If `model_save_dir` does not exist, an error is thrown with a suboptimal message

[brookslogan]

Following the README example:

library(DelphiRF)
library(ggplot2)
library(dplyr)
#> 
#> Attaching package: 'dplyr'
#> The following objects are masked from 'package:stats':
#> 
#>     filter, lag
#> The following objects are masked from 'package:base':
#> 
#>     intersect, setdiff, setequal, union
data(ma_dph)
value_col <- "confirmed_case_7d_avg"
refd_col <- "test_date"
lag_col <- "lag"
ref_lag <- 14
smoothed <- TRUE
# Preprocess the data for a specific location
df <- data_preprocessing(
  ma_dph, 
  value_col = "confirmed_case_7d_avg", 
  refd_col = "test_date", 
  lag_col = "lag", 
  ref_lag = 14, 
  smoothed = TRUE
)
# Define the test date range
test_dates <-  seq(from = as.Date("2022-06-15"),
                   to = as.Date("2022-06-30"),
                   by = "day")
# Filter training data
train_data <- df %>%
  filter(report_date < min(test_dates))
# Add weighting-related features to training data
# This should be done after the train_data is filtered with test_date
train_data <- add_weights_related(train_data)
# Filter test data
test_data <- df %>%
  filter(report_date %in% test_dates)

# Run DelphiRF for forecasting  
results <- DelphiRF(df, as.Date("2022-06-01"))
#> Warning in dir.create(path): cannot create dir
#> './receiving/testdata_count_state', reason 'No such file or directory'
#> Warning in quantile_genlasso_lp(x = x, y = y, d = d, tau = tau, lambda =
#> lambda, : gurobi R package not installed, using Rglpk instead.
#> Warning in dir.create(path): cannot create dir
#> './receiving/testdata_count_state', reason 'No such file or directory'
#> Warning in gzfile(file, mode): cannot open compressed file
#> './receiving/testdata_count_state/20220601_ma_lag0_tau_all_tw365_lambda0.1_gamma0.1.rds',
#> probable reason 'No such file or directory'
#> Error in `gzfile()`:
#> ! cannot open the connection

^{Created on 2026-06-01 with reprex v2.1.1}

For anyone who hasn't seen this pattern, with dir.create confusingly warning "No such file or directory" ("yeah, of course it doesn't exist; that's why I said to create it"), followed by something supposedly about gzfile, it may not be obvious what is going on.

Two possible approaches:

• Have DelphiRF() check for model_save_dir's existence and issue an error with a better message.
• Create model_save_dir if it doesn't exist. This may cause issues with CRAN policies, particularly with the current default model_save_dir:

Packages should not write in the user’s home filespace (including clipboards), nor anywhere else on the file system apart from the R session’s temporary directory (or during installation in the location pointed to by TMPDIR: and such usage should be cleaned up). Installing into the system’s R installation (e.g., scripts to its bin directory) is not allowed.

Limited exceptions may be allowed in interactive sessions if the package obtains confirmation from the user.

For R version 4.0 or later (hence a version dependency is required or only conditional use is possible), packages may store user-specific data, configuration and cache files in their respective user directories obtained from tools::R_user_dir(), provided that by default sizes are kept as small as possible and the contents are actively managed (including removing outdated material).

It would probably be easiest to just default to a temp directory, to avoid having to manage clean-up of files in a the R_user_dir() approach.