Left package:base

Feed marked inputs through the argument arrow, passing the rest through unchanged to the output.

Any instance of ArrowApply can be made into an instance of ArrowChoice by defining left = leftApp.

Extracts from a list of Either all the Left elements. All the Left elements are extracted in order.

Examples

Basic usage:

>>> let list = [ Left "foo", Right 3, Left "bar", Right 7, Left "baz" ]

>>> lefts list
["foo","bar","baz"]

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

Examples

Basic usage:

>>> fromLeft 1 (Left 3)
3

>>> fromLeft 1 (Right "foo")
1

Return True if the given value is a Left-value, False otherwise.

Examples

Basic usage:

>>> isLeft (Left "foo")
True

>>> isLeft (Right 3)
False

Assuming a Left value signifies some sort of error, we can use isLeft to write a very simple error-reporting function that does absolutely nothing in the case of success, and outputs "ERROR" if any error occurred. This example shows how isLeft might be used to avoid pattern matching when one does not care about the value contained in the constructor:

>>> import Control.Monad ( when )

>>> let report e = when (isLeft e) $ putStrLn "ERROR"

>>> report (Right 1)

>>> report (Left "parse error")
ERROR