CODEWITHSUNDEEP
stringlisthaskell
Sebastian Paulsen
Sebastian Paulsen

Reputation: 89

Haskell: Convert input string directly to list

Is there any way to convert an input string directly to a list?

The input string is already in the form of a list e.g.:

[((1,2),x),((3,2),y)]

I just want to convert it to [((1,2),x),((3,2),y)] which can be used in another function without having to through the string to get all the values and then make an identical list.

Is this possible?

Upvotes: 3

Views: 599

Answers (2)

dkasak
dkasak

Reputation: 2703

As mentioned, you can use read to convert your String into a value of type [((Int, Int), Animal)], provided Animal is an instance of Read. The instance can be derived automatically:

data Animal = Dog | Cat deriving (Show, Read)

Another potential pitfall is that you should specify the type annotation explicitly so read knows what to parse for:

main :: IO ()
main =
    do let list = read "[((1,2),Dog),((3,2),Cat)]" :: [((Int, Int), Animal)]
        animals = map snd list
    print animals

The output:

$ runhaskell test.hs
[Dog,Cat]

Upvotes: 2

chepner
chepner

Reputation: 530853

You just need to define a Read instance for your type.

> data Animal = Dog | Cat deriving (Read, Show)
> read "[((1,2),Dog),((3,2),Cat)]" :: [((Int, Int), Animal)]
[((1,2),Dog),((3,2),Cat)]

Upvotes: 3

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