data = list() causes expecting JSON error

[bob-carpenter]

Describe the bug

Empty data list causes JSON error.

To Reproduce

race.stan:

functions {
  real foo(int[] y, int start, int end) {
    print("hello, start = ", start, ";  end = ", end);
    return 0;
  }
}
transformed data {
  int N = 1000000;
  int y[N] = rep_array(1, N);
  int granularity = 1;
}
parameters {
  real<lower = 0, upper = 1> theta;
}
model {
  target += reduce_sum(foo, y, granularity);
}

race.R:

library('cmdstanr')
m <- cmdstan_model('race.stan',
                   cpp_options = list(stan_threads = TRUE))
fit <- m$sample(data = list(a = 5),
                chains = 1, threads_per_chain = 1,
                iter_warmup = 100, iter_sampling = 100)

In R:

> source('race.R')
...
Running MCMC with 1 chain, with 1 thread(s) per chain...

Chain 1 expecting JSON object, found array 
Warning: Chain 1 finished unexpectedly!

Warning message:
No chains finished successfully. Unable to retrieve the fit. 

If I change the data line in the script to data = list(a = 1) for a non-existent data item then it runs just fine.

Expected behavior

It treats an empty list as an empty list rather than trying to convert to JSON.

Operating system

> sessionInfo()
R version 4.0.1 (2020-06-06)
Platform: x86_64-apple-darwin17.0 (64-bit)
Running under: macOS Catalina 10.15.7

Matrix products: default
BLAS:   /Library/Frameworks/R.framework/Versions/4.0/Resources/lib/libRblas.dylib
LAPACK: /Library/Frameworks/R.framework/Versions/4.0/Resources/lib/libRlapack.dylib

locale:
[1] en_US.UTF-8/en_US.UTF-8/en_US.UTF-8/C/en_US.UTF-8/en_US.UTF-8

attached base packages:
[1] stats     graphics  grDevices utils     datasets  methods   base     

other attached packages:
[1] cmdstanr_0.2.2

loaded via a namespace (and not attached):
[1] processx_3.4.5  compiler_4.0.1  backports_1.1.8 R6_2.4.1       
[5] tools_4.0.1     checkmate_2.0.0 jsonlite_1.6.1  ps_1.3.3     

[rok-cesnovar]

Thanks. This is because jsonlite, which we use to write the json file, saves [ ] to a file when you pass it an empty list.

For cmdstan to work it should be an empty file, no file or { }. Cmdstan's input parser does not accept [].

jsonlite::write_json(
    list(),
    path = file,
    auto_unbox = TRUE,
    factor = "integer",
    digits = NA,
    pretty = TRUE
  )

stores
[ ]

[jgabry]

@bob-carpenter I think the code you shared is actually the code that runs fine, not the code that produces the warning because you included list(a=5) instead of list(), right? Or am I confused?

# this code you had doesn't result in the error/warning you showed below it
fit <- m$sample(data = list(a = 5),
                chains = 1, threads_per_chain = 1,
                iter_warmup = 100, iter_sampling = 100)

But if I go ahead and change your example to use list() instead of list(a=5) then I can reproduce it.

I think I just assumed that if a model had no data the data argument would be omitted (which works fine), but I definitely agree that it should still work if it's specified as an empty list. Should be an easy fix (@rok-cesnovar we can just immediately convert an empty list to NULL and it should work as if the argument had been omitted).

[rok-cesnovar]

we can just immediately convert an empty list to NULL

Agree.

[bob-carpenter]

I think the code you shared is actually the code that runs fine

Yup. My bad. I meant data = list() causes problems.

[jgabry]

I meant data = list() causes problems.

Ok yeah I can definitely fix that today. Thanks for pointing it out.

[jgabry]

Ok this should be fixed by #403 as soon as it's merged. Thanks again for reporting this.