json-logic-ruby

Build complex rules, serialize them as JSON, and execute them in ruby.

json-logic-ruby is a ruby parser for JsonLogic. Other libraries are available for parsing this logic for Python and JavaScript at that link!

Installation

gem install json_logic

Why use JsonLogic?

If you're looking for a way to share logic between front-end and back-end code, and even store it in a database, JsonLogic might be a fit for you.

JsonLogic isn't a full programming language. It's a small, safe way to delegate one decision. You could store a rule in a database to decide later. You could send that rule from back-end to front-end so the decision is made immediately from user input. Because the rule is data, you can even build it dynamically from user actions or GUI input.

JsonLogic has no setters, no loops, no functions or gotos. One rule leads to one decision, with no side effects and deterministic computation time.

Virtues

1. Terse.
2. Consistent. {"operator" : ["values" ... ]} Always.
3. Secure. We never eval(). Rules only have read access to data you provide, and no write access to anything.
4. Flexible. Easy to add new operators, easy to build complex structures.

Examples

simple

JSONLogic.apply({ "==" => [1, 1] }, {})
# => true

This is a simple rule, equivalent to 1 == 1. A few things about the format:

1. The operator is always in the 「key」 position. There is only one key per JsonLogic rule.
2. The values are typically an array.
3. Each value can be a string, number, boolean, array (non-associative), or null

Compound

Here we're beginning to nest rules.

JSONLogic.apply(
{ "and" => [
  { ">" => [3,1] },
  { "<" => [1,3] }]
}, {})

# => true

In an infix language (like JavaScript) this could be written as:

( (3 > 1) && (1 < 3) )

Data-Driven

Obviously these rules aren't very interesting if they can only take static literal data. Typically jsonLogic will be called with a rule object and a data object. You can use the var operator to get attributes of the data object:

 JSONLogic.apply(
  { "var" => ["a"] }, # Rule
  { "a" => 1, "b" => 2 }  # Data
 )
 # => 1

If you like, we support syntactic sugar on unary operators to skip the array around values:

JSONLogic.apply(
  { "var" => "a" },
  { "a" => 1, "b" => 2 }
 )
# => 1

You can also use the var operator to access an array by numeric index:

JSONLogic.apply(
  { "var" => 1 },
  ["apple", "banana", "carrot"]
)
# => "banana"

Here's a complex rule that mixes literals and data. The pie isn't ready to eat unless it's cooler than 110 degrees, and filled with apples.

rules = JSON.parse(%Q|{ "and" : [
  {"<" : [ { "var" : "temp" }, 110 ]},
  {"==" : [ { "var" : "pie.filling" }, "apple" ] }
] }|)

data = JSON.parse(%Q|{ "temp" : 100, "pie" : { "filling" : "apple" } }|)

JSONLogic.apply(rules, data)

# => true