letrec

[Razvan Rotaru]

Hi,

Is there a reliable implementation of letrec in clojure? Anybody using
it?
I have found a post from 2008, with an implementation which I don't
understand (and it's said to be slow), and which I don't know whether
to trust.(It's also supposed to be slow).

Thanks,
Razvan

[Kevin Downey]

lazy-seq and letfn should cover anything you would need letrec for

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--
And what is good, Phaedrus,
And what is not good—
Need we ask anyone to tell us these things?

[Razvan Rotaru]

I don't quite understand why people are saying this. Anyway, It's not
enough for me.

[David Nolen]

On Wed, Dec 14, 2011 at 2:53 PM, Razvan Rotaru <razvan...@gmail.com> wrote:

I don't quite understand why people are saying this. Anyway, It's not
enough for me.

What can't you solve your problem with what was suggested?

David 

[Razvan Rotaru]

letfn defines functions. I'm just defining some values. The values
contain anonymous functions which need to refer to other values.I know
there are workarounds for this, but this means I must change the
interface.

Razvan

On Dec 14, 9:56 pm, David Nolen <dnolen.li...@gmail.com> wrote:

[David Nolen]

Do you have a minimal example of what you are trying to do?

[Razvan Rotaru]

Yes. Assuming I have following macros:

(button :id b1 :listener #(...)) => (let [b1 (new JButton)] ...)
(panel [:id p1] (button :id b1 ...) (button :id b2 ...)) => (let [p1
(new JPanel) b1 (button :id b1 ...) b2 (button :id b2 ...)] ...)

How to make the listener in b1 refer to b2?

Razvan

On Dec 14, 11:09 pm, David Nolen <dnolen.li...@gmail.com> wrote:
> Do you have a minimal example of what you are trying to do?
>

[Daniel]

Is 'declare' possibly the missing component here?

[Bobby Eickhoff]

'declare' wouldn't be good because of the scope of vars.  There's no sense using global (albeit namespaced) variables for what probably only need to be local identifiers.

[Nils Bertschinger]

Hi,

to implement letrec in a language with eager evaluation strategy some
kind of mutability is probably needed. Consider for example a self-
referential definition such as

(let [fibo (lazy-cat [1 1] (map + fibo (rest fibo)))]
(take 10 fibo))

This will not work since fibo is not in scope when the binding is
established. The standard solution would probably be something like
this

(let [fibo (promise)]
(deliver fibo (lazy-cat [1 1] (map + @fibo (rest @fibo))))
(take 10 @fibo))

Not as nice as the original version due to explicit dereferencing, but
workable. As an alternative one could use a macro to expand to this
and use a code walker (or symbol-macros) to automatically include the
@ calls. The following is untested, but should be close:

(defmacro letrec [binding & body]
(let [[var expr] binding
g-var (gensym)]
`(let [~g-var (promise)]
(symbol-macrolet [~var @~g-var]
(deliver ~g-var ~expr)
~@body))))

and voila

(letrec [fibo (lazy-cat [1 1] (map + fibo (rest fibo)))]
(take 10 fibo))

produces (1 1 2 3 5 8 13 21 34 55).

Best,

Nils

[Meikel Brandmeyer]

Hi,

you'll probably have to rewrite your panel macro, so that it recognizes the button (and label, etc) macros ((= #'button (resolve &env sym)) in 1.3) and extracts the ids from the subform. Then you construct a global let which contains all the individual id namings. Then you group all '...' parts (from your standalone button example) in the body of the let.

Sincerely
Meikel

[Meikel Brandmeyer]

Hi,

if you always follow the let structure you outlined the following might work. Untested, though. Note, that things work recursively with this approach.

(def ours? #{#'button #'label ...})

(defmacro panel
[{:keys [id]} & components]
(let [[bindings body]
(reduce (fn [[bindings body] component]
(if (and (seq? component)
(ours? (resolve &env (first component))))
(let [[_let local-bindings & local-body]
(macroexpand-1 component)]
[(into bindings local-bindings) (into body local-body)])
; Keep stuff we don't know.
[bindings (conj body component)]))
[[id `(JPanel.)] []]
components)]
`(let ~bindings
~@body
...
~id)))

Sincerely
Meikel