This would be more fun if you provided definitions for tie and rank (and
others you use) in an addon.
Without that, it feels like deus ex machina (and does not execute for me
like it does for you).
Thanks,
--
Raul
On Thu, Feb 20, 2014 at 6:06 PM, Jose Mario Quintana <
jose.mario.quint...@gmail.com> wrote:
> "
> Anyway, thought I'd share, and see if anyone else has useful tools for
> manipulating J code (metaprogramming).
> "
>
> Dan, I will try to show some meta-programming exercises soon. I am jumping
> into the darkness hoping that I will land well; so far, so good. In the
> meantime, the following is a F^4 underground verb corresponding to Raul's
> explicit verb (using verbalized primitive conjunctions):
>
> cf=. {.@(tie&'')@(rank&_)"0 f.
>
> 100"_`(200"_)`(300"_) -: cf 100 200 300
> 1
>
>
>
> On Thu, Feb 20, 2014 at 2:39 PM, Dan Bron <j...@bron.us> wrote:
>
> > There was a question on StackOverflow today on generating constant
> > functions from the items of an array. As in, 100 200 300 <==>
> > 100"_`(200"_)`(300"_) .
> >
> > As a more general question, how can we specify code templates and then
> use
> > them to dynamically produce gerunds for later execution? *
> >
> > Here's one idiom I've found useful in such situations:
> >
> > substit =: [^:(__-:])L:_ 0
> >
> > template =: __"_ NB. The template; note the __
> > TEMPLATE =: {. template f.`'' NB. Atomic rep of template
> >
> > GERUND =: TEMPLATE substit"_1 _~ 100 * 1 + i.3
> >
> > GERUND`:6
> > (100"_) (200"_) 300"_
> >
> > GERUND@.0
> > 100"_
> >
> > GERUND@.2 'hello'
> > 300
> >
> > The basic idea is to "fill in the blanks" in the template with the
> inputs.
> > Here, the blank is __ (negative infinity), so we look for that in the
> > gerund template, and when we find it, we replace it with the input. I
> kind
> > of like how the verb does this: first, we only bother looking at level 0
> > (because __ is unboxed, so it will never appear above level 0), and
> > second, the verb is passive until __ is detected (because ^:(]-:__) is
> > ^:0, i.e. "do nothing and just return the RHA" until then).
> >
> > One possible issue is what we use to denote "blank". If we're going to
> > allow users to pass in any gerund, then he could potentially want to
> > include __ as part of the code to be produced, rather than replaced.
> There
> > are a couple possible ways to address that limitation. First, we could
> use
> > a value which a user is very unlikely to want to produce (as opposed to
> > replace), i.e. _. :
> >
> > substit =: [^:(1-:128!:5@])L:_ 0
> > SPARTA =: ({. _."_ `'') substit"_1 _~ 300
> > SPARTA`:6
> > 300"_
> >
> > Or, we could define substit as an adverb (adjective), which takes the
> > "blank" as an argument:
> >
> > Substit =: (-:&) (@]) ([^:) (L:_ 0)
> >
> > substit =: 'blank' Substit
> > substit
> > [^:(-:&'blank'@])L:_ 0
> >
> > ; , ((< {. 'blank'&;`'') substit~&.> ;:'fill in the blanks' )`:0 '!'
> > fill!in!the!blanks!
> >
> > But if we do that, we have to start wondering about the 0 in L:_ 0 ,
> > because the user's "blank" might be boxed, and therefore have L.>0 . Of
> > course, we could fix this up by making the L: dynamic:
> >
> > Substit =: adverb : '[^:(m-:])L:(_,L.m)'
> >
> > NOTNULL =: ({. ~:&a:` '') a: Substit~ <'#N/A#'
> > NOTNULL`:6
> > ~:&(<'#N/A#')
> >
> > But it might be more fun to go meta-meta:
> >
> > (({. substit f.`'') substit~ _999)`:6
> > [^:(_999 -: ])L:_ 0
> >
> > Here, define the "standard" blank to be __, and then let the user change
> it
> > by using substit :)
> >
> > Anyway, thought I'd share, and see if anyone else has useful tools for
> > manipulating J code (metaprogramming).
> >
> > -Dan
> >
> > * For a recent example of where such metaprogramming might be useful, or
> > convenient, see the postscript of:
> >
> >
> > http://www.jsoftware.com/pipermail/programming/2013-December/034373.html
> >
> > Other examples can be found with a forum archive search for the string
> L:0
> > _
> > ----------------------------------------------------------------------
> > For information about J forums see http://www.jsoftware.com/forums.htm
> >
> ----------------------------------------------------------------------
> For information about J forums see http://www.jsoftware.com/forums.htm
>
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