## What changes were proposed in this pull request?
`write.df`/`read.df` API require path which is not actually always necessary in Spark. Currently, it only affects the datasources implementing `CreatableRelationProvider`. Currently, Spark currently does not have internal data sources implementing this but it'd affect other external datasources.
In addition we'd be able to use this way in Spark's JDBC datasource after https://github.com/apache/spark/pull/12601 is merged.
**Before**
- `read.df`
```r
> read.df(source = "json")
Error in dispatchFunc("read.df(path = NULL, source = NULL, schema = NULL, ...)", :
argument "x" is missing with no default
```
```r
> read.df(path = c(1, 2))
Error in dispatchFunc("read.df(path = NULL, source = NULL, schema = NULL, ...)", :
argument "x" is missing with no default
```
```r
> read.df(c(1, 2))
Error in invokeJava(isStatic = TRUE, className, methodName, ...) :
java.lang.ClassCastException: java.lang.Double cannot be cast to java.lang.String
at org.apache.spark.sql.execution.datasources.DataSource.hasMetadata(DataSource.scala:300)
at
...
In if (is.na(object)) { :
...
```
- `write.df`
```r
> write.df(df, source = "json")
Error in (function (classes, fdef, mtable) :
unable to find an inherited method for function ‘write.df’ for signature ‘"function", "missing"’
```
```r
> write.df(df, source = c(1, 2))
Error in (function (classes, fdef, mtable) :
unable to find an inherited method for function ‘write.df’ for signature ‘"SparkDataFrame", "missing"’
```
```r
> write.df(df, mode = TRUE)
Error in (function (classes, fdef, mtable) :
unable to find an inherited method for function ‘write.df’ for signature ‘"SparkDataFrame", "missing"’
```
**After**
- `read.df`
```r
> read.df(source = "json")
Error in loadDF : analysis error - Unable to infer schema for JSON at . It must be specified manually;
```
```r
> read.df(path = c(1, 2))
Error in f(x, ...) : path should be charactor, null or omitted.
```
```r
> read.df(c(1, 2))
Error in f(x, ...) : path should be charactor, null or omitted.
```
- `write.df`
```r
> write.df(df, source = "json")
Error in save : illegal argument - 'path' is not specified
```
```r
> write.df(df, source = c(1, 2))
Error in .local(df, path, ...) :
source should be charactor, null or omitted. It is 'parquet' by default.
```
```r
> write.df(df, mode = TRUE)
Error in .local(df, path, ...) :
mode should be charactor or omitted. It is 'error' by default.
```
## How was this patch tested?
Unit tests in `test_sparkSQL.R`
Author: hyukjinkwon <gurwls223@gmail.com>
Closes#15231 from HyukjinKwon/write-default-r.
c9fe10d4ed