[Haskell-cafe] Getting started

[aditya siram]

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

On Mon, Jul 5, 2010 at 11:06 AM, aditya siram <aditya.siram at gmail.com> 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 <Jackwaters89 at googlemail.com> 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
>>>
>>> _______________________________________________
>>> Haskell-Cafe mailing list
>>> Haskell-Cafe at haskell.org
>>> http://www.haskell.org/mailman/listinfo/haskell-cafe
>>>
>>>
>>
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