CODEWITHSUNDEEP
haskell
user3097712
user3097712

Reputation: 1675

Parse error on input 'Show'

i have written the follwing code:

data Genre = Nonfiction | Novel | Biography deriving (Eq, Show)

type Name = (String, String)
type Date = (Int, Int, Int)
type Book = ABook Genre
        Name 
        String
        Date 
        Int
deriving Show

genre :: Book -> Genre 
genre (ABook g _ _ _ _) = g

author :: Book -> Name 
author (ABook _ a _ _ _) = a

title :: Book -> String 
title (ABook _ _ t _ _) = t

date :: Book -> Date
date (ABook _ _ _ d _) = d

pages :: Book -> Int
pages (ABook _ _ _ _ p) = p

year :: Book -> Int 
year b = let (_ , _ , b) = date b in y


publishedIn :: Int -> [Book] -> [Book]
publishedIn x xs = filter findYear xs 

findYear:: Book -> Bool
findYear x | year x == 2014 = True
           | otherwise      = False

The functions from genre until year are helpfunctions which I try to include in the higher order functions like filter etc. But the compiler shows me the following error message:

Parse error on input 'Show'

What can I do against that ?

Upvotes: 3

Views: 3094

Answers (2)

Sibi
Sibi

Reputation: 48664

There are number of issues with your code:

1) type is used for creating new name for an existing type. But in your case, this will result in compilation error because you are trying to define new data constructor here:

type Book = ABook Genre Name String Date Int deriving (Show)

So, use the data keyword to create proper type and data constructor. i.e:

data Book = ABook Genre Name String Date Int deriving Show

2) There is a problem with your function year :

year :: Book -> Int     
year b = let (_ , _ , b) = date b in y

As you see, there is no y there. Probably you meant something like this:

year b = let (_ , _ , y) = date b in y

With that being said, you should use the record syntax instead of manually creating helper function.

data Book = ABook {
  genre :: Genre,
  author :: Name,
  title :: String,
  date :: Date
}

Here, you get free accessor functions named genre , author, etc without writing them yourself.

Upvotes: 6

David Young
David Young

Reputation: 10783

type defines a type synonym and what you want is a new data type. You can do this by replacing type Book = ... with data Book = .... Also, there needs to be at least one space before deriving since it is part of the data type definition.

Upvotes: 2

Related Questions