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A Clojure/ClojureScript (Java/JavaScript) implementation of JUTE template language. Still in early development stage.

Build Status

Clojars Project


JUTE stands for JSON Uniform Templates and it's a small language to describe JSON documents transformations. JUTE templates are JSON documents itself. It's safe to evaluate user-provided JUTE templates, there is no way for a template to currupt a runtime environment if you use a safe YAML parser.

Few words about YAML

JSON format wasn't designed for ease of use by human beings, it's relatively hard to write JSON by hands. That's why JUTE's primary format is YAML, which is much easier to read and write, thanks to its clean syntax and indentation-based nesting. Don't be confused with it, YAML and JSON are interchangeable, and there are even online conversion tools beetween them:

  "speaker": {
    "login": "mlapshin",
    "email": ""
  "fhir?": true,
  "topics": [
  login: mlapshin
fhir?: true
  - mapping
  - dsl
  - jute
  - fhir

Quickstart Tutorial

Let's say we have a document describing a book:

    name: M. Soloviev
    title: PHD
    gender: m
  title: Approach to Cockroach
  - type: preface
    content: A preface chapter
  - type: content
    content: Chapter 1
  - type: content
    content: Chapter 2
  - type: content
    content: Chapter 3
  - type: afterwords
    content: Afterwords

And for some case we need to convert it into a slightly different format:

type: book
author: M. Soloviev
title: Approach to Cockroach
- Chapter 1
- Chapter 2
- Chapter 3

Here we're going to discard preface and afterwords as well as minor author information keepeing only his name. And we want a book's content to be an array of strings, not an array of objects with a content key. Let's write a JUTE template which will perform this transformation.

We'll start our template with a type: book flag:

type: "book"
type: "book"

This tiny document is a valid JUTE template which will always procude a {"type": "book"} result regardless the input data. Actually, everything in a JUTE is treated as a constant value unless it doesn't contain a special flag - a dollar sign. A dollar sign can appear either in a object keys or as the first character of a string. Numbers and boolean values (true/false) are always constants in JUTE templates.

Let's move to the author field. Obviously we're gonna take an author's name from an incoming data:

type: "book"
author: "$"
type: "book"
author: "M. Soloviev"

To tell JUTE that an author field will be dynamic we put a dollar sign at the beginning of a value's string. The rest of the string is a path for the data we need. Such strings starting with a dollar signs are called JUTE expressions and they have pretty rich syntax to describe various operations on an incoming data or a scope.

One of expression's abilities is an extract data by path. Every path consists of one or several path components separated by dot. In simpliest case a path component is a field name where JUTE interpreter will dig to get value. In our case it fill take the book field from the scope root, then author, then name. You can use digits as path component as well to get N-th value from an array. Array indices are starting with 0.

Please note that it's ok to omit double-quotes (") for strings in YAML, so instead of writing "$" we can just write $

We can fill the title field using similar path expression and omit double-quotes for readability:

type: book
author: $
title: $ book.title
type: book
author: M. Soloviev
title: Approach to Cockroach

Let's proceed to the content part. We need to filter out chapters where type doesn't equal to "content". There is a special type of a path element to do this called predicate search:

type: book
author: $
title: $ book.title
content: $ book.chapters.*(this.type = "content")
type: book
author: M. Soloviev
title: Approach to Cockroach
- type: content
  content: Chapter 1
- type: content
  content: Chapter 2
- type: content
  content: Chapter 3

Instead of telling an exact path, we describe a condition which an array element should met to be selected for the next step of path evaluation. Use this keyword to reference current element in an array. A result of a predicate search is always an array, even if there is only one element matching criteria.

The final step is to extract content property from every element in the content array. In most programming languages nowadays it's done using a map function which executes same code on every element in an array and returns results an array with preserved order. In JUTE we have map as well, but it's not a function, it's a directive:

type: book
author: $
title: $ book.title
  $map: $ book.chapters.*(this.type = "content")
  $as: i
  $body: $ i.content
type: book
author: M. Soloviev
title: Approach to Cockroach
- Chapter 1
- Chapter 2
- Chapter 3

A directive is an object in a template with one or several keys starting with a dollar sign. A dollar sign tells JUTE that this object needs to be evaluated in a special way depending on directive's purpose. In case of $map it takes a value from a $map key, iterates through it and executes $body on every element aliasing it with a name from $as key. Other available directives are $if, $switch, $fn and $call - you'll find all of them in the reference.

That's it, in this tutorial we wrote a simple template and touched a little bit every aspect of a JUTE language.

An FizzBuzz Example

A classicall FizzBuzz programm in JUTE:

$call: join-str
  - " "
  - $map: $ range(0, 50, 1)
    $as: num
        - s: ""

        - s:
            $if: $ num % 3 = 0
            $then: $ s + "Fizz"
            $else: $ s

        - s:
            $if: $ num % 5 = 0
            $then: $ s + "Buzz"
            $else: $ s
        $if: $ s = ""
        $then: $ num
        $else: $ toString(num) + "-" + s



Template - a JSON-like data structure to be evaluated by JUTE.

Scope - an object where JUTE looks up values and functions to evaluate expressions and directives.

Expression - a string value within a template starting with a dollar sign which will be evaluated by JUTE.

Directive - an object within a template containing one or several keys starting with a dollar sign with custom evaluation logic.


I'm quite short in time right now to describe full expressions syntax. To get some understaing of them please take a look at the expressions test suite. Commented out pieces are still need to be implemented.

Available operators are: != = ! * % <= / - >= < + > && ||



Performs conditional evaluation:

  $if: $ sex = "m"
  $then: Male
  $else: Female

If condition is true, direcitve is evaluated into a value of $then, $else otherwise. If a condition is false and $else is omitted, directive evaluates into null.

NB there is a short form of $if directive:

  $if: patient.firstName && patient.lastName
  firstName: $ patient.firstName
  lastName: $ patient.lastName

In a shortened form directrive is evaluated into itself (without the $if attribute) when condition is true, null otherwise.


$map directive evaluates into array containing results of applying it's $body on every element from a $map array. Array element is aliased by name from $as field. If $as is ommited, this is used instead.

  - 1
  - 2
  - 3
  - 4
  $as: item
  $body: $ item * 2

TODO: we need to have an index among with element itself.


To be done later.


$let directive evaluates into it's $body with scope extended with additional values:

  pi: 3.1415
  radius: 3
  area: $ pi * radius * radius
  perimeter: $ pi * 2 * radius


$fn directive returns a function which can be invoked later in an expresson. Value of an $fn key is an array containing names of function arguments. $body key contains function body.

Most likely you'll put an $fn directive into $let directive to make function accessible inside $let's body:

    $fn: ["radius"]
    $body: $ 3.1415 * radius * radius
  area: $ circleArea(circles.0.radius)


$call directive is a way to call a function outside of JUTE expression:

  $call: joinStr
    - " "
    - - $ pt.firstName
      - $ pt.middleName
      - $ pt.lastName


$switch directive takes a value of an expression and then compares it to all directive-level keys. If matching key found, directive evaluates into a value of corresponding key. $default key (if present) is used when no matching key was found. Evaluates to null if no match was found and there is no $default key.

  $switch: $
  M: male
  F: female
  U: unknown
  $default: other


$reduce directive performs standard reduce operation:

  $reduce: $ range(0, 10, 1)
  $as: ["acc", "i"]
  $start: 0
  $body: $ acc + i


To be written.

joinStr(separator, strArray)

splitStr(srt, regexp, limit?)

substr(str, start, end)







groupBy(fn, array)


range(begin, end, step)


Copyright © 2019 Health Samurai Team

Distributed under the MIT License.

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