CODEWITHSUNDEEP
haskell
Czarek
Czarek

Reputation: 679

Haskell: Function which use user definied data type

I have written following data type and its instance in Haskell:

data Plaster = Plaster [String] deriving (Read, Show)

examplePlaster :: Plaster
examplePlaster = Plaster ["..AEG", "..CD", "DC...", "A.B.", "..EFG"]

I would like to create a function, which applicated to examplePlaster returns the first String on a list. How should I do that?

Upvotes: 0

Views: 75

Answers (2)

chepner
chepner

Reputation: 532208

Like any other type, you can pattern match on the value created by your data constructor.

getFirst :: Plaster -> Maybe String
getFirst (Plaster (x:xs)) = Just x
getFirst (Plaster []) = Nothing

Upvotes: 6

moondaisy
moondaisy

Reputation: 4481

You could use the head function from Data.List

getFirst :: Plaster -> String
getFirst (Plaster []) = "" 
getFirst (Plaster xs) = head xs

Where head :: [a] -> a extract the first element of a list, which must be non-empty. As explained here.

And to call it do: getFirst examplePlaster

Upvotes: 1

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