On Tue, Aug 28, 2012 at 1:08 PM, Armando Blancas <[email protected]> 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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