Allowed delayed expressions, not just variables

[richfitz]

This is required for the malaria folk and it introduces quite a few new complexities.

After this is in, I can start on delayed delays, though first I'll probably do a bit of a triage through the odin1 tests to see what we actually do with delays

[weshinsley]

There's one typo in the instructions - and a couple of comments to help my understanding.

For a future ticket maybe - the error could be better if I try to do multiple delays on the same line -

dat <- odin_parse({
  deriv(x) <- a
  deriv(y) <- 2
  initial(x) <- 0
  initial(y) <- 0
  a <- (x + y) / 2
  c <- delay(a, 1) + delay(a, 2)
  output(z) <- c
})
Error in `odin_parse()`:
! Unsupported function 'delay'
→ Context:
c <- delay(a, 1) + delay(a, 2)
ℹ For more information, run odin2::odin_error_explain("E1027")

And we'll have some doc tickets for vignettes (eg, functions) / odin-monty later...

[weshinsley]

So does this mean:

We can't write a model that says... depending on how fat our mosquitos have grown, they will... excrete what they ate f(size) time ago - where f(size) varies in the model....

(Would anyone want such a thing...)

[richfitz]

yeah, that's right. This sort of non-constant-time delay could theoretically be interesting I'm sure, but it creates some issues in the solving and I did not add support for that into the way that delays were implemented in dust

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (04475c1) to head (7fc9c2c).
Report is 13 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main      #129      +/-   ##
===========================================
+ Coverage   99.85%   100.00%   +0.14%     
===========================================
  Files          20        20              
  Lines        3421      3504      +83     
===========================================
+ Hits         3416      3504      +88     
+ Misses          5         0       -5     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[richfitz]

the error could be better if I try to do multiple delays on the same line

Fixed now by 1d9ce17, we have a nice hook for this that I've reused