
Nothing

>>> [1..3] ^? each
Just 1

Converting Either to Maybe:

>>> Left 1 ^? _Right
Nothing

>>> Right 1 ^? _Right
Just 1

A non-operator version of (^?) is called preview, and – like view – it's a bit more general than (^?) (it works in MonadReader). If you need the general version, you can get it from microlens-mtl; otherwise there's preview available in Lens.Micro.Extras.

Flipped infix version of preview.

Returns Just the first referenced value. Returns Nothing if there are no referenced values.

(^?) :: s -> Fold s t a b -> Maybe a

Returns Just the first referenced value. Returns Nothing if there are no referenced values.

Deprecated: Use corresponding function from lens or microlens package

q ^? x. if the result of x isPrefixOf q, return True

Calculate the exclusive or of two booleans with confidence lower bounds.

Perform an *UNSAFE* head of a Fold or Traversal assuming that it is there.

>>> Left 4 ^?! _Left
4

>>> "world" ^?! ix 3
'l'

(^?!) :: s -> Getter s a     -> a
(^?!) :: s -> Fold s a       -> a
(^?!) :: s -> Lens' s a      -> a
(^?!) :: s -> Iso' s a       -> a
(^?!) :: s -> Traversal' s a -> a

(^?!) is an unsafe variant of (^?) – instead of using Nothing to indicate that there were no elements returned, it throws an exception.

Perform an *UNSAFE* head of an affine fold assuming that it is there.

>>> Left 4 ^?! _Left
4

>>> "world" ^?! ix 3
'l'

>>> [] ^?! _head
*** Exception: (^?!): empty affine fold
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