Using Maybe type in Haskell

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.

This is a duplicate: http://stackoverflow.com/questions/3375483/operating-on-a-return-from-a-maybe-that-contains-just/3375712#3375712 — Thomas M. DuBuisson, Sep 04 '10 at 19:16

score 45 · Accepted Answer · answered Sep 04 '10 at 18:09

45

Alternatively you can pattern match:

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

answered Sep 04 '10 at 18:09

Martijn

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This was more like what I was trying to do than the answer I had found. Thanks – dpsthree Sep 07 '10 at 05:22
2

What should I say in the type declaration? – qed Dec 24 '14 at 21:03

score 25 · Answer 2 · edited Dec 05 '16 at 05:52

25

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.)

edited Dec 05 '16 at 05:52

duplode

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answered Sep 04 '10 at 16:58

arsenm

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`f 3` will be `Just 7`, not `7`. – Yitz Sep 04 '10 at 23:55

score 9 · Answer 3 · edited Sep 05 '10 at 00:08

9

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)

edited Sep 05 '10 at 00:08

jrockway

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answered Sep 04 '10 at 18:21

Dario

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score 8 · Answer 4 · answered Sep 06 '10 at 09:25

8

Examples for "maybe":

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

answered Sep 06 '10 at 09:25

LennyStackOverflow

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score 5 · Answer 5 · answered Sep 04 '10 at 16:57

5

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

answered Sep 04 '10 at 16:57

dpsthree

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Yeah but make sure that is the behaviour that you want, because if you are going to, for example, use a Maybe Int and then every time that it is Nothing you return zero then you should have just used a plain old Int right from the start. Maybe is supposed to help encode the absence of a result or value and not really supposed to be worked around. Personally I try to steer as clear of the fromMaybe function. – Robert Massaioli Sep 11 '10 at 03:44

score 4 · Answer 6 · answered Sep 24 '13 at 10:14

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

Using Maybe type in Haskell

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