[SPARK-25446][R] Add schema_of_json() and schema_of_csv() to R

## What changes were proposed in this pull request?

This PR proposes to expose `schema_of_json` and `schema_of_csv` at R side.

**`schema_of_json`**:

```r
json <- '{"name":"Bob"}'
df <- sql("SELECT * FROM range(1)")
head(select(df, schema_of_json(json)))
```

```
  schema_of_json({"name":"Bob"})
1            struct<name:string>
```

**`schema_of_csv`**:

```r
csv <- "Amsterdam,2018"
df <- sql("SELECT * FROM range(1)")
head(select(df, schema_of_csv(csv)))
```

```
  schema_of_csv(Amsterdam,2018)
1    struct<_c0:string,_c1:int>
```

## How was this patch tested?

Manually tested, unit tests added, documentation manually built and verified.

Closes #22939 from HyukjinKwon/SPARK-25446.

Authored-by: hyukjinkwon <gurwls223@apache.org>
Signed-off-by: hyukjinkwon <gurwls223@apache.org>

Author: HyukjinKwon

R/pkg/NAMESPACE

@@ -351,6 +351,8 @@ exportMethods("%<=>%",
               "row_number",
               "rpad",
               "rtrim",
+              "schema_of_csv",
+              "schema_of_json",
               "second",
               "sha1",
               "sha2",

R/pkg/R/functions.R

@@ -205,11 +205,18 @@ NULL
 #'              also supported for the schema.
 #'          \item \code{from_csv}: a DDL-formatted string
 #'          }
-#' @param ... additional argument(s). In \code{to_json}, \code{to_csv} and \code{from_json},
-#'            this contains additional named properties to control how it is converted, accepts
-#'            the same options as the JSON/CSV data source. Additionally \code{to_json} supports
-#'            the "pretty" option which enables pretty JSON generation. In \code{arrays_zip},
-#'            this contains additional Columns of arrays to be merged.
+#' @param ... additional argument(s).
+#'          \itemize{
+#'          \item \code{to_json}, \code{from_json} and \code{schema_of_json}: this contains
+#'              additional named properties to control how it is converted and accepts the
+#'              same options as the JSON data source.
+#'          \item \code{to_json}: it supports the "pretty" option which enables pretty
+#'              JSON generation.
+#'          \item \code{to_csv}, \code{from_csv} and \code{schema_of_csv}: this contains
+#'              additional named properties to control how it is converted and accepts the
+#'              same options as the CSV data source.
+#'          \item \code{arrays_zip}, this contains additional Columns of arrays to be merged.
+#'          }
 #' @name column_collection_functions
 #' @rdname column_collection_functions
 #' @family collection functions
@@ -1771,12 +1778,16 @@ setMethod("to_date",
 #' df2 <- mutate(df2, people_json = to_json(df2$people))
 #'
 #' # Converts a map into a JSON object
-#' df2 <- sql("SELECT map('name', 'Bob')) as people")
+#' df2 <- sql("SELECT map('name', 'Bob') as people")
 #' df2 <- mutate(df2, people_json = to_json(df2$people))
 #'
 #' # Converts an array of maps into a JSON array
 #' df2 <- sql("SELECT array(map('name', 'Bob'), map('name', 'Alice')) as people")
-#' df2 <- mutate(df2, people_json = to_json(df2$people))}
+#' df2 <- mutate(df2, people_json = to_json(df2$people))
+#'
+#' # Converts a map into a pretty JSON object
+#' df2 <- sql("SELECT map('name', 'Bob') as people")
+#' df2 <- mutate(df2, people_json = to_json(df2$people, pretty = TRUE))}
 #' @note to_json since 2.2.0
 setMethod("to_json", signature(x = "Column"),
           function(x, ...) {
@@ -2285,6 +2296,32 @@ setMethod("from_json", signature(x = "Column", schema = "characterOrstructType")
             column(jc)
           })
 
+#' @details
+#' \code{schema_of_json}: Parses a JSON string and infers its schema in DDL format.
+#'
+#' @rdname column_collection_functions
+#' @aliases schema_of_json schema_of_json,characterOrColumn-method
+#' @examples
+#'
+#' \dontrun{
+#' json <- "{\"name\":\"Bob\"}"
+#' df <- sql("SELECT * FROM range(1)")
+#' head(select(df, schema_of_json(json)))}
+#' @note schema_of_json since 3.0.0
+setMethod("schema_of_json", signature(x = "characterOrColumn"),
+          function(x, ...) {
+            if (class(x) == "character") {
+              col <- callJStatic("org.apache.spark.sql.functions", "lit", x)
+            } else {
+              col <- x@jc
+            }
+            options <- varargsToStrEnv(...)
+            jc <- callJStatic("org.apache.spark.sql.functions",
+                              "schema_of_json",
+                              col, options)
+            column(jc)
+          })
+
 #' @details
 #' \code{from_csv}: Parses a column containing a CSV string into a Column of \code{structType}
 #' with the specified \code{schema}.
@@ -2315,6 +2352,32 @@ setMethod("from_csv", signature(x = "Column", schema = "characterOrColumn"),
             column(jc)
           })
 
+#' @details
+#' \code{schema_of_csv}: Parses a CSV string and infers its schema in DDL format.
+#'
+#' @rdname column_collection_functions
+#' @aliases schema_of_csv schema_of_csv,characterOrColumn-method
+#' @examples
+#'
+#' \dontrun{
+#' csv <- "Amsterdam,2018"
+#' df <- sql("SELECT * FROM range(1)")
+#' head(select(df, schema_of_csv(csv)))}
+#' @note schema_of_csv since 3.0.0
+setMethod("schema_of_csv", signature(x = "characterOrColumn"),
+          function(x, ...) {
+            if (class(x) == "character") {
+              col <- callJStatic("org.apache.spark.sql.functions", "lit", x)
+            } else {
+              col <- x@jc
+            }
+            options <- varargsToStrEnv(...)
+            jc <- callJStatic("org.apache.spark.sql.functions",
+                              "schema_of_csv",
+                              col, options)
+            column(jc)
+          })
+
 #' @details
 #' \code{from_utc_timestamp}: This is a common function for databases supporting TIMESTAMP WITHOUT
 #' TIMEZONE. This function takes a timestamp which is timezone-agnostic, and interprets it as a

R/pkg/R/generics.R

@@ -1206,6 +1206,14 @@ setGeneric("rpad", function(x, len, pad) { standardGeneric("rpad") })
 #' @name NULL
 setGeneric("rtrim", function(x, trimString) { standardGeneric("rtrim") })
 
+#' @rdname column_collection_functions
+#' @name NULL
+setGeneric("schema_of_csv", function(x, ...) { standardGeneric("schema_of_csv") })
+
+#' @rdname column_collection_functions
+#' @name NULL
+setGeneric("schema_of_json", function(x, ...) { standardGeneric("schema_of_json") })
+
 #' @rdname column_aggregate_functions
 #' @name NULL
 setGeneric("sd", function(x, na.rm = FALSE) { standardGeneric("sd") })

R/pkg/tests/fulltests/test_sparkSQL.R

@@ -1620,14 +1620,20 @@ test_that("column functions", {
   expect_equal(collect(select(df, bround(df$x, 0)))[[1]][1], 2)
   expect_equal(collect(select(df, bround(df$x, 0)))[[1]][2], 4)
 
-  # Test from_csv()
+  # Test from_csv(), schema_of_csv()
   df <- as.DataFrame(list(list("col" = "1")))
   c <- collect(select(df, alias(from_csv(df$col, "a INT"), "csv")))
   expect_equal(c[[1]][[1]]$a, 1)
   c <- collect(select(df, alias(from_csv(df$col, lit("a INT")), "csv")))
   expect_equal(c[[1]][[1]]$a, 1)
 
-  # Test to_json(), from_json()
+  df <- as.DataFrame(list(list("col" = "1")))
+  c <- collect(select(df, schema_of_csv("Amsterdam,2018")))
+  expect_equal(c[[1]], "struct<_c0:string,_c1:int>")
+  c <- collect(select(df, schema_of_csv(lit("Amsterdam,2018"))))
+  expect_equal(c[[1]], "struct<_c0:string,_c1:int>")
+
+  # Test to_json(), from_json(), schema_of_json()
   df <- sql("SELECT array(named_struct('name', 'Bob'), named_struct('name', 'Alice')) as people")
   j <- collect(select(df, alias(to_json(df$people), "json")))
   expect_equal(j[order(j$json), ][1], "[{\"name\":\"Bob\"},{\"name\":\"Alice\"}]")
@@ -1654,6 +1660,12 @@ test_that("column functions", {
     expect_true(any(apply(s, 1, function(x) { x[[1]]$age == 16 })))
   }
 
+  df <- as.DataFrame(list(list("col" = "1")))
+  c <- collect(select(df, schema_of_json('{"name":"Bob"}')))
+  expect_equal(c[[1]], "struct<name:string>")
+  c <- collect(select(df, schema_of_json(lit('{"name":"Bob"}'))))
+  expect_equal(c[[1]], "struct<name:string>")
+
   # Test to_json() supports arrays of primitive types and arrays
   df <- sql("SELECT array(19, 42, 70) as age")
   j <- collect(select(df, alias(to_json(df$age), "json")))