[lpw25]

> Also, has anyone ever used gadts to implement type safe syntax trees like that?

I believe the main use is for embedded DSLs. I'm afraid I don't have any practical examples on hand to link to.

[moomin]

Consider an HTTP request. A get can't have a body, a post can.

So you could define

    data Request b where
      GET :: Headers -> Request ()
      POST :: Headers -> body -> Request body

As the article says, building up your intuition is the hard bit.

You could go further by typing the headers to recognize the type of body you're dealing with.