> On Tue, Aug 28, 2012 at 1:08 PM, Armando Blancas <abm2...@gmail.com<javascript:>>
> wrote:
> > I'm playing around with a parser combinator library from the paper
> Monadic
> > Parser Combinators by Hutton and Meijer [1] and came up with this:

> > https://gist.github.com/3501273

> > That's just enough to show the error I'm getting when (expr) calls
> (factor):

> > Clojure 1.4.0
> > user=> (load-file "expr.clj")
> > #'user/expr
> > user=> (run expr "(5)")
> > IllegalStateException Attempting to call unbound fn: #'user/expr
> > clojure.lang.Var$Unbound.throwArity (Var.java:43)

> > To avoid this error, I coded a new version of the parser (factor) but in
> > this case I use inline calls to (bind) instead of using parser
> (between),
> > which makes it work:

> > Now using (parser*) inside the definition of (expr): (def expr (bind
> facfor*
> > ...)

> > Clojure 1.4.0
> > user=> (load-file "expr.clj")
> > #'user/expr
> > user=> (run expr "(5)")
> > 5

> > I thought it was a bug but this may have to do with the forward
> declaration
> > of "expr" and when is deref'ed. After much trying I can't see why this
> won't
> > work:

> > (def factor
> >   (choice (between (match \() (match \)) expr)
> >  integer))

> (def factor ...) immediately gets the value of the body to assign to
> the var #'factor.  (choice ...) is a function, so has to get the
> values of it's arguments before invoking.  This means it gets the
> value of expr, which unbound and uses that.

> > But this will:

> > (def factor*
> >   (choice (bind (match \() (fn [_]
> >           (bind expr       (fn [e]
> >  (bind (match \)) (fn [_] (result e)))))))
> >  integer))

> This occurs similar to the above, except it hits a fn.  fn is a
> "special form" defined to wait to get the value of its body until it
> is executed.  By that time the (def expr ...) has occurred and #'expr
> has a bound value.  A similar effect can be achieved with

> (def factor
>   (choice (between (match \() (match \)) (fn [x] (expr x)))
>               integer))

> I ran into the same thing re-exploring that paper in clojure.