Ternary Conditional Expression

Abstract

This JEP introduces a new expression to support an it-then-else ternary operation.

Motivation

Most languages do have a ternary operator that is akin to an if-then-else syntax as a single statement.
For instance, many languages do have the following syntax:

<condition> ? <true-result> : <false-result>

While JMESPath does not have a ternary operator, the closest approximation using an expression like ( x || y ) && z unfortunately suffers from a limitation that makes it not really a true ternary operator. Specifically, if both x and y are false, the expression still falls back to evaluating z.

More complicated, "object-wrapping" expressions are possible such as (x && [y] || [z])[0]. Unfortunately, these appear to be more like a trick and are not user-friendly, especially for newcomers to the language.

For those reasons, we propose introducing a true ternary operator to the language.

Grammar Changes

The following updates to the grammar support a ternary operation:

expression =/ ternary-expression

ternary-expression = expression "?" expression ":" expression

A ternary-expression (short for ternary conditional expression) is an expression taking the following form:

condition ? true-expression : false-expression

Where condition, true-expression and false-expression are JMESPath expressions.
A ternary conditional expression will evaluate to either its "true" expression or its "false" expression.
If the evaluation of the condition is not false, then the "true" expression is evaluated and used as the return value.
Otherwise, the "false" epression is evaluated and used as the return value.

The following values are considered to be false values in JMESPath:

• The boolean false JSON value
• The null JSON value
• An empty "" JSON string
• An empty {} JSON objects
• An empty [] JSON array

Expressions within a ternary conditional expression are evaluated using a right-associative reading.
This means that a chain of ternary expression can be expressed without parentheses:

a ? b :
c ? d :
e

Precedence

The ternary condition expression is a new infix operator that has a higher precedence than the | pipe expression and lower precedence than the || and && logical expressions. Thus the precedence of various expressions goes like this, from lowest to highest:

• | pipe expression
• … ? … : … ternary conditional expression
• || OR logical expression
• && AND logical expression

Compliance Tests

A new ternary.json test file will be added to the suite of compliance tests.

[
	{
		"given": {
			"true": true, "false": false,
			"foo": "foo",  "bar": "bar", "baz": "baz", "qux": "qux", "quux": "quux"
		},
		"cases": [
			{ "expression": "true ? foo : bar", "result": "foo" },
			{ "expression": "false ? foo : bar", "result": "bar" },
			{ "expression": "`null` ? foo : bar", "result": "bar" },
			{ "expression": "`[]` ? foo : bar", "result": "bar" },
			{ "expression": "`{}` ? foo : bar", "result": "bar" },
			{ "expression": "'' ? foo : bar", "result": "bar" },
			
                        { "expression": "false ? foo | bar | @ : baz", "result": "baz" },
			{ "expression": "foo ? bar ? baz : qux : quux", "result": "baz" },

			{ "expression": "true || false ? foo : bar", "result": "foo" },
			{ "expression": "true && false ? foo : bar", "result": "bar" }
		]
	}
]

Alternatives

Although achieving the same result is possible within the current specification, the "array-wrapping" trick referred to in the motivation section is not user-friendly.

Other alternatives considered introducing a more general "switch-case-when-else" syntax but we feel that a ternary operator is so ubiquitous in most languages that it is worth introducing in the language. This does not preclude revisiting a more general syntax in the future.

Finally, we considered adding this feature as a function. Although this can be done today in most implementations that provide an extension point, we think a ternary operator is the most user-friendly least surprising way to include this feature.

Implementation Survey

Implementation is quite straighforward:

• TypeScript
• Python