lua-schema

A simple package to check LUA-data against schemata. The package is written entirely in Lua (5.2) and has no further dependencies. It is designed to be easily extensible.

The problem tackled by this package is the following: Suppose your program is reading data via Lua. Verifying that the data fulfills the requirements of your program is tedious; Lua's lack of types requires you to write repetitive code to verify the input data.

The solution proposed by this package are schemata: You specify a schema for the data and the program automatically verifies the data for you.

Installation

Put schema.lua somewhere where your program can find it. Then use

local schema = require "schema.lua"

Example

We read a bunch of tables, each of them looks like this:

user = {
    id        = 12, -- id is a number
    usertype  = "admin", -- one of 'admin', 'moderator', 'user'
    nicknames = { "Nick1", "Nick2" }, -- nicknames used by this user
    rights    = { 4, 1, 7 } -- table of fixed length of types
}

A schema describing such a table would be

local s = require "schema.lua"

rights = s.AllOf(s.NumberFrom(0, 7), s.Integer)

userSchema = s.Record {
    id        = s.Number,
    usertype  = s.OneOf("admin", "moderator", "user"),
    nicknames = s.Collection(s.String),
    rights    = s.Tuple(rights, rights, rights) 
}

This schema can now be used to check the data:

local err = s.CheckSchema(user, userSchema)

-- 'err' is nil if no error occured
if err then
    print(s.FormatOutput(err))
end

Usage

Specify a schema for your data using the built-in schemata or by writing custom schemata. A schema is either a function or a non-function value. The program takes an object to be checked and a schema. If the schema is a non-function value, the program uses Lua's comparison operator to compare the object to the schema. If the schema is a function, the program returns the result of the function applied to the object and the path of the object in the main-object to check. Schema functions always return list of errors.

The main function to get things going is schema.CheckSchema(obj, schema). It returns nil iff the data matches the schema. Otherwise it returns a list of errors (which in turn may contain suberrors). Use schema.FormatOutput(output) on a non-nil result of schema.CheckSchema to get a string describing all errors (or just use tostring on the result).

The package comes with a set of builtin schemata (see below). It is very easy to extend the package with additional schemata (see 'Custom Schemata' below).

Builtin Schemata

The schemata built into the package are (mostly) designed to check local properties of the data (context free properties). While it is possible to write custom schemata to handle context sensitive data (such as uniqueness of certain values), the schemata shipping with the library feature only a single context sensitive schema ('Case', see below).

All non-function values are schemata. The following schemata are built into the package (sorted alphabetically):

• AllOf(...)

  Takes a list of schemata and accepts any object that is accepted by all of the schemata. Example:

  local exampleSchema = schema.AllOf(schema.NonNegativeNumber, schema.Integer)
  local posExample = 3
  local negExample = 2.4
  -- Invalid value: '<val>' must be an integral number
  print(schema.CheckSchema(negExample, exampleSchema))
  local negExample2 = -2.4
  -- Invalid value: '<val>' must be >= 0
  -- Invalid value: '<val>' must be an integral number
  print(schema.CheckSchema(negExample2, exampleSchema))
  

• Any

  Matches anything. Example:

  local exampleData = { "test" }
  -- err is always nil
  local err = schema.CheckSchema(exampleData, schema.Any)
  

• Boolean

  Matches booleans. Example:

  local posExample = true
  local negExample = { true }
  -- Type mismatch: '<val>' should be boolean, is table
  print(schema.CheckSchema(negExample, schema.Boolean))
  

• Case(path, ...)

  Takes a relative path and a list of entries of the form {c, s}, whereby c (condition) and s (consequence) are both schemata. It then navigates to the value denoted by the relative path and checks it against every condition schema. If the condition schema matches, then it tries to apply the consequence schema to the value.

  The relative path is either a non-table value or a path constructed using schema.Path. In such paths, '..' denotes the parent of a value. This parent must be part of the object to be checked. If a value v is given instead of a path, the program constructs a path as in schema.Path("..", v). Example:

  local exampleSchema = schema.Record {
    kind   = schema.OneOf("user", "admin"),
    rights = schema.Case("kind", {"user", "000"}, {"admin", "777"})
  }
  local posExample = {
    kind   = "user",
    rights = "000"
  }
  
  local negExample = {
    kind = "user",
    rights = "777"
  }
  -- Case failed: Condition 1 of 'rights' holds but the consequence does not
  --   Invalid value: rights should be 000
  print(schema.CheckSchema(negExample, exampleSchema))
  local negExample2 = {
    kind  = "test", -- invalid kind!
    rights = "777" 
  }
  
  -- Case failed: No condition on 'rights' holds
  -- No suitable alternative: No schema matches 'kind'
  print(schema.CheckSchema(negExample2, exampleSchema))
  

• Collection(valSchema)

  Alias for Map(Any,valSchema): Takes a schema and matches all tables which have values matching the given schema. The keys of the table are ignored. Also accepts the empty table. Example:

  local exampleSchema = schema.Collection(schema.Boolean)
  local posExample = {}
  local posExample2 = { test = true, false, false }
  
  local negExample = "test"
  -- Type mismatch: '<val>' should be a map (table), is string
  print(schema.CheckSchema(negExample, exampleSchema))
  
  local negExample2 = { "true", test = 1, false }
  -- Type mismatch: '1' should be boolean, is string
  -- Type mismatch: 'test' should be boolean, is number
  print(schema.CheckSchema(negExample2, exampleSchema))
  

• Function

  Matches functions. Example:

  local posExample = table.concat
  
  local negExample = "test"
  -- Type mismatch: '<val>' should be function, is string
  print(schema.CheckSchema(negExample, schema.Function))
  

• Integer

  Matches integers. Example:

  local posExample = 42
  
  local negExample = "test"
  -- Type mismatch: '<val>' should be number, is string
  print(schema.CheckSchema(negExample, schema.Integer))
  
  local negExample2 = 42.1
  -- Invalid value: '<val>' must be an integral number
  print(schema.CheckSchema(negExample2, schema.Integer))
  

• Map(keySchema, valSchema)

  Matches all tables whose keys match the keySchema and whose values match the valSchema. Example:

  local exampleSchema = schema.Map(schema.Integer, true)
  local posExample = { [1] = true, [42] = true }
  
  local negExample = { test = true }
  -- Invalid map key
  --   Type mismatch: 'test' should be number, is string
  print(schema.CheckSchema(negExample, exampleSchema))
  

• Nil

  Matches nil. Note that you could just as well use the value nil as a schema in most situations. If an argument list is used, you should prefer Nil to nil due to the way that Lua deals with nil values in tables. Example:

  local posExample = nil
  
  local negExample = 1
  -- Type mismatch: '<val>' should be nil, is number
  print(schema.CheckSchema(negExample, schema.Nil))
  

• NonNegativeNumber

  Matches all non-negative numbers (i.e, number >= 0). Example:

  local posExample = 42.3
  
  local negExample = -14
  -- Invalid value: '<val>' must be >= 0
  print(schema.CheckSchema(negExample, schema.NonNegativeNumber))
  

• Nothing

  Does not match anything. Always returns an error.

  local negExample = { "test" }
  -- Failure: '<val>' will always fail.
  print(schema.CheckSchema(negExample, schema.Nothing))
  

• Number

  Matches all numbers.

  local posExample = 42
  
  local negExample = "test"
  -- Type mismatch: '<val>' should be number, is string
  print(schema.CheckSchema(negExample, schema.Integer))
  

• NumberFrom(lower, upper)

  Matches all numbers in the interval [lower, upper].

  local exampleSchema = schema.NumberFrom(0, 42)
  local posExample = 42
  
  local negExample = -1
  -- Invalid value: '<val>' must be between 0 and 42
  print(schema.CheckSchema(negExample, exampleSchema))
  

• OneOf(...)

  Takes a list of schemata and accepts any object that is accepted by at least one of the schemata.

  local exampleSchema = schema.OneOf(schema.String, schema.Number)
  local posExample = 1
  local posExample2 = "test"
  
  local negExample = true
  -- No suitable alternative: No schema matches '<val>'
  print(schema.CheckSchema(negExample, exampleSchema))
  

• Optional(s)

  Alias for OneOf(s, Nil). Represents optional values. Example:

  local exampleSchema = schema.Optional(schema.Integer)
  local posExample = 1
  local posExample2 = nil
  
  local negExample = "test"
  -- No suitable alternative: No schema matches '<val>'
  print(schema.CheckSchema(negExample, exampleSchema))
  

• Pattern(patt)

  Checks that the value is a string matching a given Lua pattern. The entire string must match the pattern: if ^ and $ markers in the beginning and end of string are not present, they are internally added to the pattern. Example:

  local exampleSchema = schema.Pattern("[A-Za-z_][A-Za-z0-9_]*")
  local posExample = "test"
  local posExample2 = "_"
  
  local negExample = "0var"
  -- Invalid value: '<val>' must match pattern '[A-Za-z_][A-Za-z0-9_]*'
  print(schema.CheckSchema(negExample, exampleSchema))
  

• PositiveNumber

  Matches all positive numbers (i.e, number > 0). Example:

  local posExample = 42.3
  
  local negExample = -14
  -- Invalid value: '<val>' must be >= 0
  print(schema.CheckSchema(negExample, schema.PositiveNumber))
  

• Record(tableSchema, additionalValues = false)

  Takes a table schema. The table schema consists of keys (strings only) and schemata for the corresponding values. If the object contains additional values to those mentioned in the schema, the schema fails. This behavior can be changed by setting the second argument to true. Example:

  local exampleSchema = schema.Record {
    data = schema.String,
    data2 = schema.Record {
      test = schema.Number
    }
  }
  local posExample = {
    data = "",
    data2 = {
      test = 15
    }
  }
  
  local negExample = {
    [1] = "",
    data2 = {
      test = "12"
    }
  }
  -- Type mismatch: 'data' should be string, is nil
  -- Type mismatch: 'data2.test' should be number, is string
  -- Invalid key: '1' must be of type 'string'
  -- Superfluous value: '1' does not appear in the record schema
  print(schema.CheckSchema(negExample, exampleSchema))
  

• MixedTable(tableSchema, additionalValues = false)

  Takes a table schema. The table schema consists of keys (strings or numbers only) and schemata for the corresponding values. Additional values treated in the same way as in a Record. Example:

  local exampleSchema = schema.MixedTable {
    [1] = schema.Number,
    [2] = schema.Number,
    count = schema.Number,
  }
  
  local matchingExample = {
    [1] = 4,
    [2] = 5,
    count = 2,
  }
  
  local failExample = {
    [1] = 30,
    count = true,
    data = { 1, 2, 3 }
  }
  
  -- Type mismatch: 'count' should be number, is boolean
  -- Type mismatch: '2' should be number, is nil
  -- Superfluous value: 'data' does not appear in the table schema
  print(schema.CheckSchema(failExample, exampleSchema))
  

  Also can be used to check dynamic schemata via metatable. Example:

  local exampleSchema_mt = {
    __index = function(_, key)
      -- we want to handle only integer keys
      if type(key) ~= 'number' or math.floor(key) ~= key then
        -- unknown field, will report error
        return
      end
  
      -- can be stored in upvalue to avoid creating new schemata
      -- on every metamethod call
      return schema.MixedTable {
        args = schema.Record {
          min = schema.Optional(schema.Number),
          max = schema.Optional(schema.Number),
          assume = schema.Optional(schema.Collection(schema.String)),
        },
        [1] = schema.Function,
      }
    end
  }
  
  local exampleSchema = schema.MixedTable(setmetatable({
    name = schema.String,
  }, exampleSchema_mt))
  
  local failExample = {
    name = 'abc', -- matched via 'name' schemata
    -- following matched via schemata returned from __index metamethod of exampleSchema_mt
    { args = { min = 1, max = 1, assume = { 'number' } }, function() print('number') end },
    { args = { min = 1, assume = { 'string' } }, function() print('string') end },
    { args = { -- empty table ok: all field in args Record are optional
             }, function() end },
    -- only integer args expected in __index metamethod
    [3.5] = { args = { }, function() end },
  }
  
  -- Superfluous value: '3.5' does not appear in the table schema
  print(schema.CheckSchema(failExample, exampleSchema))
  

• String

  Matches strings. Example:

  local posExample = "test"
  
  local negExample = 42
  -- Type mismatch: '<val>' should be string, is number
  print(schema.CheckSchema(negExample, schema.String))
  

• Table

  Matches tables (as in: everything that is of type table). Example:

  local posExample = {}
  local posExample2 = { test = true }
  
  local negExample = "42"
  -- Type mismatch: '<val>' should be table, is string
  print(schema.CheckSchema(negExample, schema.Table))
  

• Test(fn, [msg])

  Runs an arbitrary test function on the value. This is useful for quickly creating custom validations. Example:

  local negExample = {
    plugin = "invalid.module"
  }
  
  -- Invalid value: '<plugin>': not an existing module
  print(schema.CheckSchema(negExample, schema.Test(function(v) return pcall(require, v) end, "not an existing module")))
  

• Tuple(...)

  Takes schemata and matches against a tuple of those schemata in the order passed to the constructor. Example:

  local exampleSchema = schema.Tuple(schema.Number, schema.String)
  local posExample = { 1, "42" }
  
  local negExample = { "42", 1}
  -- Type mismatch: '1' should be number, is string
  -- Type mismatch: '2' should be string, is number
  print(schema.CheckSchema(negExample, exampleSchema))
  
  local negExample2 = {1, "42", 14}
  -- Invalid length: '<val> should have exactly 2 elements
  print(schema.CheckSchema(negExample, exampleSchema))
  

• UserData

  Matches user data.

Custom Schemata

Adding custom schemata is straight-forward. We will start with an example. Suppose we want to have a schema for even integers. We will design a function that matches just those:

-- All schemata get the object to check and a 'path' variable as arguments.
--   - obj is what we want to check
--   - path is the path to obj from the main object to check
function EvenInteger(obj, path)
    -- first check that the obj is an integer.
    local err = schema.Integer(obj, path)
    -- got an error? propagate it.
    if err then return err end
    if obj % 2 ~= 0 then
        return schema.Error("Invalid value: "..path.." must be even", path)
    end
    return nil
end

Alternatively, you can use schema.Test:

EvenInteger = schema.AllOf(
                 schema.Integer,
                 schema.Test(function(obj) return obj % 2 == 0 end, "must be even")
              )

There are a few things to note:

• schema.Error(msg, path, suberrors)

  Takes a message, the current path and a list of errors. Returns a list of errors. If you want to aggregate errors, use append:

  local err = schema.Integer(obj, path)
  -- add another error
  err:append(schema.Error(...))
  

• The path argument is not a string, but a table. Use path:push(key) add a key to the path; path:pop() to remove the last key. Note that path:push does not return a new path but modifies the original path. Your function is expected to get path back into its original state when it returns. Use path:copy() to get a copy of the path. path:getBase() returns the base object (i.e. the main object to be checked). path:target() returns the value the path points to relative to its base object..