The
maybe function takes a default value, a function, and a
Maybe value. If the
Maybe value is
Nothing, the
function returns the default value. Otherwise, it applies the function
to the value inside the
Just and returns the result.
Examples
Basic usage:
>>> maybe False odd (Just 3)
True
>>> maybe False odd Nothing
False
Read an integer from a string using
readMaybe. If we succeed,
return twice the integer; that is, apply
(*2) to it. If
instead we fail to parse an integer, return
0 by default:
>>> import Text.Read ( readMaybe )
>>> maybe 0 (*2) (readMaybe "5")
10
>>> maybe 0 (*2) (readMaybe "")
0
Apply
show to a
Maybe Int. If we have
Just n,
we want to show the underlying
Int n. But if we have
Nothing, we return the empty string instead of (for example)
"Nothing":
>>> maybe "" show (Just 5)
"5"
>>> maybe "" show Nothing
""