[Haskell-beginners] Data declarations

[Christopher Howard]

I'm trying to understand the Haskell type system, because it is
portrayed as a rather powerful tool when properly harnessed. There are a
few things, however, I could use some clarification on. For starters,
I'm a bit confused about the "data" declarations:

I understand (correctly, I hope) that the purpose of a "data"
declaration is to create a new type. So this makes sense:

data KeyValuePair = KeyValuePair String String

However, this also compiles:

data KeyValuePair a b = KeyValuePair String String

As well as does this:

data KeyValuePair a b = KeyValuePair String

And this:

data KeyValuePair a b = KeyValuePair

So you can see how I'm getting a bit confused?

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