fromMaybe -package:rattletrap -package:ghc -package:colonnade

The fromMaybe function takes a default value and a Maybe value. If the Maybe is Nothing, it returns the default value; otherwise, it returns the value contained in the Maybe.

Examples

Basic usage:

>>> fromMaybe "" (Just "Hello, World!")
"Hello, World!"

>>> fromMaybe "" Nothing
""

Read an integer from a string using readMaybe. If we fail to parse an integer, we want to return 0 by default:

>>> import Text.Read ( readMaybe )

>>> fromMaybe 0 (readMaybe "5")
5

>>> fromMaybe 0 (readMaybe "")
0

Given a default value and a Maybe, yield the default value if the Maybe argument is Nothing and extract the value out of the Just otherwise.

The fromMaybe function takes a default value and and Maybe value. If the Maybe is Nothing, it returns the default values; otherwise, it returns the value contained in the Maybe.

Examples

Basic usage:

>>> fromMaybe "" (Just "Hello, World!")
"Hello, World!"

>>> fromMaybe "" Nothing
""

Read an integer from a string using readMaybe. If we fail to parse an integer, we want to return 0 by default:

>>> import Text.Read ( readMaybe )

>>> fromMaybe 0 (readMaybe "5")
5

>>> fromMaybe 0 (readMaybe "")
0

The fromMaybe function takes a default value and and Maybe value. If the Maybe is Nothing, it returns the default values; otherwise, it returns the value contained in the Maybe.

Examples

Basic usage:

>>> fromMaybe "" (Just "Hello, World!")
"Hello, World!"

>>> fromMaybe "" Nothing
""

Read an integer from a string using readMaybe. If we fail to parse an integer, we want to return 0 by default:

>>> import Text.Read ( readMaybe )

>>> fromMaybe 0 (readMaybe "5")
5

>>> fromMaybe 0 (readMaybe "")
0

Return the value of an optional value. The default is returned if Nothing. Compare to fromJust.

>>> fromMaybe 2 (sNothing :: SMaybe Integer)
2 :: SInteger

>>> fromMaybe 2 (sJust 5 :: SMaybe Integer)
5 :: SInteger

>>> prove $ \x -> fromMaybe x (sNothing :: SMaybe Integer) .== x
Q.E.D.

>>> prove $ \x -> fromMaybe (x+1) (sJust x :: SMaybe Integer) .== x
Q.E.D.

Convert between Nothing and a default value, or Just and its value (not a true bijection).

fromMaybe default m is the a in m if it exists and the default otherwise

The fromMaybe function takes a default value and and Maybe value. If the Maybe is Nothing, it returns the default values; otherwise, it returns the value contained in the Maybe.

Examples

Basic usage:

>>> fromMaybe "" (Just "Hello, World!")
"Hello, World!"

>>> fromMaybe "" Nothing
""

Read an integer from a string using readMaybe. If we fail to parse an integer, we want to return 0 by default:

>>> import Text.Read ( readMaybe )

>>> fromMaybe 0 (readMaybe "5")
5

>>> fromMaybe 0 (readMaybe "")
0

Monadic generalisation of fromMaybe.

Convert a Maybe value to a concrete value, by suppling a default

Monadic version of fromMaybe.

Translates Maybe to MonadPlus.

A lifted version of fromMaybe.

The Opaleye analogue of fromMaybe

Drop bin with no events

'fromMaybes; is similar to fromList, but allows values to be optional using the Maybe type, so that Nothing results in gaps in the pattern.