Haskell abstract datatypes and retrieving characters/defining as strings

Question

I had a datatype, example:

data MyData = Something1 String

and then I had a function

myFunction :: MyData -> String
myFunction x = x

within myFunction I want to refer to the characters ie ['S','o','m','e'......'1'] which are in my data type MyData. However, I get the following error:

Couldn't match expected type [Char]' with actual typeMyData' Expected type: String Actual type: MyData

As far as I understand [Char] is the same as String, and I have declared 'Something1' as String, so it should work?

hammar · Accepted Answer · 2011-11-07 22:17:19Z

2

[Char] is the same as String, but neither is the same as MyData. To access the string stored within your data type, you'll need to use pattern matching:

myFunction :: MyData -> String
myFunction (Something1 xs) = xs

This is because the data keyword makes a completely new data type. If you only wanted an alias, you could also use the type keyword:

type MyData = String

myFunction :: MyData -> String
myFunction x = x

answered Nov 7, 2011 at 22:17

hammar

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2 Comments

user997112 Over a year ago

I should just tell you my overall problem, i want to create custom data types, but I need to be able to access the characters composing them (like Something1) in my code.

hammar Over a year ago

Ah, I think I see what you're trying to do. No, you cannot access the name of the constructor itself directly, although you can define a function myFunction (Something1 _) = "Something1". A more advanced option is to derive the Data and Typeable type classes. This will allow you to write myFunction x = showConstr (toConstr x), but it's the same thing under the hood, with the compiler creating the boilerplate for you.

Max · Accepted Answer · 2011-11-07 22:22:12Z

2

MyData is not the same as String. It is just very similar.

You can declare a type synonym like this:

type MyData = String

and then MyData and String are two names for the same type. In fact, String is already a type synonym of [Char]. In this case, myFunction is just the identity function id.

Or you can use pattern matching to extract the String from a MyData like this:

myFunction :: MyData -> String
myFunction (Something1 xs) = xs

Alternatively, you can use the record syntax to make the accessor automatically:

data MyData = Something1 { myFunction :: String}

(this is practically identical to declaring myFunction as above, except you can now construct MyDatas using the syntax Something1 { myFunction = x } as well as Something1 x)

answered Nov 7, 2011 at 22:22

Max

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2 Comments

user997112 Over a year ago

Hi Max, if i were to use type MyData = String, could I then declare which abstract types could be part of MyData using constructors? I want to declare MyData only allowed say Object1 | Object2 | Object3 and then being able to access those Objectx's, to retrieve their characters?

Max Over a year ago

No. If you use type MyData = String, then MyData and String are exactly the same. I'm not sure what you mean by "retrieving Objectx's characters" though.

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