fromRight -package:optics-core

Return the contents of a Right-value or a default value otherwise.

Examples

Basic usage:

>>> fromRight 1 (Right 3)
3

>>> fromRight 1 (Left "foo")
1

Extract the right value or a default.

fromRight b ≡ either (const b) id

>>> fromRight "hello" (Right "world")
"world"

>>> fromRight "hello" (Left 42)
"hello"

Take a Right to a value, crashes on a Left

Extracts the element out of a Right and throws an error if the argument is a Left.

Return the contents of a Right-value or a default value otherwise. @since base-4.10.0.0

Examples

Basic usage:

>>> fromRight 1 (Right 3)
3

>>> fromRight 1 (Left "foo")
1

The fromRight function extracts the element out of a Right and throws an error if its argument take the form Left _.

Extracts value from Right or return given default value.

>>> fromRight 0 (Left 3)
0

>>> fromRight 0 (Right 5)
5

Return the value from the right component. The behavior is undefined if passed a left value, i.e., it can return any value.

>>> fromRight (sRight (literal 'a') :: SEither Integer Char)
'a' :: SChar

>>> prove $ \x -> fromRight (sRight x :: SEither Char Integer) .== (x :: SInteger)
Q.E.D.

>>> sat $ \x -> x .== (fromRight (sLeft (literal 2) :: SEither Integer Char))
Satisfiable. Model:
s0 = 'A' :: Char

Note how we get a satisfying assignment in the last case: The behavior is unspecified, thus the SMT solver picks whatever satisfies the constraints, if there is one.

Analogue of fromMaybe.

Construct a VEither from an a. Exists for symmetry with fromLeft. Indeed, this is just another name for VRight (and for pure).

The fromRight' function extracts the element out of a Right and throws an error if its argument is Left. Much like fromJust, using this function in polished code is usually a bad idea.

\x -> fromRight' (Right x) == x
\x -> fromRight' (Left  x) == undefined

Extracts the element out of a Right and throws an error if its argument take the form Left _. Using Control.Lens:

fromRight' x ≡ x^?!_Right

>>> fromRight' (Right 12)
12

Return the contents of a Right'-value or errors out.

Analogue of fromMaybeM.

This is the same as from rightMonoidalUpdate but doesn't depend on the lens Library.

fromRight but with a better error message if it fails. Use this only where it shouldn't fail!

Pad a builder from the right side to the specified length with the specified character.

>>> padFromRight 5 ' ' "123"
"123  "

>>> padFromRight 5 ' ' "123456"
"123456"