[Haskell-cafe] Getting started

[Holger Siegel]

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