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, which breaks everything.
My idea was to define the different currencies as their own data types and then add them to the Currency type, which I tried with:
data Euro = MkEuro Integer Integer
data Dollar = MkDollar Integer Integer
data Yen = MkYen Integer
data Currency = Euro | Dollar | Yen
If I want to use the type Currency in any function I get a variation of the following error message:
Couldn't match expected type `Currency' with actual type `Dollar'
Sadly I have to use the type Currency and can't create different functions for all three currencies.
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.
The issue with what you attempted is that the Euro, Dollar and Yen defined on the first 3 lines (on the left-hand-side of the = signs) are types, whereas those in the last line, on the right-hand-side, are data constructors. These are different things, which is why you're allowed to give them the same names - but also means that the value Dollar has the type Currency, not the type Dollar.
You had the right idea to create a union type combining the other three - but you are confusing the data constructors (which are just "tags") with the types of the values they hold. You can fix it like this:
data Currency = Euro Euro | Dollar Dollar | Yen Yen
That might look confusing to have the same name twice (three times, too) - but the two occurrences of the name have different meanings. In Euro Euro, for example, the first Euro is a data constructor (a function which "tags" the value), while the second is the type on which the constructor function operates.
