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Programming constructs

So far, we have introduced all the parts of the language that allow us to extract data from an input JSON document, combine the data using string and numeric operators, and format the structure of the output JSON document. What follows are the parts that turn this into a Turing complete, functional programming language.

Comments

JSONata expressions can be interleaved with comments using 'C' style comment delimeters. For example,

/* Long-winded expressions might need some explanation */
(
  $pi := 3.1415926535897932384626;
  /* JSONata is not known for its graphics support! */
  $plot := function($x) {(
    $floor := $string ~> $substringBefore(?, '.') ~> $number;
    $index := $floor(($x + 1) * 20 + 0.5);
    $join([0..$index].('.')) & 'O' & $join([$index..40].('.'))
  )};

  /* Factorial is the product of the integers 1..n */
  $product := function($a, $b) { $a * $b };
  $factorial := function($n) { $n = 0 ? 1 : $reduce([1..$n], $product) };

  $sin := function($x){ /* define sine in terms of cosine */
    $cos($x - $pi/2)
  };
  $cos := function($x){ /* Derive cosine by expanding Maclaurin series */
    $x > $pi ? $cos($x - 2 * $pi) : $x < -$pi ? $cos($x + 2 * $pi) :
      $sum([0..12].($power(-1, $) * $power($x, 2*$) / $factorial(2*$)))
  };

  [0..24].$sin($*$pi/12).$plot($)
)

Produces this, if you're interested!

Conditional logic

If/then/else constructs can be written using the ternary operator "? :".

predicate ? expr1 : expr2

The expression predicate is evaluated. If its effective boolean value (see definition) is true then expr1 is evaluated and returned, otherwise expr2 is evaluated and returned.

Examples

Account.Order.Product.{ `Product Name`: $.Price > 100 ? "Premium" : "Basic" }
[ { "Bowler Hat": "Basic" }, { "Trilby hat": "Basic" }, { "Bowler Hat": "Basic" }, { "Cloak": "Premium" } ]

Variables

Any name that starts with a dollar '$' is a variable. A variable is a named reference to a value. The value can be one of any type in the language's type system.

Built-in variables

  • $ The variable with no name refers to the context value at any point in the input JSON hierarchy. Examples
  • $$ The root of the input JSON. Only needed if you need to break out of the current context to temporarily navigate down a different path. E.g. for cross-referencing or joining data. Examples
  • Native (built-in) functions. See function library.

Variable binding

Values (of any type in the type system) can be bound to variables

$var_name := "value"

The stored value can be later referenced using the expression $var_name.

The scope of a variable is limited to the 'block' in which it was bound. E.g.

Invoice.(
  $p := Product.Price;
  $q := Product.Quantity;
  $p * $q
)

Returns Price multiplied by Quantity for the Product in the Invoice.

Functions

The function is a first-class type, and can be stored in a variable just like any other data type. A library of built-in functions is provided (link) and assigned to variables in the global scope. For example, $uppercase contains a function which, when invoked with a string argument, str, will return a string with all the characters in str changed to uppercase.

Invoking a function

A function is invoked by following its reference (or definition) by parentheses containing a comma delimited sequence of arguments.

Examples

  • $uppercase("Hello") returns the string "HELLO".
  • $substring("hello world", 0, 5) returns the string "hello"
  • $sum([1,2,3]) returns the number 6

Defining a function

Anonymous (lambda) functions can be defined using the following syntax:

function($l, $w, $h){ $l * $w * $h }

and can be invoked using

function($l, $w, $h){ $l * $w * $h }(10, 10, 5) which returns 500

The function can also be assigned to a variable for future use (within the block)

(
  $volume := function($l, $w, $h){ $l * $w * $h };
  $volume(10, 10, 5);
)

Function signatures

Functions can be defined with an optional signature which specifies the parameter types of the function. If supplied, the evaluation engine will validate the arguments passed to the function before it is invoked. A dynamic error is thown if the argument list does not match the signature.

A function signature is a string of the form <params:return>. params is a sequence of type symbols, each one representing an input argument's type. return is a single type symbol representing the return value type.

Type symbols work as follows:

Simple types:

  • b - Boolean
  • n - number
  • s - string
  • l - null

Complex types:

  • a - array
  • o - object
  • f - function

Union types:

  • (sao) - string, array or object
  • (o) - same as o
  • u - equivalent to (bnsl) i.e. Boolean, number, string or null
  • j - any JSON type. Equivalent to (bnsloa) i.e. Boolean, number, string, null, object or array, but not function
  • x - any type. Equivalent to (bnsloaf)

Parametrised types:

  • a<s> - array of strings
  • a<x> - array of values of any type

Some examples of signatures of built-in JSONata functions:

  • $count has signature <a:n>; it accepts an array and returns a number.
  • $append has signature <aa:a>; it accepts two arrays and returns an array.
  • $sum has signature <a<n>:n>; it accepts an array of numbers and returns a number.
  • $reduce has signature <fa<j>:j>; it accepts a reducer function f and an a<j> (array of JSON objects) and returns a JSON object.

Each type symbol may also have options applied.

  • + - one or more arguments of this type
    • E.g. $zip has signature <a+>; it accepts one array, or two arrays, or three arrays, or...
  • ? - optional argument
    • E.g. $join has signature <a<s>s?:s>; it accepts an array of strings and an optional joiner string which defaults to the empty string. It returns a string.
  • - - if this argument is missing, use the context value ("focus").
    • E.g. $length has signature <s-:n>; it can be called as $length(OrderID) (one argument) but equivalently as OrderID.$length().

Recursive functions

Functions that have been assigned to variables can invoke themselves using that variable reference. This allows recursive functions to be defined. Eg.

( $factorial:= function($x){ $x <= 1 ? 1 : $x * $factorial($x-1) }; $factorial(4) )
24

Note that it is actually possible to write a recursive function using purely anonymous functions (i.e. nothing gets assigned to variables). This is done using the Y-combinator which might be an interesting diversion for those interested in functional programming.

Tail call optimization (Tail recursion)

A recursive function adds a new frame to the call stack each time it invokes itself. This can eventually lead to stack exhaustion if the function recuses beyond a certain limit. Consider the classic recursive implementation of the factorial function

(
  $factorial := function($x) {
    $x <= 1 ? 1 : $x * $factorial($x-1)
  };
  $factorial(170)
)

This function works by pushing the number onto the stack, then when the stack unwinds, multiplying it by the result of the factorial of the number minus one. Written in this way, the JSONata evaluator has no choice but to use the call stack to store the intermediate results. Given a large enough number, the call stack will overflow.

This is a recognised problem with functional programming and the solution is to rewrite the function slightly to avoid the need for the stack to store the itermediate result. The following implementation of factorial achieves this

(
  $factorial := function($x){(
    $iter := function($x, $acc) {
      $x <= 1 ? $acc : $iter($x - 1, $x * $acc)
    };
    $iter($x, 1)
  )};
  $factorial(170)
)

Here, the multiplication is done before the function invokes itself and the intermediate result is carried in the second parameter $acc (accumulator). The invocation of itself is the last thing that the function does. This is known as a 'tail call', and when the JSONata parser spots this, it internally rewrites the recursion as a simple loop. Thus it can run indefinitely without growing the call stack. Functions written in this way are said to be tail recursive.

Higher order functions

A function, being a first-class data type, can be passed as a parameter to another function, or returned from a function. Functions that process other functions are known as higher order functions. Consider the following example:

(
  $twice := function($f) { function($x){ $f($f($x)) } };
  $add3 := function($y){ $y + 3 };
  $add6 := $twice($add3);
  $add6(7)
)
  • The function stored in variable $twice is a higher order function. It takes a parameter $f which is a function, and returns a function which takes a parameter $x which, when invoked, applies the function $f twice to $x.
  • $add3 stores a function that adds 3 to its argument. Neither $twice or $add3 have been invoked yet.
  • $twice is invoked by passing the function add3 as its argument. This returns a function that applies $add3 twice to its argument. This returned function is not invoked yet, but rather assigned to the variable add6.
  • Finally the function in $add6 is invoked with the argument 7, resulting in 3 being added to it twice. It returns 13.

Functions are closures

When a lambda function is defined, the evaluation engine takes a snapshot of the environment and stores it with the function body definition. The environment comprises the context item (i.e. the current value in the location path) together with the current in-scope variable bindings. When the lambda function is later invoked, it is done so in that stored environment rather than the current environment at invocation time. This property is known as lexical scoping and is a fundamental property of closures.

Consider the following example:

Account.(
  $AccName := function() { $.'Account Name' };

  Order[OrderID = 'order104'].Product.{
    'Account': $AccName(),
    'SKU-' & $string(ProductID): $.'Product Name'
  }
)

When the function is created, the context item (referred to by '$') is the value of Account. Later, when the function is invoked, the context item has moved down the structure to the value of each Product item. However, the function body is invoked in the environment that was stored when it was defined, so its context item is the value of Account. This is a somewhat contrived example, you wouldn't really need a function to do this. The expression produces the following result:

{
  "Account": "Firefly",
  "SKU-858383": "Bowler Hat",
  "SKU-345664": "Cloak"
}

Partial function application

Functions can partially applied by invoking the function with one or more (but not all) arguments replaced by a question mark ? placeholder. The result of this is another function whose arity (number of parameters) is reduced by the number of arguments supplied to the original function. This returned function can be treated like any other newly defined function, e.g. bound to a variable, passed to a higher-order function, etc.

Examples

  • Create a function to return the first five characters of a string by partially applying the $substring function

    ( $first5 := $substring(?, 0, 5); $first5("Hello, World") )
    "Hello"

  • Partially applied function can be further partially applied

    ( $firstN := $substring(?, 0, ?); $first5 := $firstN(?, 5); $first5("Hello, World") )
    "Hello"

Function chaining

Function chaining can be used in two ways:

  1. To avoid lots of nesting when multiple functions are applied to a value

  2. As a higher-order construct for defining new functions by combining existing functions

Invocation chaining

value ~> $funcA ~> $funcB
is equivalent to
$funcB($funcA(value))

Examples

  • Customer.Email ~> $substringAfter("@") ~> $substringBefore(".") ~> $uppercase()

Function composition

Function composition is the application of one function to another function to produce a third function.

$funcC := $funcA ~> $funcB
is equivalent to
$funcC := function($arg) { $funcB($funcA($arg)) }

Examples

  • Create a new function by chaining two existing functions
    ( $normalize := $uppercase ~> $trim; $normalize(" Some Words ") )
    "SOME WORDS"

Functions as first class values

Function composition can be combined with partial function application to produce a very compact syntax for defining new functions.

Examples

  • Create a new function by chaining two partially evaluated functions
    ( $first5Capitalized := $substring(?, 0, 5) ~> $uppercase(?); $first5Capitalized(Address.City) )
    "WINCH"

Advanced example - The Y-combinator

There is no need to read this section - it will do nothing for your sanity or ability to manipulate JSON data.

Earlier we learned how to write a recursive function to calculate the factorial of a number and hinted that this could be done without naming any functions. We can take higher-order functions to the extreme and write the following:

λ($f) { λ($x) { $x($x) }( λ($g) { $f( (λ($a) {$g($g)($a)}))})}(λ($f) { λ($n) { $n < 2 ? 1 : $n * $f($n - 1) } })(6)

which produces the result 720. The Greek lambda (λ) symbol can be used in place of the word function which, if you can find it on your keyboard, will save screen space and please the fans of lambda calculus.

The first part of this above expression is an implementation of the Y-combinator in this language. We could assign it to a variable and apply it to other recursive anonymous functions:

(
  $Y := λ($f) { λ($x) { $x($x) }( λ($g) { $f( (λ($a) {$g($g)($a)}))})};
  [1,2,3,4,5,6,7,8,9] . $Y(λ($f) { λ($n) { $n <= 1 ? $n : $f($n-1) + $f($n-2) } }) ($)
)

to produce the Fibonacci series [ 1, 1, 2, 3, 5, 8, 13, 21, 34 ].

But we don't need to do any of this. Far more sensible to use named functions:

(
  $fib := λ($n) { $n <= 1 ? $n : $fib($n-1) + $fib($n-2) };
  [1,2,3,4,5,6,7,8,9] . $fib($)
)
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