[racket] Using `dynamic-require` for optional user-supplied "template" code?

Every function is jitted the first time it is called if you're running on a
platform that the JIT supports (currently x86 and ARM).

Robby


On Thu, May 23, 2013 at 1:25 PM, Greg Hendershott <greghendershott at gmail.com
> wrote:

> p.s.
>
> > Also: Am I correct that the user-supplied function gets JIT-ed, and
> > there's no big performance hit from this?  (I think the answer is no,
>
> That question had mixed polarities. :)  I should have written, I think
> the answer is No there's no big performance hit, and Yes it gets
> JIT-ed.
>
> On Thu, May 23, 2013 at 2:22 PM, Greg Hendershott
> <greghendershott at gmail.com> wrote:
> > I've never had a reason to use `dynamic-require`, but now I think I do.
> >
> > For my Frog static blog generator, I'd like to provide a light
> > "template" feature, where the user can override the default layout of
> > the main container <div> in the page.
> >
> > I have an existing function that returns an xexpr for this. So my idea
> > is to let the user optionally supply a `template.rkt` file that
> > `provide`s a function named (say) `container` that returns (listof
> > xexpr?).
> >
> > So I think what I want to do is something like this:
> >
> > (define (container-proc)
> >   (define p (build-path (src-path) "template.rkt"))
> >   (cond [(file-exists? p)
> >          (dynamic-require p
> >                           'container
> >                           (thunk default-container))]
> >         [else default-container]))
> >
> > The built-in `default-container` looks something like this:
> >
> > (define (default-container bootstrap-row-class ;"row" or "row-fluid"
> >                            bodies              ;listof xexpr?: main
> content
> >                            tocs                ;listof xexpr?: TOC
> >                            tags/feeds          ;listof xexpr?: tag/feed
> links
> >                            follow)             ;listof xexpr?: Twitter,
> etc.
> >   `((div ([class ,bootstrap-row-class])
> >          ;; Left column
> >          (div ([id "left-sidebar"]
> >                [class "span2 bs-docs-sidebar"])
> >               , at tocs
> >               (p nbsp))
> >          ;; Main column
> >          (div ([id "content"]
> >                [class "span8"])
> >               , at bodies)
> >          ;; Right column
> >          (div ([id "right-sidebar"]
> >                [class "span2"])
> >               ,@(tags/feeds)
> >               ,@(follow)))))
> >
> > The user-supplied template.rkt would provide some variation on it. For
> > example they don't want any left column with a TOC, so they supply
> > this:
> >
> > ;; template.rkt
> > #lang racket
> > (provide container)
> > (define (container bootstrap-row-class ;"row" or "row-fluid"
> >                    bodies              ;listof xexpr?: main content
> >                    tocs                ;listof xexpr?: TOC
> >                    tags/feeds          ;listof xexpr?: tag/feed links
> >                    follow)             ;listof xexpr?: Twitter, etc.
> >   `((div ([class ,bootstrap-row-class])
> >          ;; Don't want any left column; ignore `tocs`
> >          ;; Main column
> >          (div ([id "content"]
> >                [class "span9"]) ;wider
> >               , at bodies)
> >          ;; Right column: Tags/feeds/follow
> >          (div ([id "right-sidebar"]
> >                [class "span3"]) ;wider
> >               ,@(tags/feeds)
> >               ,@(follow)))))
> >
> > In a quick experiment this seems to work fine. I could also go further
> > and make this a tiny s-exp #lang, to make the end user experience even
> > simpler. But meanwhile, and before that, I just want to make sure that
> > `dynamic-require` is a reasonable way to do this?  Does anyone
> > disagree?
> >
> > Also: Am I correct that the user-supplied function gets JIT-ed, and
> > there's no big performance hit from this?  (I think the answer is no,
> > because from reviewing Racket's own source, I got the impression that
> > all modules are built on dynamic-require, but I might have
> > understood.)
> >
> > Thank you in advance for any advice or suggestions!
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