Thank you for your help. Can you elaborate a little more on your
explanation? So, would "a" be some data type representing the atoms of
the grammar? Or some kind of recursive data type that could represent
any kind of valid structure? Or...?

The type parameter "a" is the output of that particular parser. So, a parser for an integer might have type String -> Integer, while a parser for some complicated data type Foo would have type String -> Foo.

I'll wait until I'm clear on that point before asking about how I would
combine parsers. (Presumably, two combined parsers would both have to
have the same "a" type.)

Since the parsers in this scheme are just functions, there are endless ways they could be combined, and the input and output types may or may not match.

Some combinators you will probably want:

andThen :: Parser a -> (a -> Parser b) -> Parser b

orElse :: Parser a -> Parser a -> Parser a

And you might also want:

succeedWith :: a -> Parser a

-Karl