[Ur] Using Variant.weaken in a foldUR

[Patrick Hurst]

On Tuesday, February 12, 2013 at 6:40 PM, Adam Chlipala wrote:
> On 02/12/2013 06:11 PM, Patrick Hurst wrote:
> > On Tue, Feb 12, 2013 at 10:03 AM, Adam Chlipala <adamc at csail.mit.edu (mailto:adamc at csail.mit.edu)> wrote:
> > > On 02/12/2013 12:48 AM, Patrick Hurst wrote:
> > > > fun formResultToRows  (fl : folder assns) (result : $(mapU assignmentData assns)) : list assignmentVariant =
> > > >     @foldUR [assignmentData] [fn cols => list (variant (mapU unit cols))]
> > > >     (fn [nam :: Name] [rest ::_] [[nam] ~ rest] res acc =>
> > > >         make [nam] () :: List.mp Variant.weaken acc
> > > >     )
> > > >     [] fl result
> > > In particular, I bet [Variant.fold] from the meta library is just what you need here.
> > > 
> > > Also, if you are building a list of rows to insert into the database, it will probably work better to instead execute one SQL INSERT per fold iteration, so that no intermediate list needs to be materialized.  Something like [Variant.app] would be appropriate for such a pattern.  (Actually, it seems I only implemented a fancier version [Variant.appR] so far, but I bet you could implement the natural [Variant.app] and submit a patch with it. :])
> > 
> > I'm not sure how [Variant.fold] helps me out here, since the thing I'm folding over with the [@foldUR] isn't a variant. I'm fine with inserting something directly into the database inside the fold as opposed to creating the list and then iterating over it, but again I'm not sure how to use [Variant.app]  (what would that be? I'm not sure what the R suffix means exactly) to implement it since I'm not folding over a variant. 
> > 
> > Do you mean I want to use [Variant.app] or [Variant.fold] inside the [@foldUR]? If so, I'm not sure how to do so by directly inserting inside the fold, since the type doesn't guarantee me [nam] is in the relevant variant as far as I can tell, so I can't insert it. 
> I'm suggesting using [Variant.appR] instead of [foldUR], rather than within the function passed to [foldUR].  (I just noticed the parameter [result] that seems not to influence the result above, but probably is meant to influence the result in the real code, so it seems that actually [Variant.appR] is exactly what you need.)
> 
> The [R] suffix means that the fold is over a (value-level) _R_ecord.
After playing around with it a bit [Variant.appR]was indeed the solution; thanks!
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://www.impredicative.com/pipermail/ur/attachments/20130212/3c0d56a9/attachment.html>