Data types basics

[Thomas L. Bevan]

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