[Ur] Using Variant.weaken in a foldUR

[Adam Chlipala]

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.
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