Re: [Haskell-cafe] Getting started

Yes there are a few more problems. Instead of point out each one here is some modified code that does compile. Compare it to what you have and let me know if you have questions: type Title = String type Artist = String type Sold = Int type Record = (Title, (Artist, Sold)) testDatabase :: [Record] testDatabase = [("Jack", ("Waters", 2))] newRecord :: Record -> [Record] -> [Record] newRecord record dbase = [record] ++ dbase getTitle :: Record -> Title getTitle (title,_) = title addSale :: Record -> Record addSale (title, (artist,sold)) = (title, (artist, sold + 1)) recordSale :: Record -> [Record] -> [Record] recordSale r@(title, (artist,sold)) sales = case (Prelude.lookup title sales) of Nothing -> error "This record does not exist" Just x -> [addSale r] ++ (filter (\a -> (getTitle a) /= title) sales) addRecord :: IO Record addRecord = do putStrLn "Please enter a title: " title <- getLine putStrLn "Please enter an artist name: " artist <- getLine putStrLn "Please enter the number sales: " sales <- getInt return (title , (artist, sales)) where getInt :: IO Int getInt = getLine >>= return . read On Mon, Jul 5, 2010 at 11:36 AM, Mrwibbly wrote:

I changed that line to say type Sales = Sales Record. But unfortunately it still fails to compile. Do you have any idea why this might be the case?

-Jack

aditya siram-2 wrote:

...

You said it didn't compile. I somehow missed that , sorry. -deech

On Mon, Jul 5, 2010 at 11:06 AM, aditya siram wrote:

...

Does this code compile? The line  "type Sales = Sales Record" for instance is wrong - it should be "data Sales = Sales Record". Additionally "recordSale" returns an Int, not [Sales].

-deech

On Mon, Jul 5, 2010 at 10:50 AM, Mrwibbly wrote:

...

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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