If_else special form

cpp/src/arrow/CMakeLists.txt

@@ -802,6 +802,8 @@ if(ARROW_COMPUTE)
        compute/row/grouper.cc
        compute/row/row_encoder_internal.cc
        compute/row/row_internal.cc
+       compute/special/conditional_special.cc
+       compute/special/if_else_special.cc
        compute/util.cc
        compute/util_internal.cc)
 

cpp/src/arrow/compute/CMakeLists.txt

@@ -179,3 +179,5 @@ add_arrow_compute_benchmark(function_benchmark)
 add_subdirectory(kernels)
 
 add_subdirectory(row)
+
+add_subdirectory(special)

cpp/src/arrow/compute/api.h

@@ -39,8 +39,14 @@
 #include "arrow/compute/registry.h"          // IWYU pragma: export
 #include "arrow/datum.h"                     // IWYU pragma: export
 
+/// \defgroup expressions Expressions of computation
+/// @{
+/// @}
+
 #include "arrow/compute/expression.h"  // IWYU pragma: export
 
+#include "arrow/compute/api_special.h"  // IWYU pragma: export
+
 /// \defgroup execnode-row Utilities for working with data in a row-major format
 /// @{
 /// @}

cpp/src/arrow/compute/api_special.h

@@ -0,0 +1,52 @@
+// Licensed to the Apache Software Foundation (ASF) under one
+// or more contributor license agreements.  See the NOTICE file
+// distributed with this work for additional information
+// regarding copyright ownership.  The ASF licenses this file
+// to you under the Apache License, Version 2.0 (the
+// "License"); you may not use this file except in compliance
+// with the License.  You may obtain a copy of the License at
+//
+//   http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing,
+// software distributed under the License is distributed on an
+// "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+// KIND, either express or implied.  See the License for the
+// specific language governing permissions and limitations
+// under the License.
+
+#include "arrow/compute/expression.h"
+
+namespace arrow::compute {
+
+/// The concept of "special form" is borrowed from Lisp
+/// (https://courses.cs.northwestern.edu/325/readings/special-forms.html). A special form
+/// is used to implement evaluation strategies
+/// (https://en.wikipedia.org/wiki/Evaluation_strategy) other than the default
+/// call-by-value strategy used by Arrow expression evaluation. Velox also uses this term.
+///
+/// In a call-by-value strategy, all arguments are evaluated before the function
+/// invocation. For example, consider a regular function call
+///   if_else(a, b, c)
+/// Under call-by-value semantics, the expressions `a`, `b`, and `c` are all evaluated
+/// before calling `if_else`. This can lead to unintuitive behavior when subexpressions
+/// have observable side effects. For instance,
+///   if_else(not_equal(a, 0), divide(b, a), 0)
+/// should never produce a divide-by-zero error in most programming languages. However,
+/// under call-by-value semantics, `divide(b, a)` is evaluated regardless of the
+/// condition, so a divide-by-zero error can still occur. To address this, a special form
+/// for `if_else` would be needed, namely `if_else_special`, that follows a
+/// call-by-name-like evaluation strategy, where, for each row in a batch, only one of the
+/// branches is evaluated based on the corresponding value of condition.
+///
+/// Each API in this file is intended to refer to a concrete special form. In addition to
+/// the aforementioned `if_else_special`, the design anticipates variants of conditional
+/// constructs such as `case_when_special` and `coalesce_special`, as well as boolean
+/// operators with short-circuit semantics, such as `and_special` and `or_special`, some
+/// of which may not be implemented yet.
+
+/// @brief Construct an Expression representing an if-else special form.
+ARROW_EXPORT
+Expression if_else_special(Expression cond, Expression if_true, Expression if_false);
+
+}  // namespace arrow::compute