CODEWITHSUNDEEP
haskell
Alan Coromano
Alan Coromano

Reputation: 26048

Representing data as a string

I have the following simple code:

data Shape = Circle Float Float Float | Rectangle Float Float Float Float deriving (Show)
surface :: Shape -> Float
surface (Circle _ _ r) = pi * r ^ 2

main = putStrLn $ surface $ Circle 10 20 30

It complains:

Couldn't match expected type `String' with actual type `Float'
In the second argument of `($)', namely `surface $ Circle 10 20 30'

How do I get rid of the error? I also would like to "add" show method to Shape and override it so that I can represent Shape on the screen (printed) whatever I want.

Upvotes: 1

Views: 105

Answers (1)

seanmcl
seanmcl

Reputation: 9946

You need to add show:

main = putStrLn $ show $ surface $ Circle 10 20 30

If you want your own Show method, don't derive Show:

data Shape = Circle Float Float Float
           | Rectangle Float Float Float Float

instance Show Shape where   
  show (Circle _ _ r) = show r   
  show (Rectangle r _ _ _) = show r

main = putStrLn $ show $ Circle 10 20 30

Upvotes: 4

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