felipecrv commented on code in PR #42106:
URL: https://github.com/apache/arrow/pull/42106#discussion_r1638354632
##########
cpp/src/arrow/compute/expression.cc:
##########
@@ -72,11 +72,12 @@ Expression field_ref(FieldRef ref) {
}
Expression call(std::string function, std::vector<Expression> arguments,
- std::shared_ptr<compute::FunctionOptions> options) {
+ std::shared_ptr<compute::FunctionOptions> options, bool
is_special_form) {
Review Comment:
This is exactly the kind of added complexity around calls that I'm trying to
convince you is not a good idea. This boolean is effectively saying the
arguments are passed in call-by-name fashion and not call-by-value [1].
We should keep the Arrow rewrite system a strict-evaluated system with just
1 or 2 special forms. That means calls are always called by value. We keep the
`if_else` and `case_when` functions as they are and introduce a
`ConditionalSpecialForm` in the context of `compute::Expression`.
C++ is a strictly evaluated rewrite system. You can't declare a function and
say "all the parameters are call-by-name" like you're trying to do with this
boolean parameter. So you can implement `if_else` just like Arrow does here and
has to use special syntax to get a conditional that takes two expression in a
call by name fashion.
```cpp
template<typename T>
T if_else(bool cond, T then_case, T else_case) {
if (cond) { return then_case; } else { return else_case; }
}
// Non-strict evaluation with special form syntax. The branches of the
// if are non-strictly evaluated in a way that can't be achieved with
// the `if_else` Call.
if (den > 0) {
return num / den;
} else {
return 0;
}
```
A general conditional special form can serve for if-else, `and`, and `or`.
Probably the single special form we need.
https://en.wikipedia.org/wiki/Evaluation_strategy#Non-strict_binding_strategies
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