rstan::stanc doesn't always return an object

Summary:

stanc doesn't do as the doc says. It only returns an object when it is successful. When it is unsuccessful, it doesn't return.

Description:

It's hard to use stanc programmatically due to inconsistent output.

Reproducible Steps:

library(rstan)

r <- rstan::stanc(model_code = "foo")
str(r)

Current Output:

> r <- rstan::stanc(model_code = "foo")
SYNTAX ERROR, MESSAGE(S) FROM PARSER:


ERROR at line 1

  1:    foo
       ^

PARSER EXPECTED: <model declaration (or perhaps an earlier block)>
Error in rstan::stanc(model_code = "foo") : 
  failed to parse Stan model 'foo' due to the above error.
> str(r)
Error in str(r) : object 'r' not found

Expected Output:

> r <- rstan::stanc(model_code = "foo")
SYNTAX ERROR, MESSAGE(S) FROM PARSER:


ERROR at line 1

  1:    foo
       ^

PARSER EXPECTED: <model declaration (or perhaps an earlier block)>
Error in rstan::stanc(model_code = "foo") : 
  failed to parse Stan model 'foo' due to the above error.
> str(r)
List of 5
 $ status       : logi FALSE
 $ model_cppname: <not sure what goes here>
 $ cppcode      :  <should be empty>
 $ model_name   : chr "anon_model"
 $ model_code   : atomic [1:1] model { }
  ..- attr(*, "model_name2")= chr "anon_model"

RStan Version:

2.12.1

R Version:

3.3.1

Operating System:

Mac OS X 10.11.6.

Start by reproducing the failure with rstan::stanc(model_code = "foo") and compare it with the documented return shape. Trace the stanc entry point to see how unsuccessful parsing is handled; done means the call returns a five-element result with status FALSE and empty generated code instead of leaving the assignment unset.