In Haskell, data types and contructors must be designated by names whose first letter is capitalised. So,

data currency = ...

is illegal. Instead use,

data Currency = Dollar Double | Pound Double | Zloty Double | Euro Double

Above are four data constructors and they can be used to contruct a datum of the type Currency. They can be exploited in functions through pattern matching as follows,

toDollar :: Currency -> Currency toDollar (Pound d) = Dollar ( d / 2 ) toDollar (Euro d) = Dollar ( d / 1.01 ) ...

I think, to answer your last question, that they are functions, however they can only be defined using a much more limited syntax. I'm sure someone else can give a fuller answer here. Tom