I'm relatively new to Haskell and began to read "Real World Haskell".
I Just stumbled over the type Maybe and have a question about how to receive the actual value from a Just 1 for example.
I have written the following code:
combine a b c = (eliminate a, eliminate b, eliminate c)
where eliminate (Just a) = a
eliminate Nothing = 0
This works fine if I use:
combine (Just 1) Nothing (Just 2)
But if I change, for example, 1 to a String it doesn't work.
I think I know why: because eliminate has to give back one type, which is, in this case, an Int. But how can I change eliminate to deal at least with Strings (or maybe with all kind of types)?
[edit from Author, 6 years later] This is a needlessly long answer, and I'm not sure why it was accepted. Use maybe or Data.Maybe.fromMaybe as suggested in the highest upvoted answer. What follows is more of a thought experiment rather than practical advice.
So you're trying to create a function that works for a bunch of different types. This is a good time to make a class. If you've programmed in Java or C++, a class in Haskell is kind of like an interface in those languages.
class Nothingish a where
nada :: a
This class defines a value nada, which is supposed to be the class's equivalent of Nothing. Now the fun part: making instances of this class!
instance Nothingish (Maybe a) where
nada = Nothing
For a value of type Maybe a, the Nothing-like value is, well, Nothing! This will be a weird example in a minute. But before then, let's make lists an instance of this class too.
instance Nothingish [a] where
nada = []
An empty list is kind of like Nothing, right? So for a String (which is a list of Char), it will return the empty string, "".
Numbers are also an easy implementation. You've already indicated that 0 obviously represents "Nothingness" for numbers.
instance (Num a) => Nothingish a where
nada = 0
This one will actually not work unless you put a special line at the top of your file
{-# LANGUAGE FlexibleInstances, UndecidableInstances, OverlappingInstances #-}
Or when you compile it you can set the flags for these language pragmas. Don't worry about them, they're just magic that makes more stuff work.
So now you've got this class and these instances of it...now let's just re-write your function to use them!
eliminate :: (Nothingish a) => Maybe a -> a
eliminate (Just a) = a
eliminate Nothing = nada
Notice I only changed 0 to nada, and the rest is the same. Let's give it a spin!
ghci> eliminate (Just 2)
2
ghci> eliminate (Just "foo")
"foo"
ghci> eliminate (Just (Just 3))
Just 3
ghci> eliminate (Just Nothing)
Nothing
ghci> :t eliminate
eliminate :: (Nothingish t) => Maybe t -> t
ghci> eliminate Nothing
error! blah blah blah...**Ambiguous type variable**
Looks great for values and stuff. Notice the (Just Nothing) turns into Nothing, see? That was a weird example, a Maybe in a Maybe. Anyways...what about eliminate Nothing? Well, the resultant type is ambiguous. It doesn't know what we are expecting. So we have to tell it what type we want.
ghci> eliminate Nothing :: Int
0
Go ahead and try it out for other types; you'll see it gets nada for each one. So now, when you use this function with your combine function, you get this:
ghci> let combine a b c = (eliminate a, eliminate b, eliminate c)
ghci> combine (Just 2) (Just "foo") (Just (Just 3))
(2,"foo",Just 3)
ghci> combine (Just 2) Nothing (Just 4)
error! blah blah Ambiguous Type blah blah
Notice you still have to indicate what type your "Nothing" is, or indicate what return type you expect.
ghci> combine (Just 2) (Nothing :: Maybe Int) (Just 4)
(2,0,4)
ghci> combine (Just 2) Nothing (Just 4) :: (Int, Int, Int)
(2,0,4)
Or, you could restrict the types that your function allows by putting its type signature explicitly in the source. This makes sense if the logical use of the function would be that it is only used with parameters of the same type.
combine :: (Nothingish a) => Maybe a -> Maybe a -> Maybe a -> (a,a,a)
combine a b c = (eliminate a, eliminate b, eliminate c)
Now it only works if all three Maybe things are the same type. That way, it will infer that the Nothing is the same type as the others.
ghci> combine (Just 2) Nothing (Just 4)
(2,0,4)
No ambiguity, yay! But now it is an error to mix and match, like we did before.
ghci> combine (Just 2) (Just "foo") (Just (Just 3))
error! blah blah Couldn't match expected type blah blah
blah blah blah against inferred type blah blah
Well, I think that was a sufficiently long and overblown answer. Enjoy.
From the standard Prelude,
maybe :: b -> (a -> b) -> Maybe a -> b
maybe n _ Nothing = n
maybe _ f (Just x) = f x
Given a default value, and a function, apply the function to the value in the Maybe or return the default value.
Your eliminate could be written maybe 0 id, e.g. apply the identity function, or return 0.
From the standard Data.Maybe,
fromJust :: Maybe a -> a
fromJust Nothing = error "Maybe.fromJust: Nothing"
fromJust (Just x) = x
This is a partial function (does not return a value for every input, as opposed to a total function, which does), but extracts the value when possible.
I'm new to Haskell too, so I don't know if this exists in the platform yet (I'm sure it does), but how about a "get or else" function to get a value if it exists, else return a default?
getOrElse::Maybe a -> a -> a
getOrElse (Just v) d = v
getOrElse Nothing d = d
This is the answer I was looking for when I came to this question:
https://hackage.haskell.org/package/base-4.9.0.0/docs/Data-Maybe.html#v:fromJust
...and similarly, for Either:
https://hackage.haskell.org/package/either-unwrap-1.1/docs/Data-Either-Unwrap.html
They provide functions I would have written myself which unwrap the value from its context.
The eliminate function's type signature is:
eliminate :: Maybe Int -> Int
That's because it returns 0 on Nothing, forcing the compiler to assume that a :: Int in your eliminate function. Hence, the compiler deduces the type signature of the combine function to be:
combine :: Maybe Int -> Maybe Int -> Maybe Int -> (Int, Int, Int)
and that's precisely why it doesn't work when you pass a String to it.
If you wrote it as:
combine a b c = (eliminate a, eliminate b, eliminate c)
where eliminate (Just a) = a
eliminate Nothing = undefined
then it would have worked with String or with any other type. The reason relies on the fact that undefined :: a, which makes eliminate polymorphic and applicable to types other than Int.
Of course, that's not the aim of your code, i.e., to make the combine function total.
Indeed, even if an application of combine to some Nothing arguments would succeed (that's because Haskell is lazy by default), as soon as you try to evaluate the results you would get a runtime error as undefined can't be evaluated to something useful (to put it in simple terms).
来源：https://stackoverflow.com/questions/4940349/how-to-get-the-value-of-a-maybe-in-haskell