On Fri, Mar 29, 2013 at 2:22 AM, Jamie F Olson
None of the tutorials/manuals were entirely clear to me, nor was the wiki(http://www.haskell.org/haskellwiki/Constructor), which uses the same statement as the example of both type and data constructors.
Indeed, it's a very common gotcha: JSObject is a type constructor in the sense that it takes a type and returns a type. It so happens that JSObject is also a data constructor that takes a value and returns another value of the datatype of which it is a constructor. Why is this ambiguity allowed? Because JSObject as type constructor can't be used at the term level. You can only write type signatures with it, you can't use it in an expression. JSObject as data constructor is the reverse. It's a function like any other. In this case, it has type signature JSObject :: JSObject JSValue -> JSValue. Note how JSObject after the double colon is JSObject as the one-parameter type constructor (see newtype definition). Confusing, eh? After a bit of practice, it should be clear that this is a means of conserving the namespace. You can always start with a different naming convention on your own code until you get used to the separation between types and terms (which does tend to become muddled as dependent types loom, but that's a story for later). -- Kim-Ee