如何在 haskell 中返回同一 function 中的不同类型？

Question

I cannot understand why, although I have declared the data types, the function does not accept the different return. I also tried to create a "Result" type but it was not enough to solve the problem.

Take a look at my code:

type Set a = [a]
type Bag a = [(a, Int)]
type Seq a = [a]

data ComposedType a = Set a | Bag [a] | Seq a deriving(Eq, Show)
data ComposedResult a = Res1 [a] | Res2 [(a, Int)]

-- This function get a list [a] and returns a list of tuples [(a, Int)]
occrs :: (Ord a) => [a] -> [(a, Int)]
occrs [] = []
occrs xs = toList (fromListWith (+) [(x, 1) | x <- xs])

--getElements :: ComposedType a -> ComposedResult a
getElements (Set a) = (Set (nub a))
getElements (Seq a) = (Seq (sort a))
getElements (Bag a) = (Bag (occrs a))

The error:

• Couldn't match type ([a], Int) with '[a]' Expected type: ComposedType [a] Actual type: ComposedType ([a], Int)

• In the expression: (Bag (occrs a)) In an equation for 'getElements': getElements (Bag a) = (Bag (occrs a))

Answer 1

The reason this does not work is because Set (num a) for example has type ComposedType [a] , whereas for the Bag (occrs) , it will require ComposedTYpe (a, Int) , since the Bag uses, in contrast to the other data constructors a list of a s: [a] .

If you make use of a ComposedResult , then you will need to use the Res1 and Res2 data constructors:

getElements :: (Eq a, Ord a) => ComposedType [a] -> ComposedResult a
getElements (Set b) =  (nub b)
getElements (Seq b) =  (sort b)
getElements (Bag b) =  (occrs b)

but this will still not work. Indeed Bag a has a parameter b , but that b has as type [[a]] , and although occrs can work with a list of lists (since these are also an instance of Ord if the element is an instance of ord , this would mean that occrs b has type [([a], Int)] whereas the others ( nub b and sort b ), have type [a] , so that would again raise trouble.

We can fix this by concatenating for example the elements of b together:

getElements :: (Eq a, Ord a) => ComposedType [a] -> ComposedResult a
getElements (Set b) = Res1 (nub b)
getElements (Seq b) = Res1 (sort b)
getElements (Bag ) = Res2 (occrs ())

but likely you made a mistake by specifying [a] as parameter for bag, you thus can fix the type and use:

data ComposedType a = Set a | Bag  | Seq a deriving (Eq, Show)

getElements :: (Eq a, Ord a) => ComposedType [a] -> ComposedResult a
getElements (Set b) = Res1 (nub b)
getElements (Seq b) = Res1 (sort b)
getElements (Bag ) = Res2 (occrs )