One thing I haven't seen mentioned here is that web development is often not a one-man endeavor. At its most basic, you commonly have separation between front- and back-end. So unless you plan on doing it all yourself, you will need to find a front-end developer who is either familiar with Clay, JMacro and Fay; or who is willing to learn; and frankly, I don't think that's a very likely scenario, given the scant usage of Haskell as a web development language.

Also, the example of the compiled JavaScript from Fay is horribly obfuscated:

  return Fay$$_(Fay$$_(
            Fay$$cons)(i))(Fay$$_(
            Prelude$enumFrom)(
            Fay$$_(Fay$$_(
                Fay$$add)(i))(1)
        ));

Oh, God, my eyes. Any idea at all what that does? I can tell you one thing: I would not want to debug that on a Friday night. Or a Tuesday afternoon. Or ever, really.

Kind of have to agree with a few of the other commenters here who are saying things along the lines of "You can do anything with a hammer, if a hammer is what you have."

Also: this reminds me of a "LISP for Web Development" talk I went to a few years ago. The speaker was talking to a bunch of web devs about how LISP has an undeserved reputation for being domain specific (academia, astronomy, whatever). He then went on to explain how you could use it for calculating the results of some kind of physics experiment and output it as an HTML table. Uh-huh. My buddy and I walked out after about 25 minutes.

Am I being unfair?

>> As an example of we’ll use JMacro to implement a simple translator for the untyped typed lambda calculus.

Nope.