On 02/12/2013 06:11 PM, Patrick Hurst wrote:
On Tue, Feb 12, 2013 at 10:03 AM, Adam Chlipala <ad...@csail.mit.edu
<mailto:ad...@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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