contract

contract is a module that checks whether or not function arguments satisfy a specified "contract" string. These strings define the argument type requirements for a given function using a custom mini-language.

contract provides two main benefits:

1. It provides a simple mechanism for checking argument datatypes.
2. It helps document the intended use of each function.

contract is NOT:

• A full unit-testing solution.
• A compile-time evaluator. All checks are performed at run-time, however they can be turned off for non-development builds. If you want something that evaluates your code prior to execution, consider something like TypeScriptToLua.

Installation

contract can be installed using luarocks:

luarocks install contract

You may also download the latest release and include contract.lua in your project folder.

contract is compatible with LUA versions 5.1 and up. It does not have any external dependencies.

Usage

Consider the following function:

local function sum(a, b)
    assert(type(a) == 'number', 'arg "a" must be a number')
    assert(type(b) == 'number', 'arg "b" must be a number')
    return a + b
end

This function expects two numbers to be passed to it. Note the two assert calls - if any other value types are passed in, the asserts will fail and an error will be raised. We can think of the sum() function as having a "contract" that says, "I expect two number values to be passed to me, and as long as that is true I will run correctly."

contract() can be used as an alternative to writing out these asserts manually. For example:

local contract = require('contract')

local function sum2(a,b)
    contract('rn, rn', a, b)
    return a + b
end

Function sum2() is equivalent to sum() in that any non-number values for a and b will be rejected.

The first argument passed to contract() is the contract string "rn, rn", which represents a contract for two required number arguments.

The arguments a and b are then passed after the contract string to verify if they satisfy the contract. If sum2() is called with arguments that violate this contract, an error will be raised:

sum2(1, 2)        --<passes>
sum2('one', 2)    --Contract violated: arg "1" is type "string", but must be "number".
sum2(1, 'two')    --Contract violated: arg "2" is type "string", but must be "number".
sum2(1)           --Contract violated: arg pos "2" is required.

Contract string syntax

The syntax for contracts is quite simple:

• Each argument has a rule specifying its allowed datatype(s). For example, in the contract string above "rn" stands for "required number".
• The rules for each argument are listed in position order separated by commas.
• Contracts are not case-sensitive.
• All whitespace is ignored.

contract can be used to check any of the primitive Lua datatypes:

local function callIfTrue(bl, fnc)
    contract('rb, rf', bl, fnc)
    if bl then
        fnc()
    end
end

callIfTrue(true, function() print('hello') end)    --'hello'
callIfTrue(true, 'not a function')    --Contract violated: arg "2" is type "string", but must be "function".

There are multiple specifiers associated with each datatype. This allows you to be as brief or as explicit as you want:

contract('number', 1)    --<passes>
contract('s, str, string', 'one', 'two', 'three')    --<passes>

Here is a table listing all the acceptable specifiers for each Lua type:

datatype	specifiers
number	"n", "num", "number"
string	"s", "str", "string"
boolean	"b", "bool", "boolean"
table	"t", "tbl", "table"
function	"f", "fnc", "func", "function"
thread	"th", "thread"
userdata	"u", "usr", "user", "userdata"

Note that contract is intended for evaluating arguments passed to a calling function, but you can actually pass any values to it:

contract('rt, rs', {}, '')           --<passes>
contract('ru', 'not userdata')    --Contract violated: arg "1" is type "string", but must be "userdata".
contract('rf', print)              --<passes>

Required and optional arguments

contract allows arguments to be specified as "required" or "optional". A type specifier preceeded by an "r" flags it as required. Any specifiers without an "r" are treated as optional.

Optional arguments can be omitted, but if they are passed they still must match their specified type for the contract to pass:

local function config(tbl, name, op1, val)
    contract('rt, s, rb, n', tbl, name, op1, val)
    tbl.name = name or 'default'
    tbl.op1 = op1
    tbl.val = val or 42
end

config({}, 'mytable', true, 19)    --<passes>
config({}, 'mytable', true)        --<passes>
config({}, nil, true)              --<passes>
config({}, 'mytable', true, 'not a number')    --Contract violated: arg "4" is type "string" but must be "number".
config({}, true)    --Contract violated: arg "2" is type "boolean" but must be "string".

Optional args at the end of the list can be completely omitted. Also, note that optional args can come before required args, but if they are being omitted they must have nil passed explicitly - just leaving these out will not work.

Multi-type arguments

You can specify an argument that can be one of multiple types using the '|' operator:

contract('rn|s', 1)    --<passes>
contract('rn|s', 'one')    --<passes>
contract('rn|s', true)    --Contract violated: arg "1" is type "boolean" but must be one of: "number|string".

You can also use the "a" or "any" specifiers to accept values of any type:

contract('a', 1)    --<passes>
contract('any', 'one')    --<passes>
contract('a', true)    --<passes>
contract('ra, ra', 1)    --Contract violated: arg pos "2" is required.

Extra arguments are ignored

If more arguments are passed than are specified in the contract, then as long as the contract holds the extra arguments do not matter:

contract('rn', 1, 'two', 'three')    --<passes>

Implicit argument lookup

There are actually two ways to use contract(). The first is by explicitly passing the argument values you wish to check against the contract (all of the examples above use this method). The second method automatically looks up the arguments from the function that called contract() without needing to pass them in:

local function sum(a, b)
    contract('rn, rn')    --implicit lookup of a & b
    return a + b
end

sum(1, 2)    --<passes>
sum(1, 'two')    --Contract violated: arg "2" is type "string" but must be "number".

While this method uses less typing, it unfortunately takes longer to execute compared to the explicit method. You are free to use whichever method best suits your needs.

Enabling/disabling checks

The contract module can be globally disabled by calling contract.off(). Once disabled, any calls to contract() will essentially be no-ops. This is helpful for production builds since it removes the extra overhead needed to process the contracts.

The module can be re-enabled by calling contract.on(). There is also contract.toggle(), which toggles between the on/off state.

Performance

To help minimize execution time, contract makes use of an internal cache of intermediate objects tied to each contract string. The first time a contract string is passed, it is parsed and built into a corresponding object. The next time that contract string is passed in, the arguments are checked against the previously-built object rather than creating a new one.

Sadly, the contract module still adds a bit of execution overhead compared to just checking arguments with assert(). You can test performance on your system using the performance.lua file included in this repository.

Contract language in EBNF

Here is the complete grammar for the contract string mini-language:

contract = '' | (argRule , (',' , argRule)*)
argRule = ['r'] , type , ('|' , type)*
type = num|str|bool|user|fnc|th|tbl|any
num = 'n'|'num'|'number'
str = 's'|'str'|'string'
bool = 'b'|'bool'|'boolean'
user = 'u'|'usr'|'user'|'userdata'
fnc = 'f'|'fnc'|'function'
th = 'th'|'thread'
tbl = 't'|'tbl'|'table'
any = 'a'|'any'

API

contract.check(input, ...)

Checks the argument list against the contract string input. If no arguments are passed, attempts to look up the arguments passed to the function that called contract.check(). Raises an error if the contract is violated.

contract(input, ...)

Alias for contract.check().

contract.on()

Enables all contract checking (module is "on" by default).

contract.off()

Turns off all contract checking.

contract.isOn()

Returns true if contract checking is currently enabled; otherwise, returns false.

contract.toggle()

Switches the on/off state of the module.

contract.clearCache()

Clears the contract cache.

Credits

contract is written and maintained by Matt Rogge.

Portions of the parser code were inspired by this great series of tutorials written by Ruslan Spivak.

License

contract is licensed under the MIT license.