fromMaybe -package:xlsx-tabular -package:opaleye -package:Agda

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.

Lift a column over a Maybe. For example, if some people have houses and some do not, the data that pairs them together could be represented as:

>>> :{
let owners :: [(Person,Maybe House)]
owners =
[ (Person "Jordan" 18, Nothing)
, (Person "Ruth" 25, Just (House Red 125000))
, (Person "Sonia" 12, Just (House Green 145000))
]
:}

The column encodings defined earlier can be reused with the help of fromMaybe:

>>> :{
let colOwners :: Colonnade Headed (Person,Maybe House) String
colOwners = mconcat
[ lmap fst colPerson
, lmap snd (fromMaybe "" colHouse)
]
:}

>>> putStr (ascii colOwners owners)
+--------+-----+-------+---------+
| Name   | Age | Color | Price   |
+--------+-----+-------+---------+
| Jordan | 18  |       |         |
| Ruth   | 25  | Red   | $125000 |
| Sonia  | 12  | Green | $145000 |
+--------+-----+-------+---------+

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

A lifted version 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.

Decode an image using the supplied function or throw an error in case of failure.