Here is the data type:

data Fruit = Apple | Banana | Orange
data Basket = Basket Fruit Int

now in the code ,there is a list of Basket which could have any of Fruit inside. How to filter out all the Baskets with Apple in it ? in Python there is a function type or instanceof that may help but not sure how Haskell can handle this ?

baskets = [Baseket (Apple 10),Baseket (Banana 2),Baseket (Apple 6)]

CodePudding user response：

First, there are a couple of typos in your code. If Basket is meant to have a fruit and and integer inside it, then it should be

data Basket = Basket Fruit Int

Second, the constructor for Basket should be called with two arguments and no parentheses around the arguments, so your list is

baskets :: [Basket]
baskets = [Basket Apple 10, Basket Banana 2, Basket Apple 6]

Finally, you use the word "filter" so I suspect you've already surmised that the operation you're looking for is the filter function.

filter :: (a -> Bool) -> [a] -> [a] 

In your case, a ~ Basket. So we need to give it a mapping from baskets to Bool which returns true if (and only if) the basket has apples in it. Since we're comparing fruits for equality, we need an Eq instance on Fruit (technically, we could pattern match, but since Fruit is clearly an enumeration to begin with, it should have Eq, Ord, Enum, and all of the nice instances).

data Fruit = Apple | Banana | Orange deriving (Eq)

Now our filter.

appleBaskets :: [Basket]
appleBaskets = filter (\(Basket fruitType _) -> fruitType == Apple) baskets

We destructure the Basket inside of the lambda using pattern matching and then compare the fruit type against the Apple value.