Other ternary operators besides ternary conditional (?:)

Question

The "ternary operator" expression is now almost equivalent to the ternary conditional operator:

condition ? trueExpression : falseExpression;

However, "ternary operator" only means that it takes three arguments. I'm just curious, are there any languages with any other built-in ternary operators besides conditional operator and which ones?

Answer 1

There’s the slice operator in Python:

a = [1, 2, 3]
b = a[1 : 2 : 2]

With the operands meaning start, stop and step.

Answer 2

These operators are often called mixfix operators (also, sometimes triadic operators). The Maude rewriting logic language allows user-defined mixfix operators, as does the dependently typed functional programming language, Agda.

In general, they're not common, because they can be tricky to parse and because, if there is no deferred evaluation, you can usually find a nice, readable decomposition into binary infix operators.

Answer 3

Mathematica has the "TagSet" operator

f /: g = h

which associates g to h and differentiate this assignment using f, e.g.

a /: f[a] = 2
b /: f[b] = 8
c f[c] + b f[b] + a f[a]
(* # prints 'c f[c] + 8 b + 2 a' *)

Answer 4

CLU's element update syntax works on any abstract type that provides a store method. The full syntax is "T$store(primary, e1, e2)", but it has an elegant ternary syntactic sugar:

primary[e1] := e2

What I especially like about this is that any type can participate in this syntax; it's not limited to arrays. So you can define it on hash tables, search trees, basically whatever you like.

Answer 5

SQL's foo BETWEEN 0 AND 10 is another example.

Answer 6

Scala's foldLeft (/:) operator:

val list = List(4, 1, 8, 9)
def add(a: Int, b: Int) = a + b
val sum = (0 /: list)(add)

Takes three parameters: a seed value, a collection and an operation to use for reduction.

It should however be noted that technically Scala doesn't have operators. All the "operators" in Scala are actually just regular methods.

Answer 7

Python's a < b < c comes to mind, it's equivalent to a < b and b < c - not to (a < b) < c as someone may think.

I suppose we can say in Python there is whole class of ternary comparison operators of the type expr1 op1 expr2 op2 expr3, where op1 and op2 are binary comparison operators?

Answer 8

Just to do my bit for Lisp today, many of the binary operators are actually n-ary operators in Common Lisp,

(< a b c) => a < b and b < c

(+ 1 2 3) => 6

so you can read < as monotonically increasing if it tickles your fancy.