A staple of compact code, the ternary operator
?: feels like it’s earned a place any programming language. I use it often; you should use it often. However, I don’t like the operator itself. It feels complex and incomplete. Though I’ve stopped development on Leaf, I did find a better option.
Let’s take a deeper look at what this ternary operator is, then show how a language could avoid it.
What’s this ternary operator?
The name of this operator raises questions. A ternary operator, in a typical math sense, or from the view of a parser, is an operator that takes three arguments. An operator is a function that has a special syntax, not the generic call syntax
foo(a,b,c). Most common are binary operators, we see them everywhere.
//binary operators x + y x / y x * y
There are also a handful of unary operators we see commonly. Unary meaning they have only one argument.
//unary operators -a ~bits !cond
There’s no shortage of unary and binary operators. But what examples of ternary operators do we have?
//ternary operators cond ? true_value : false_value
Are you scratching your head trying to think of another one? As far as I know, there aren’t any. This is why we end up calling this the “ternary operator” instead of using a proper name, like “conditional operator”. There aren’t any other ternary operators in programming languages.
Let’s step aside for a second. There are operator sequences and combinations that may look like ternary operators but aren’t.
For example, this Python comparison appears to involve three arguments.
if a < b < c: go()
That looks similar to a ternary operator. It must consider all three arguments to evaluate correctly.
However, digging deeper this is more of a syntactic sugar than a true ternary operator. It’s equivalent to the following.
once_b = b if a < once_b and once_b < c: go()
You may find several examples that show only
a < b and b < c, but those are incorrect. The original form guarantees that
b is evaluated only once. I use the
once_b = b to show this guarantee; I assume a different internal feature achieves this in practice.
This python construct can be extended to include even more values.
if a < b < c < d <e < e: go()
It’s a chained operator. There’s no limit to what can be added.
<< stream operator in C++ can also be chained.
cout << "Hello " << first_name << " " << last_name << endl;
Unlike Python’s chained comparisons, this C++ sequence doesn’t require any parser support.
<< is a normal binary operator that happens to be chainable. Basic math has this same property, for example
a + b + c + d.
Messy syntax and parsing
I said there aren’t any other ternary operators in programming languages. There’s a good reason for this.
cond ? true_value : false_value. Is there anything that makes it look like three arguments are involved? What if I change the operators slightly, to unknown ones:
expr ⋇ value_a ⊸ value_b. That looks like two binary operators. Even if I keep the same symbols, but add nested expressions, it looks off:
cond ? value_a + value_b : value_c * value_d. There’s no visual indication that
? ... : is one unified operator. This syntax limitation prevents any new ternary operators from being introduced. It’d create a backlash.
?: is already in some coder’s bad books as it’s easy to abuse.
Despite providing a valuable syntax, the ternary operator is syntactically messy. It’s just weird to have two split symbols define three arguments. A function with three arguments is not a problem since we have a robust syntax for it
foo( a, b, c ).
Adding to the complexity is precedence. All operators require precedence. For example,
a + b * c is evaluated as
a + (b*c). Multiplication has higher precedence than addition. Its result is evaluated before addition.
Operators of the same precedence also have associativity. For example
5 - 2 + 3 is evaluated as
(5 - 2) + 3 = 6, which is left associative. Right associativity would be evaluated as
5 - (2 + 3) = 0, which is wrong.
Right associativity is generally reserved for unary operators, which only have a right argument, and assignment operators. For example, if you’re crazy enough to write
a = b += c, right associativity evaluates this as
a = (b += c).
The ternary operator is right associative. It doesn’t feel right though. How can an operator with three arguments and two symbols have associativity defined merely as left or right?
cond_a ? val_one : cond_b ? val_two : val_three //right? associative cond_a ? val_one : (cond_b ? val_two : val_three) //left? associative (wrong) (cond_a ? val_one : cond_b) ? val_two : val_three //strict-left? associative (wacky) (((cond_a ? val_one) : cond_b) ? val_two) : val_three //strict-right? associative (nonsensical) cond_a ? (val_one : (cond_b ? (val_two : val_three)))
The two strict forms, which apply associativity on each operator symbol, are wacky. Think for a moment from the point of the view of the parser. It’s one of those two that make the most sense syntactically. The first two forms require a bit of trickery to bypass regular associative parsing.
Try to imagine what happens in nested ternary operators:
a ? b ? c : d : e. You won’t find nested ternaries often in code. They are too hard to parse mentally.
You find lots of code depending on the quasi-right associativity. That’s what allows chaining.
int a = cond_a ? val_one : cond_b ? val_two : cond_c ? val_three : val_four;
A binary solution
When I was working on Leaf, I wanted to avoid the ternary operator. I liked the feature it provided, but I wanted to find a more fundamental approach to providing it.
The solution came in the form of language optionals. These were fundamental in Leaf, thus had operator support.
The first operator was a defaulting one. If the optional value were set, it’d evaluate to that. Otherwise, it’d evaluate to the provided default.
var a : optional print( a | 1 ) //prints 1, since `a` has no value var b : optional = 2 print( b | 3 ) //prints 2, since that's the value in `b`
?? that uses
null in the same way as Leaf uses unset optionals.
This sounds like half of what the ternary operator does. We could look at like this:
cond ? some_val | default_val. That is, consider the entire left part, the
cond ? some_val to produce an optional value. Given an optional, we already have operators to default that value when unset.
Leaf incorporated the
? operator to create an optional.
var a = true ? 5 //a is an optional set to value 5 var b = false ? 6 //b is an optional without a value
On its own, this operator is often useful for optionals. Combined with
| default operator it mimics the traditional ternary operator.
var a = cond ? true_value | false_value
int a = cond ? true_value : false_value;
| are both binary operators. There is no need to have an actual ternary operator to produce the same conditional evaluation. We get the same feature without the syntactic messiness. It’s a lot clearer what happens, and the parser doesn’t require any trickery.
It also improves expressiveness without introducing more symbols, as both operators are useful on their own.
Alas, I’ve not seen any plans to add this to any language. If you know of one, please leave a comment. I’d be curious to see it.