void value discards or ignores the result of
evaluation, such as the return value of an
IO action.
Examples
Replace the contents of a
Maybe Int with
unit:
>>> void Nothing
Nothing
>>> void (Just 3)
Just ()
Replace the contents of an
Either Int
Int with unit, resulting in an
Either
Int '()':
>>> void (Left 8675309)
Left 8675309
>>> void (Right 8675309)
Right ()
Replace every element of a list with unit:
>>> void [1,2,3]
[(),(),()]
Replace the second element of a pair with unit:
>>> void (1,2)
(1,())
Discard the result of an
IO action:
>>> mapM print [1,2]
1
2
[(),()]
>>> void $ mapM print [1,2]
1
2