Nelson, that explained the case quite nicely. I appreciate it. On Tuesday, August 28, 2012 1:01:32 PM UTC-7, Nelson Morris wrote: > > 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. >
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