[plt-scheme] redex: meta-function with alternatives

I think you want something like this for the case with two options:

(pick   ;; try that first
 (lookup os_2 f (store (l_3 os_3) ...))

 ;; if above fails (exception or #f) then try that:
 (lookup os_1 f (store (l_3 os_3) ...)))

and then to use this pick metafunction:

(define-metafunction foo
  [(pick #f any) any]
  [(pick any_1 any_2) any_1])

hth,
Robby

On Fri, Sep 11, 2009 at 6:46 PM, Eric Tanter <etanter at dcc.uchile.cl> wrote:
> Hi,
>
> I want to define a simple recursive metafunction, but which has the
> particularity of needing to somehow backtrack. It is doing a lookup in
> terms, but some terms have two branches, and if the first one fails, it
> should go in the second one.
>
> I see two options to do it:
> - use exception handling (the "not found in first branch" case being caught
> as a "no match for clause...")
> - have a base case returns #f, and then somehow distinguish whether the
> recursive call returned a normal term or #f, if normal term return it,
> otherwise go through second branch.
>
> I actually can't figure out how to make either case work... I imagine the
> latter solution is cleaner than exceptions, but couldn't make it work (with
> term-let).
>
> This is a simplified version of what I have:
>
> (define-metafunction foo
>  lookup : os f S -> any
>  ;; lookup f in os
>  ; os = (def f v)
>  ((lookup (def f v) f (store (l os) ...)) v)
>
>  ; 'fail case'
>  ((lookup (def f_1 v) f_2 (store (l os) ...)) #f
>   (side-condition (not (eq? (term f_1) (term f_2)))))
>
>  ; os = (with os1 os2)
>  ((lookup (with os_1 os_2) f (store (l_3 os_3) ...))
>
>    ;; try that first
>     (lookup os_2 f (store (l_3 os_3) ...)
>
>    ;; if above fails (exception or #f) then try that:
>     (lookup os_1 f (store (l_3 os_3) ...))
> )
>
> any idea?
>
> Thanks in advance,
>
> -- Éric
>
>
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