A library for validating YAML documents against a schema.

yaml-schema is a schema validation library that allows you to enforce type safety and structural constraints on YAML files. It works directly with an AST (which can be obtained from Psych) representation to provide strict validation without implicit type coercion.

require "yaml-schema"

schema = {
  "type" => "object",
  "properties" => {
    "name" => { "type" => "string" },
    "age" => { "type" => "integer" },
    "active" => { "type" => "boolean" }
  }
}

yaml_string = <<~YAML
  name: "John Doe"
  age: 30
  active: true
YAML

node = Psych.parse(yaml_string)
YAMLSchema::Validator.validate(schema, node.children.first)

• string - Scalar string values
• integer - Numeric integers
• boolean - Boolean values (true/false)
• null - Null values
• array - Sequences with optional constraints
  • items - Schema for all array elements
  • prefixItems - Fixed-position tuple validation
  • maxItems - Maximum array length
• object - Mappings with properties
  • properties - Named properties with schemas
  • items - Schema for arbitrary key-value pairs
  • tag - Optional YAML tag requirement

Specify multiple valid types for a field:

schema = {
  "type" => "object",
  "properties" => {
    "value" => { "type" => ["null", "string"] }
  }
}

schema = {
  "type" => "object",
  "properties" => {
    "users" => {
      "type" => "array",
      "items" => {
        "type" => "object",
        "properties" => {
          "name" => { "type" => "string" },
          "tags" => {
            "type" => "array",
            "items" => { "type" => "string" }
          }
        }
      }
    }
  }
}

Validation errors include detailed path information:

begin
  YAMLSchema::Validator.validate(schema, node)
rescue YAMLSchema::Validator::UnexpectedType => e
  puts e.message  # e.g., "Expected integer at root -> age"
end

• UnexpectedType - Node doesn't match expected type
• UnexpectedProperty - Object has unexpected properties
• UnexpectedTag - YAML tag doesn't match schema
• UnexpectedValue - Value doesn't match constraints
• InvalidSchema - Schema definition is invalid