在Haskell中使用Maybe类型

Question

I'm trying to utilize the Maybe type in Haskell. I have a lookup for key, value tuples that returns a Maybe. How do I access the data that was wrapped by Maybe? For example I want to add the integer contained by Maybe with another integer.

Answer 1

Alternatively you can pattern match:

case maybeValue of
  Just value -> ...
  Nothing    -> ...

Answer 2

You could use Data.Maybe.fromMaybe , which takes a Maybe a and a value to use if it is Nothing . You could use the unsafe Data.Maybe.fromJust , which will just crash if the value is Nothing . You likely want to keep things in Maybe . If you wanted to add an integer in a Maybe , you could do something like

f x = (+x) <$> Just 4

which is the same as

f x = fmap (+x) (Just 4)

f 3 will then be Just 7 . (You can continue to chain additional computations in this manner.)

Answer 3

Just as a side note: Since Maybe is a Monad , you can build computations using do -notation ...

sumOfThree :: Maybe Int
sumOfThree = do
  a <- someMaybeNumber
  b <- someMaybeNumber
  c <- someMaybeNumber
  let k = 42 -- Just for fun
  return (a + b + c + k)

Answer 4

Examples for "maybe":

> maybe 0 (+ 42) Nothing
0
> maybe 0 (+ 42) (Just 12)
54

Answer 5

Many people are against the use of fromJust , however it can be convenient if you are aware of what will happen when the lookup fails (error!!)

Firstly you will need this:

import Data.Maybe

And then your lookup from a list of tuples will look like this

Data.Maybe.fromJust $ lookup key listOfTuples

For example, successful lookup:

Data.Maybe.fromJust $ lookup "a" [("a",1),("b",2),("c",3)]
1

And horrible failure looks like this:

Data.Maybe.fromJust $ lookup "z" [("a",1),("b",2),("c",3)]
*** Exception: Maybe.fromJust: Nothing

Answer 6

Sorry, I should have googled better.

using the fromMaybe function is exactly what I need. fromMaybe will return the value in Maybe if it is not nothing, otherwise it will return a default value supplied to fromMaybe.

http://www.haskell.org/ghc/docs/6.12.2/html/libraries/base-4.2.0.1/Data-Maybe.html