Building lists of data types in Haskell

Question

I'm really confused about how your supposed to get data out of typeclasses in haskell. I'm coming for a C background so finding it really difficult that you can't just access the data. What I have is something like this:

data MyType = MyType String deriving (Show)


display :: [MyType] -> IO ()
display ((MyType name):xs) = do
       display xs
       putStr name

Basically here I want to access 'name' however it just doesn't seem to work. Can I access the data within an instance of a typeclass by just having a reference to the object in my code or do I have to map its contents to variables? and if so how?

Links to good tutorials on this would be appreciated, I've read 'Learn you a Haskell for great good' but when I try to deviate from the examples given there always seems to be to much I need to know to get it done. -A

Don Stewart · Accepted Answer · 2011-05-04 19:42:00Z

6

I think you might just be missing some little pieces that tie it all together.

Firstly, you have a perfectly fine data type, MyType, that holds strings:

data MyType = MyType String deriving (Show)

Now, you want to write a function that walks a list of such type, printing each element as it goes. We do this via recursion over the list data type.

Since lists have two cases, the empty list, [], and the cons case, (:), we have two branches:

display :: [MyType] -> IO ()
display []                 = return ()
display ((MyType name):xs) = do
       putStrLn name
       display xs

Now, where I think you might have go stuck was building some data of this type. You already know how to take it apart with pattern matching, and you build the data using almost the same syntax. Here's a list of MyType:

table = 
    [ MyType "john"
    , MyType "don"
    , MyType "eric"
    , MyType "trevor"
    ]

Finally, you can run your program from main

main = display table

Note, there's no typeclasses here, just algebraic data types (introduced with data).

answered May 4, 2011 at 19:42

Don Stewart

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1 Comment

lesolorzanov Over a year ago

What about having a function that dynamically adds MyType s to the list. Imagine a function that processes something and I take some strings that I convert into MyType and I have to add them to a global list of MyTypes, how can I do that?

fuz · Accepted Answer · 2011-05-04 19:45:35Z

1

First of all, I am a bit confused about the words you use. A typeclass is a way to overload functions. What you have is an algebraic data type. The problem you have (if I understood it correctly) is well known. For the purpose of accessing data easier, you could use record syntax:

data Foo = Foo {
    bar :: Int
  , baz :: String
}

Do you see the similarity to a struct in C? Using record syntax, some interesting things are possible:

bar y -- access member bar of y
y { bar = z } -- Make a new record but with field bar changed to the value of z

and some other thing too.

answered May 4, 2011 at 19:45

fuz

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1 Comment

pat Over a year ago

Record syntax departs from C struct syntax because every field gives rise to a function of the same name for extracting that field from an instance of the data type. In the case above, you will get bar :: Foo -> Int and baz :: Foo -> String. As a result, you cannot create another record in the same module with fields named bar or baz since you can't have more than one type for the resulting function.

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