APL Data-Binding Syntax
Data-binding expressions
APL data binding expressions occur inside JSON strings and take the form "${expression}". Any number of expressions may occur inside a string, such as "${2}+${2} = ${2+2}". Expressions are evaluated within the current data-binding context. The data-binding context is a global dictionary that supports booleans, numbers, strings, arrays, objects, null, and references to defined resources.
Supported value types
String literals
Strings are defined using either single or double quotes. The starting and ending quote must match. Quotes, carriage returns, and line-feeds may be escaped.
${"Double-quoted string"}
${'Single-quoted string'}
${"Inner quote: \" or '"}
Expressions may be nested inside of a string.
${"Two plus two is ${2+2}"}
Numbers
Positive, negative, and floating point numbers are supported. Scientific notation is not supported. In JavaScript, all numbers are doubles.
Booleans
Boolean values of true and false are supported.
${true}
${false}
null
The null constant is supported.
${null}
Resources
Resources defined in the data-binding context use the reserved character "@". For example:
${@myBlue}
${@isLandscape ? @myWideValue : @myNarrowValue}
APL packages define resources exposed in data-binding.
Truthy and Coercion
A truthy value is a value that is considered true when evaluated in a boolean context. All values are truthy except for false, 0, "", and null.
Operators
Arithmetics operators
The standard arithmetic operations for addition, subtraction, multiplication, division, and remainder are supported:
${1+2} // 3
${1-2} // -1
${1*2} // 2
${1/2} // 0.5
${1%2} // 1.
The addition operator also acts as a string-concatenation operator if either the left or right operand is a string.
${27+""} // '27'
${1+" dog"} // '1 dog'
${"have "+3} // 'have 3'
The remainder operator behaves as in JavaScript. That is,
${10 % 3} // 1
${-1 % 2} // -1
${3 % -6} // 3
${6.5 % 2} // 0.5
Logical operators
The standard logical and/or/not operators are supported.
${true || false} // true
${true && false} // false
${!true} // false
The && returns the first operand if it is not truthy and the second otherwise. The || operator returns the first operand if it is truthy and the second otherwise.
${7 && 2} // 2
${null && 3} // null
${7 || 2} // 7
${0 || -16} // 0
Comparison Operators
Comparison operators return boolean values.
${1 < 2}
${75 <= 100}
${3 > -1}
${4 >= 4}
${myNullValue == null}
${(2>1) == true}
${1 != 2}
The comparison operators do not apply to arrays and objects.
The ?? operator is the null-coalescing operator. It returns the left-hand operand if the operand is not null, and otherwise it returns the right-hand operand.
${person.name ?? person.surname ?? 'Hey, you!'}
Ternary Operator
The ternary conditional operator ${a ? b : c} evaluates the left-hand operand. If it evaluates to true, the middle operand is returned. Otherwise the right-hand operand is returned.
${person.rank > 8 ? 'General' : 'Private'}
Array and Object access
Array
Array access uses the [] operator, where the operand should be an integer. Arrays also support the .length operator to return the length of the array. Accessing an element outside of the array bounds returns null.
${myArray[4]} // 5th element in the array (0-indexed)
${myArray.length} // Length of the array
${myArray[-1])} // Last element in the array
Passing a negative index counts backwards through the array.
${a[-1] == a[a.length - 1]} // True
Object
Objects support the . operator and the [] array access operator with string values.
${myObject.name} // The 'name' property of myObject
${myObject['name']} // The 'name' property of myObject
If the property is not defined, null is returned.
Calling the . or [] operator on null returns null.
${myNullObject.address.zipcode} // Returns null