`Yojson.Safe.Util.to_string` fails on most JSON objects

[camlfriend]

Is there a function available, that should not exist?
Using Yojson.Safe.Util.to_string compiles, seems to be valid function.

But I get:

Used string:
{"foo":"bar"}
Fatal error: exception Yojson__Safe.Util.Type_error("Expected string, got object", _)
Raised at Yojson__Safe.Util.typerr in file "lib/read.ml", line 2901, characters 20-60
Called from Main in file "main.ml", line 8, characters 21-50

The Code:

 let js_str = {|{"foo":"bar"}|}                                                 
                                                                                
let () =                                                                        
  print_endline "Used string:";                                                 
  print_endline js_str;                                                         
                                                                                
  let js = Yojson.Safe.from_string js_str in                                    
  let again_js_str = Yojson.Safe.Util.to_string js in (* exception *)           
  (*                                                                            
  let again_js_str = Yojson.Safe.to_string js in (* works well *)               
  *)                                                                            
  print_endline again_js_str   

Using Yojson.Safe.to_string instead blows away the problem.

[camlfriend]

This happens with yojson 3.0.0.

[camlfriend]

The problem within some real world code is, when replacing the functions as mentioned above, I get crap data.

[Leonidas-from-XIV]

It's a bit confusing and old-school but to_string in Util is about extracting a string value out of JSON into an OCaml string, not turning a JSON object into a string (Yojson.Safe.to_string).

It only works if the value that you're trying to extract is a string (or use to_string_option to get an option type):

let j = Yojson.Safe.from_string {|"foo"|} in
let s = Yojson.Safe.Util.to_string j in
print_endline s

[camlfriend]

I would guess a better name for this function would be extract_string.

The to_string implies converting of any yojson-object into string-representation,
as is the name convention in functions like string_of_float, Array.to_list etc. and also used in examples in the OCaml reference manual, where Modulename.to_string converts the main type of Modulname into it's string representation.

For extracting a value that is expected to be there, extract_... is a much better fitting.
It also reduces a source of confusion, if there already is a function with the same name but different meaning/semantics in a module of the same library.

[Leonidas-from-XIV]

I completely agree that the name is confusing, however the name goes back to Yojson 1.0.3 from 14 years ago. So changing it would potentially create a lot of breakage for (I would argue) little gain.

So, maybe you can change it (but then the other to_* need to be changed), but then it would need to be a major version bump as well as submitting a lot of PRs to FOSS codebases to update to the new version (to avoid splitting the ecosystem in two, some using older Yojson, some only supporting newer Yojson) or at least start by adding the new names and adding deprecation warnings, and removing the old names at a later major release.

This is a potentially multi-year process (see the deprecation of Yojson.Safe.json), so I am personally not particularly keen on it, but if you have a good plan doing so (e.g. identify how many projects would be affected), I am not ruling it out.