Haskell return Either Double Bool

Question

I have simple code

delta a b c = b*b - (4*a*c)

mz1 :: Double -> Double -> Double -> Double
mz1 a b c = (-b - sqrt(delta a b c)) / (2*a)

mz2 :: Double -> Double -> Double -> Double
mz2 a b c = (-b + sqrt(delta a b c)) / (2*a)

mz0 :: Double -> Double -> Double -> Double
mz0 a b c = (-b) / (2*a)

kwad :: (Enum a) => a -> a -> a -> Either Double Bool
kwad a b c 
    | delta a b c == 0 = Left mz0 a b c
    | delta a b c < 0 = Right False
    | otherwise = Left mz1 a b c

I'm just starting to learn Haskell and I have no idea how to return either Bool or Double, could anybody help?

Error is:

Couldn't match expected type `a -> a -> a -> Either Double Bool'
            with actual type `Either (Double -> Double -> Double -> Double) >b0'
The function `Left' is applied to four arguments,
but its type `(Double -> Double -> Double -> Double)
              -> Either (Double -> Double -> Double -> Double) b0'
has only one

Answer 1

Your kwad function has three parameters of type Enum a => a. In other words, they can take any a as long as it is enumerable. Your mz functions expects their values to be of type Double. I would suggest changing the type signature of kwad so it takes Double arguments instead, like so:

kwad :: Double -> Double -> Double -> Either Double Bool

This guarantees that the values you put into kwad are Doubles, and therefore suitable for your mz functions as well!

---

You also need to wrap the argument to Left in parentheses, like so:

Left (mz0 a b c)

This means that you want to put the entire mz0 a b c result into a Left, and not anything else.

Answer 2

It is not about how to correct your code, but to show what is the main (but not only) purpose of Either a b.

As a rule it is used as Right answer , yes, like right answer and Left error_message - like something is not right.

Sure, this meaning of Either a b it is only inside the brain, mathematically Left is the same as Right. But not every time. For example, standard instance Monad (Either a) uses Left as error.

So, more pleasant code would be Either Bool Double in you example.

But ...

If you want to write something like Left doesn'tMatter or more simply Left () it is another data for this case - Maybe a.

You could rewrite your example to ... -> Maybe Double:

| wrong_pattern -> Nothing
| right_pattern -> Just (mz0 a b c)