Question

I have my own data type:

type Types = String
data MyType = MyType [Types]

I have a utility function:

initMyType :: [Types] -> MyType
initMyType types = Mytype types

Now I create:

let a = MyType ["1A", "1B", "1C"]

How can I get the list ["1A", "1B", "1C"] from a? And in general, how can I get data from a data constructor?

Answer 1

Besides using pattern matching, as in arrowdodger’s answer, you can also use record syntax to define the accessor automatically:

data MyType = MyType { getList :: [Types] }

This defines a type exactly the same as

data MyType = MyType [Types]

but also defines a function

getList :: MyType -> [Types]

and allows (but doesn’t require) the syntax MyType { getList = ["a", "b", "c"] } for constructing values of MyType. By the way, initMyTypes is not really necessary unless it does something else besides constructing the value, because it does exactly the same as the constructor MyType (but can’t be used in pattern matching).