[Haskell-cafe] Getting started

[Mrwibbly]

This really helped, but now I am trying to add a new track to the database
using a menu but it won't compile. I have tried a lot of different things
but to no avail. 

When I get rid of the menu I am able to run, for example, newRecord "This
Charming Man" "The Smiths" 1 []

This adds the data to an empty database but I can't seem to call newRecord
again and add another record to the existing database.

Thanks for your help previously,

Jack

type Title = String
type Artist = String
type Sold = Int
type Sales = Sales Record
type Record = (Title, Artist, Sold)


testDatabase :: [Sales]
testDatabase = [(Sales "Jack" "Waters" 2)]

--recordSale :: Sales -> String -> String -> Sales
--recordSale title artist = (title, artist)

newRecord :: Record -> [Sales] -> [Sales]
newRecord title artist sold dbase = (title, artist, sold):dbase

recordSale :: Record -> [Sales]
recordSale record sales = sold + 1
 
main :: [Sales] -> IO()
main dbase = do 
	putStrLn "1 = Add a new record: "
	input <- getLine 
	let x = read input :: Int
	if x == 1 
		then do putStrLn "Please enter a title: "
			title <- getLine
			putStrLn "Please enter an artist name: "
			artist <- getLine
			putStrLn "Please enter the number sales: "
			sales <- getInt
			newRecord (Sales title artist sales []) dbase


Holger Siegel wrote:
> 
> 
> Am 01.07.2010 um 21:56 schrieb Mrwibbly:
> 
>> 
>> I'm having real trouble starting this project. Basically I have to create
>> a
>> record store that store information about artists and albums and also the
>> number of sales that they have had. It also needs to generate a list of
>> the
>> top 5 sellers.
>> 
>> So far I have: recordSale :: Sales -> String -> String -> Sales
>> 
>> where recordSale sales anArtist aTrack returns a modified version of the
>> sales. 
>> 
>> Any help getting started on this would be gratefully received. I don't
>> want
>> answers, I just want help getting started.
> 
> First, I would state explicitly what a record is: It is a tuple of an
> artist's name and a record's name
> 
>   type Record = (String, String)
> 
> Now function recordSale has type
> 
>   recordSale :: Sales -> Record -> Sales
> 
> This is the an "uncurried" equivalent of your definition. You can read it
> as "from a sales object you get to another sales object via a (sold)
> record". That already seems to be a good abstraction, but we can do
> better: If you flip the arguments, you get
> 
>   recordSale :: Record -> Sales -> Sales
> 
> Now you can create a sale (recordSale ("Zappa", "Apostrophe")). This sale
> is a function of type (Sales -> Sales) that modifies your sales. We state
> this by defining
> 
>   type Sale = Sales -> Sales
> 
>   recordSale :: Record -> Sale
> 
> Sales can be concatenated with the dot operator (.) and there is even a
> "neutral sale", the function 'id'. Thus, you know immediately that for any
> sales x,y,z there is (x . (y . z) == (x . y) . z) and (x . id == x). In
> other words, it forms a monoid - just like the number of sales together
> with (+) and 0!
> 
> If you're only interested in the number of sales, you can simply define
> 
> type Sales = Integer
> 
> recordSale record sales = sales + 1
> 
> But you don't want to keep track of the whole number of sales - you want a
> number for every record you have sold. That means, you need a data
> structure that maps records to their number of sales:
> 
> import Data.Map
> 
> type Sales = Map Record Integer
> 
> It's a bit tricky to find an implementation for recordSale. Think of how
> you can combine two arbitrary Sales objects before you try to implement
> it.
> 
> Regards, Holger
> 
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> 

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