[GitHub] [spark] HyukjinKwon commented on a change in pull request #27433: [SPARK-30682][SPARKR][SQL] Add SparkR interface for higher order functions

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HyukjinKwon commented on a change in pull request #27433: [SPARK-30682][SPARKR][SQL] Add SparkR interface for higher order functions
URL: https://github.com/apache/spark/pull/27433#discussion_r373898428
 
 

 ##########
 File path: R/pkg/R/functions.R
 ##########
 @@ -3281,6 +3322,126 @@ setMethod("row_number",
 
 ###################### Collection functions######################
 
+#' Create o.a.s.sql.expressions.UnresolvedNamedLambdaVariable,
+#' convert it to o.s.sql.Column and wrap with R Column.
+#' Used by higher order functions.
+#'
+#' @param ... character of length = 1
+#'        if length(...) > 1 then argument is interpreted as a nested
+#'        Column, for example \code{unresolved_named_lambda_var("a", "b", "c")}
+#'        yields unresolved \code{a.b.c}
+#' @return Column object wrapping JVM UnresolvedNamedLambdaVariable
+unresolved_named_lambda_var <- function(...) {
+  jc <- sparkR.newJObject(
+    "org.apache.spark.sql.Column",
+    sparkR.newJObject(
+      "org.apache.spark.sql.catalyst.expressions.UnresolvedNamedLambdaVariable",
+      list(...)
+    )
+  )
+  column(jc)
+}
+
+#' Create o.a.s.sql.expressions.LambdaFunction corresponding
+#' to transformation described by func.
+#' Used by higher order functions.
+#'
+#' @param fun R \code{function} (unary, binary or ternary)
+#'        that transforms \code{Columns} into a \code{Column}
+#' @param expected_nargs numeric a vector of the expected
+#'        number of arguments. Used for validation
+#' @return JVM \code{LambdaFunction} object
+create_lambda <- function(fun, expected_nargs) {
+  as_jexpr <- function(x) callJMethod(x@jc, "expr")
+
+  # Process function arguments
+  parameters <- formals(fun)
+  nparameters <- length(parameters)
+
+  stopifnot(nparameters %in% expected_nargs)
+
+  stopifnot(
+    nparameters >= 1 &
+    nparameters <= 3 &
+    !"..." %in% names(parameters)
+  )
+
+  args <- lapply(c("x", "y", "z")[seq_along(parameters)], function(p) {
+      unresolved_named_lambda_var(p)
+  })
+
+  # Invoke function and validate return type
+  result <- do.call(fun, args)
+  stopifnot(class(result) == "Column")
+
+  # Convert both Columns to Scala expressions
+  jexpr <- as_jexpr(result)
+
+  jargs <- callJStatic(
 
 Review comment:
   Can you use handledCallJStatic (https://github.com/apache/spark/blob/9514b822a70d77a6298ece48e6c053200360302c/R/pkg/R/utils.R#L787-L803) ?

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