ARROW-7179: [C++][Python][R] Consolidate coalesce/fill_null

cpp/src/arrow/CMakeLists.txt

@@ -403,7 +403,6 @@ if(ARROW_COMPUTE)
        compute/kernels/scalar_string.cc
        compute/kernels/scalar_temporal.cc
        compute/kernels/scalar_validity.cc
-       compute/kernels/scalar_fill_null.cc
        compute/kernels/scalar_if_else.cc
        compute/kernels/util_internal.cc
        compute/kernels/vector_hash.cc

cpp/src/arrow/compute/api_scalar.cc

@@ -472,10 +472,6 @@ Result<Datum> Compare(const Datum& left, const Datum& right, CompareOptions opti
 SCALAR_EAGER_UNARY(IsValid, "is_valid")
 SCALAR_EAGER_UNARY(IsNan, "is_nan")
 
-Result<Datum> FillNull(const Datum& values, const Datum& fill_value, ExecContext* ctx) {
-  return CallFunction("fill_null", {values, fill_value}, ctx);
-}
-
 Result<Datum> IfElse(const Datum& cond, const Datum& if_true, const Datum& if_false,
                      ExecContext* ctx) {
   return CallFunction("if_else", {cond, if_true, if_false}, ctx);

cpp/src/arrow/compute/api_scalar.h

@@ -787,21 +787,6 @@ Result<Datum> IsNull(const Datum& values, NullOptions options = NullOptions::Def
 ARROW_EXPORT
 Result<Datum> IsNan(const Datum& values, ExecContext* ctx = NULLPTR);
 
-/// \brief FillNull replaces each null element in `values`
-/// with `fill_value`
-///
-/// \param[in] values input to examine for nullity
-/// \param[in] fill_value scalar
-/// \param[in] ctx the function execution context, optional
-///
-/// \return the resulting datum
-///
-/// \since 1.0.0
-/// \note API not yet finalized
-ARROW_EXPORT
-Result<Datum> FillNull(const Datum& values, const Datum& fill_value,
-                       ExecContext* ctx = NULLPTR);
-
 /// \brief IfElse returns elements chosen from `left` or `right`
 /// depending on `cond`. `null` values in `cond` will be promoted to the result
 ///

cpp/src/arrow/compute/kernels/CMakeLists.txt

@@ -29,7 +29,6 @@ add_arrow_compute_test(scalar_test
                        scalar_string_test.cc
                        scalar_temporal_test.cc
                        scalar_validity_test.cc
-                       scalar_fill_null_test.cc
                        scalar_if_else_test.cc
                        test_util.cc)