[GitHub] [arrow] nealrichardson commented on issue #13803: Get the table schema as json?

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nealrichardson commented on issue #13803:
URL: https://github.com/apache/arrow/issues/13803#issuecomment-1206399291

   The trouble is: what do you expect to do with this JSON? Round-trip and re-create the Schema from the JSON? That gets tricky with things like nested types, extension types, metadata, etc. It really becomes an Arrow specification question, not a JSON serialization problem. As such, if you want to pursue it, you should raise it on the dev mailing list. I seem to recall this idea coming up in the past and it not getting much support, but maybe I misremember, or maybe the times are different. 
   
   Naively, in R you can get a JSON string version of the Schema with some manipulation. Here's an ugly base-R one-liner, just to illustrate:
   
   ```
   > s
   Schema
   name: string
   height: int32
   mass: double
   hair_color: string
   skin_color: string
   eye_color: string
   birth_year: double
   sex: string
   gender: string
   homeworld: string
   species: string
   films: list<item: string>
   vehicles: list<item: string>
   starships: list<item: string>
   
   > cat(jsonlite::toJSON(setNames(lapply(s, function (x) x$type$ToString()), s$names), pretty = TRUE, auto_unbox = TRUE))
   {
     "name": "string",
     "height": "int32",
     "mass": "double",
     "hair_color": "string",
     "skin_color": "string",
     "eye_color": "string",
     "birth_year": "double",
     "sex": "string",
     "gender": "string",
     "homeworld": "string",
     "species": "string",
     "films": "list<item: string>",
     "vehicles": "list<item: string>",
     "starships": "list<item: string>"
   }
   ```
   
   But if you wanted to create a Schema from that, you're still in the world of parsing things like `"list<item: string>"`.
   
   On the R side, there is this nice code generating utility that Romain added recently:
   
   ```
   > s$code()
   schema(name = utf8(), height = int32(), mass = float64(), hair_color = utf8(), 
       skin_color = utf8(), eye_color = utf8(), birth_year = float64(), 
       sex = utf8(), gender = utf8(), homeworld = utf8(), species = utf8(), 
       films = list_of(utf8()), vehicles = list_of(utf8()), starships = list_of(utf8()))
   ```


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