Re: [Haskell-cafe] Getting started

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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