How do I define a data type consisting of other data types?

Question

I want to define the data type Currency, which consists of three other data types. I have the problem that Haskell doesn\'t recognize the data types as a part of currency, w

Answer 1

Currently your Currency is constructed as three values, that take no parameters. So Euro is a value, Dollar is a value, and Yen is a value, but not MkYen 15.

You can add parameters to your data constructors like:

data Currency = Euro Euro | Dollar Dollar | Yen Yen

Then you thus can construct a Currency with:

Euro (MkEuro 14 25) :: Currency

The :: Currency is not necessary. The MkEuro will thus construct an object of the Euro type, and then we use the Euro data constructor with type Euro -> Currency to construct a Currency.

Sadly I have to use the type Currency and can't create different functions for all three currencies.

You might want to make a Currency typeclass however, that provides an interface that says what functions a currency should implement, and then make Euro, Dollar, and Yen instances of that typeclass.