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    Racket-style Higher-Order Contracts in Plain JavaScript


    npm install rho-contracts


    (scroll down to Tutorial to skip the intro)

    rho-contracts.js is an implementation of Racket's higher-order contracts library in JavaScript. It is an attempt to bring to JavaScript the reliability benefits we usually get from static types, namely:

    • Types detect bugs early, loudly, and provide clear error messages with precise blame.

    • Types establish powerful invariants that guarantee that certain kinds of bugs do not exists in certain sections of the code.

    • Types act as a checked documentation for the expected input-output type of functions.

    • Types provide a fulcrum against which we can leverage a refactoring.

    Among the dynamic languages, JavaScript is suffering from the absence of static types quite, because of it propensity for implicitly converting everything into everything else, and its habit turning anything into a null at a moment's notice. When I couldn't stand it anymore, I wrote this contract library.

    Run-time vs Compile-time

    rho-contracts.js is purely a run-time checker. It will never give a compile-time error; it will never refuse to run your code. rho-contracts.js is an assert library where the assertions are written in a style similar to that of a static type system, and whose checking discipline is sufficiently strict to provide similar guarantees as a type system (though not the same.)

    Higher-order contracts

    rho-contracts.js is an higher-order contract library, as opposed to run-of-the-mill assertion library, which means that it provides the ability to check assertions on functions received as an arguments, and on function returned from functions. When implementing derive(fn, deltaX), it is trivial to add an assert that checks that deltaX is a number. It is harder to check that fn is a function that always returns a number. Without higher-order contracts, the only way to implement this check is to pollute the code with if (!isNumber(result_from_fn)) ... everywhere fn is called -- that's not great. Higher-order contracts make it possible to place the specification next to the definition of derive, where it belongs, like this:

    var c = require('rho-contracts')
    // derive: returns a function that is the numerically-computed derivative
    //         of the given function.
    var derive =
      /* This is the specification: */ { fn: { x: c.number } ).returns(c.number) },
             { deltaX: c.number                                   } )
             /* And the implementation goes here: */ 
             function(fn, deltaX) { 
               return function(x) { 
                return (fn(x+deltaX/2) - fn(x-deltaX/2))/deltaX

    In this example, we use to instantiate a contract stating that derive is a function of two arguments. The first argument, which is named fn, must be a function of one argument, called x, which must be a number. The contract also specifies that derive's second argument is named deltaX, which must be a number.

    The derive function itself is created as an anonymous function using JavaScript's own function keyword. The newly created anonymous function is then immediately wrapped with a contract-checking shell, using rho-contracts.js' .wrap() method on contracts. The result of .wrap() is a function that:

    1. checks that the given arguments passes their contracts (aka, that fn is a function and that deltaX is a number),
    2. calls the original function, then
    3. checks that the result of calling the function matches the contract specified in the .returns() clause, then
    4. passes the result through to the to the original caller.

    In addition, at the moment of the call to the original function (Step 2 above), rho-contracts.js will .wrap() the function passed-in for fn. This way fn itself will be protected by a contract shell during the entire duration of the execution of the body of derive, and all its invocations will be checked against the contract.

    Given the definition for derive above:

      > function quadratic(x) { return 5*x*x + 3*x + 2 }
      // When `derive` is called correctly, there is no error:
      > var linear = derive(quadratic, 1.0)
      > linear(0)
      > linear(1)
      > linear(10)
      // Error: calling with the arguments flipped:
      > derive(1.0, quadratic)
      ContractError: Expected fun, but got 1
      For the `fn` argument of the call.
      // Error: forgetting an argument:
      > derive(quadratic)
      ContractError: Wrong number of arguments, expected 2 but got 1
      // Error: calling with the wrong kind of function:
      > var fprime = derive(function(x) { return "**" + x + "**" }, 1.0)
      // There is now a contract-checking shell installed around `fprime` that
      // throws an error when `fprime` is called:
      > fprime(100)
      ContractError: `fn()` broke its contract
      Expected number, but got '**100.5**'
      for the return value of the call.

    Note how these contract errors are triggered much faster than JavaScript's native error, they provide clearer error messages, and they highlight the exactly line where the error is, rather than some line deep inside the implementation of derive.

    Blame, Blame-correctness, and Blame Tracking

    In the last example, when fn fails to return a number, which code is responsible for the failure? A normal assertion library used as desbribed earlier would raise an exception: assertion failed: expected a number for variable result_from_fn but got a string. This exception would contain a stack trace whose first frame would be pointing the blame on the shoulders of the implementation of derive. But that is incorrect. The error is not that derive assigned a wrong value to the result_from_fn variable. Rather, fn broke its contract -- or more precisely, the module calling derive was contractually required to provide a function that would only return numbers when called, but it failed to abide to its responsibility. The error message should make it clear that the failure comes from fn, not from derive. rho-contracts.js's error messages do indeed makes this clear. The error printed is:

      `fn()` broke its contract. Expected a number, but got '**100.5**'
      for the return value of the call.

    rho-contracts.js is an implementation of the paper Contracts for higher-order functions, by Findler and Felleisen, ICFP 2002. The paper formalizes the notion of blame, describes the blame-tracking algorithm necessary to report blame correctly, and proves the algorithm correct.

    This implementation follows the paper closely, though without Racket's macro system it was not possible to implement the report of blame in term of the name of the module interacting. rho-contracts.js only reports the function names.

    Contracts on Functions-as-Values

    rho-contracts.js's higher-order contracts can also be used to check the correctness of functions used as values (aka, stored inside data structures.) This is clearly very useful in JavaScript where functions-in-data are used everywhere. In JavaScript, objects are constructed by putting functions into a hash table, then passing that hash table around. It would be impossible to check these functions against their specification without higher-order contracts.

    For example:

    // Define a contract for position objects with two methods, `moveX` and `moveY`:
    > var posContract = 
                x: c.number,
                y: c.number,
                moveX:{dx: c.number}),
                moveY:{dx: c.number})
    // Define a constructor for position objects. Objects returned 
    // will have their methods `.wrap()`-ed with contract-checking shells:
    > var makePos ={x: c.number}, { y: c.number })
        function(x, y) { 
          return { x: x, y: y,
                   moveX: function(dx) { return makePos(this.x + dx, this.y) }
                   moveY: function(dy) { return makePos(this.x, this.y + dy) }
    // Try to misuse the object:
    > makePos(5, 7).moveX("left")
    ContractError: on `moveX()`
    Expected number, but got 'left'
    for the `dx` argument of the call.


    (In a delightful instance of self-reference, the contract library is documented and checked using the contract library itself. If reading tutorials is not your thing, you may want to instead look at the contracts placed on rho-contracts.js's functions and methods by reading contract.face.js directly.)

    The contract library is typically require'd and bound to a variable called c:

    > var c = require('rho-contracts')

    Basic Value Contracts

    Some fields of c are contract objects you can use directly, such as the c.number contract:

    > c.number.toString()
    > c.number.check(5)       // everything is fine, no error, returns the given value.
    > c.number.check("five")  // boom, because a string is not a number.
    ContractError: Expected number, but got 'five'

    The ContractError being thrown is a normal JavaScript Error. It can be caught and rethrown like normal exceptions. Other useful basic contracts are c.string, c.integer, c.bool, and c.regexp.

    • c.string : accepts only strings, according to Underscore.js's _.isString()
    • c.integer : accepts only numbers v that satisfy Math.floor(v) === v
    • c.bool : accepts only booleans, according to Underscore.js's _.isBoolean()
    • c.regexp : accepts only regular expressions, according to Underscore.js's _.isRegExp()

    For completeness, there are also

    • c.falsy : accepts only values that selects the else branch of a JavaScript conditional
    • c.truthy : accepts only values that select the if branch.
    • c.value() : accepts only the given value and nothing else.
    • c.any : the contract that accepts everything
    • c.nothing : the contract that rejects everything

    Other fields of c are functions that construct interesting contracts, such as c.oneOf() which returns a contract that only accepts the values enumerated:

    > var anwserContract = c.oneOf("y", "yes", "n", "no")
    > anwserContract.toString()
    'c.oneOf(y, yes, n, no)'
    > answerContract.check("yes")   // good, no error
    > answerContract.check("bunny")    // boom
    ContractError: Expected oneOf(y, yes, n, no), but got 'bunny'

    On particularly powerful contract is c.or(), which is a contract that takes two or more contracts as argument, and returns a contract that accept a value if it passes any one of the given contracts:

    > c.or(c.number, c.string).check(10)        // good
    > c.or(c.number, c.string).check("ten")     // good
    > c.or(c.number, c.string).check( { x: 10 } )
    ContractError: none of the contracts passed:
     - c.number
     - c.string
    The failures were:
    c.number: Expected number, but got { x: 10 }
    c.string: Expected string, but got { x: 10 }

    The c.or() contracts makes it possible to specify types for the kind of heterogeneous functions that are common in idiomatic JavaScript, but that would be refused outright by most static type systems (that is so awesome.)

    Storing Custom Contracts

    The contract library provides a rich collection of contract function to construct sophisticated contracts from simple one, such as:

    • c.or() : as we just saw, accepts value that passes at least one of the given contracts.

    • c.and() : accepts only values that pass all of the given contracts.

    • c.matches() : accepts only strings that match the given regular expressions.

    In all likelihood, you will be instantiating a large number of custom contracts. It is customary to create a hash to contain the custom contract created in an application or in a particular module:

    > var cc = {}  // custom contracts
    > cc.numberAsAString = c.matches(/^[0-9]+(\.[0-9]+)?$/)
    > cc.numberAsAString.check("42")            // ok
    > cc.numberAsAString.check("10.7")          // ok
    > cc.numberAsAString.check("10.")           // boom
    ContractError: Expected matches(/^[0-9]+(\.[0-9]+)?$/), but got '10.'

    Another option is to make a clone of the contract library at the top of your NodeJs module and keep the contracts created and used in that module in the clone:

    > var __ = require('underscore')
    > var c = __.clone(require('rho-contracts'));
    > c.numberAsString = c.matches(/^[0-9]+(\.[0-9]+)?$/)
    > c.or(c.falsy, c.numberAsString).check(null)     // ok, null is falsy

    To prevent the toString() output of custom contracts from become unwieldy long and render the rho-contracts.js's error messages difficult to read, call .rename() before storing them:

    > c.numberAsString = c.matches(/^[0-9]+(\.[0-9]+)?$/)
    > c.numberAsString.check("o_0.")           // boom
    ContractError: Expected numberAsString, but got 'o_0.'

    Data Structure Contracts

    A c.array() contract checks that all items in the array passes the given contract:

    > c.array(c.integer).check([1, 2, 3, 45.2, 5, 6])
    ContractError: Expected integer, but got 45.2
    for the 4th element of the array.
    The full value being checked was:
    [ 1, 2, 3, 45.2, 5, 6 ]

    A c.tuple() contract checks that the array has at least the given number of items (having extra items is OK). Then it checks that each item passes its corresponding contract:

    > c.tuple(c.number, c.string).check([10, "ten"])   // ok
    [ 10, 'ten' ]
    > c.tuple(c.number, c.string).check([10, 20])      // boom
    ContractError: Expected string, but got 20 
    for the 2nd element of the tuple.
    The full value being checked was:
    [ 10, 20 ]
    > c.tuple(c.number, c.string).check([10])          // boom
    ContractError: Expected tuple of size 2, but got [ 10 ]

    A c.hash() contract checks that all right-hand values of a hash table passes the given contract:

    > c.hash(c.bool).check({ a: true, b: true, c: false, d: null, e: false })
    ContractError: Expected bool, but got null
    for the key `d` of the hash.
    The full value being checked was:
    { a: true, b: true, c: false, d: null, e: false }

    Contracts on Functions

    Contract on functions are implemented by wrapping the implementing function with a contract-checking shell. This is achieved with the .wrap() method on contracts:

    > function square_implementation(x) { return x * x }
    > var square_contract = ( { x: c.number } )
    > var square = square_contract.wrap(square_implementation)
    > square(25)

    The contract-checking shell checks all invocations of the square function. It will raise an error if either the wrong number of arguments is provided, or if any argument fails to check against its contract:

    > square(10, 11, 12)
    ContractError: Wrong number of arguments, expected 1 but got 3
    > square("cat")
    Expected number, but got 'cat'
    for the `x` argument of the call.

    Usually, the implementation, the contract, and the wrapped function are all created at once in one expression, like this:

    var square = { x: c.number } )
      function (x) { return x * x } )

    Each argument's contract is specified in the call to using a hash table containing exactly one field. The name of that field is used by rho-contracts.js's error messages when the argument's check fails. Note that the name of the argument in the contract can be different from the name of the argument in the implementation. This is sometime useful -- at time the implementation might want to uses a short name internally, yet still prefer to give users a long-form variable name in the error messages:

    var normalizeTime = { secondSinceEpoc: c.number } )
                           function (s) { return s % 60 } )
    > normalizeTime(124526)
    > normalizeTime(null)
    ContractError: Expected number, but got null
    for the `secondSinceEpoc` argument of the call.

    Contracts for function of more than one arguments are specified by passing additional one-field hashes, separated by commas:

    var area = { x: c.number }, { y: c.number } )
      function(x, y) { return x * y }

    Attempting to pass all arguments as a single hash is an error:

    > var area = { x: c.number,  y: c.number } )
    >             .wrap(          // ^---- THIS IS WRONG
    >   function(x, y) { return x * y }
    ContractLibraryError: fun: expected exactly one
    key to specify the name of the 1st arguments, but got 2

    This style of specifying arguments names when calling is necessary because JavaScript does maintain the order of fields in hashes.

    Contracts returned by have three additional methods not found on other contracts:

    • : This will check that the function returns only numbers.

    • : This will allow a variable number of arguments, so long as they are all numbers. Generally, the contract passed to .extraArgs() will be matched against an array containing the extra arguments beyond those specified explicitly. This opens the possibility of checking overloaded function and other rich combinations of extra arguments by using c.or() contract along with c.tuple() contracts.

    Like all other methods on contract, these thwo methods, .returns() and .extraArgs() do not modify the original contract. Instead they return a new contract which checks everything the original contract checks, plus their additional check. They are used like this:

    > var triceWord ={s:c.any}).returns(c.string)
                        //     ^---- This is a bug, should be `c.string`        
                          function (s) { return s + s + s })
    > triceWord("bork")
    > triceWord(35)
    ContractError: Expected string, but got 105
    for the return value of the call.
    • ---- ) : We mention .ths() for completeness. This contract checks that the method was invoked on an object of the right form. (Note, this method name is missing an "i" to avoid clashing with the JavaScript reserved word "this"). However, usages of are rare. It is more customary to use the .method() method on object contacts (See Contracts on Objects below.) is useful when using the Apply Invocation Pattern described in Chapter 4 of Douglas' Crockford' JavaScript, The Good Parts.

           > var makeStatus = function(string) { return { status:  string } }
           > var get_status = 
     {status: c.string})).returns(c.string)
                   function() { return this.status })
           > get_status.apply({ status: 'A-OK' }) // OK
           > get_status.apply({ statosstratos: 'I have a typo' }) // not OK
           ContractError: Field `status` required, got { statosstratos: 'I have a typo' }
           for this `this` argument of the call.

    Contracts for Optional Arguments

    Contracts can be marked optional using c.optional() When used for a function's argument, a contract that has been marked optional makes that argument optional (the contract itself is not affected otherwise). All arguments to the right of an optional argument must be optional as well.

    > var c = require('rho-contracts')
    > var util = require('util')
    > var x = 0
    > var incrementIt ={ i: c.optional(c.number) } ).returns(c.number)
          function(i) { if (i) x+=i; else x++; return x })
    > incrementIt(10)
    > incrementIt()   // calling with the argument omitted
    > incrementIt(10, 20) // too many arguments!
    ContractError: Too many arguments, expected at most 1 but got 2

    Wrapping vs Checking

    Recall, we cannot tell if a function will be miscalled until it is called, and we cannot tell if a function will return a value of the wrong type until it tries to return. Thus function contracts cannot be checked without wrapping the targeted function with a contract-checking shell. Concretely, this means it is an error to call .check() on a function contract:

        >{ n: c.integer }).check(function(n) { return n+1 })
        ContractLibraryError: check: This contract requires wrapping. 
        Call wrap() instead and retain the wrapped result.

    The requirement to call .wrap() instead of .check() carries over to contracts over data structures containing functions:

        > var operations = [function (x) { return x + 1 }, 
                            function (x) { return x * 2 },
                            function (x) { return x * x } ]
        // Check whether `operations` is indeed an array of functions from number to number:
        > c.array({ x: c.number }).returns(c.number))
        ContractLibraryError: check: This contract requires wrapping. 
        Call wrap() instead and retain the wrapped result.

    By replacing .check() with .wrap(), rho-contracts.js will recur down the array and wrap each function with the function contract:

        > var operations_wrapped = 
            c.array({ x: c.number }).returns(c.number))

    Here, .wrap() returns a new array containing the wrapped functions. So long as the array's functions are used correctly, the presence of the contract checking-shells is unnoticeable:

        > operations_wrapped.foreach(function(fn) { util.debug(fn(5)) }
        DEBUG: 6
        DEBUG: 10
        DEBUG: 25

    But if we misuse one of the functions, the checking shell throws an exception. The error provided clearly identifies the source of the fault:

        > operations_wrapped.foreach(function(fn) { fn("five") }
        ContractError: Expected number, but got 'five'
        for the `x` argument of the call.
        The full value being checked was:
        [ [Function], [Function], [Function] ]

    Meanwhile, the original functions rest unmodified in the original operations array, and continue to fail silently:

        > operations.foreach(function(fn) { util.debug(fn("five")) }
        DEBUG: five1
        DEBUG: NaN
        DEBUG: NaN

    The .wrap() method wraps recursively all JavaScript's data structures, array, hashes, tuples, and objects.

    Object Contracts

    Since objects in JavaScript are constructed out of normal hash tables containing normal functions, contracts on objects follow the usage described in the previous three sections Data Structure Contracts, Contracts on Functions and Wrapping vs Checking.

    > String.prototype.repeat = function( num ) {   // A helper function on
                                                       String, just for fun.
        return new Array(num + 1).join(this);
    > c.animal = c.object({ nLegs: c.number, 
                            name:  c.string,
                            speak:{n: c.number}).returns(c.string) })
    > var makeCat ={ name: c.string }).returns(c.animal)
                    .wrap(function (name) { 
                      return {
                        nLegs: 4,
                        name: name,
                        speak: function(n) { return + " says " + "meow".repeat(n) }
    > var makeBird ={ name: c.string }).returns(c.animal)
                    .wrap(function (name) { 
                      return {
                        nLegs: 2,
                        name: name,
                        speak: function(n) { return + " says " + "tweet".repeat(n) }
    > var tweetie = makeBird("tweetie")
    > tweetie.speak(3)
    tweetie says tweettweettweet.

    In this example, the contract on the .speak() method will correctly verify that the method returns a string. However, it does not verify whether it was correctly invoked on an animal -- an error could go undetected:

    > var speak = tweetie.speak
    > speak(2)
    undefined says tweettweet.     // Yikes!

    The .ths() method on function contracts can be used to add this additional check. In order to distinguish functions intended be used as methods, rho-contracts.js provides c.method(), which is a variant of that takes the contract on this as its first argument:

     > c.animal = c.object({ nLegs: c.number, 
                             name:  c.string,
                             speak: c.method(c.animal, { n: c.number}).returns(c.string) }) 
                                           // ^--- Ousp, this doesn't actually work.

    However, this attempt fails due to the cyclic reference: the line of code defining the contract for animals refers to the contract for animals. When the c.animal is looked up on the third line the first line has not returned yet, so c.animal is not defined and the look up returns of c.animal returns undefined.

    rho-contracts.js provides a way to establish this cyclic reference in large part to make it possible to fully specify such contract on objects. The function c.cyclic() creates a temporary placeholder until we can close the cycle:

    > c.animal = c.cyclic()

    The placeholder returned by c.cyclic() has only one useful method: .closeCycle(), which must be called with the actual contract:

    > c.animal.closeCycle(c.object({ nLegs: c.number, 
                                     name:  c.string,
                                     speak: c.method(c.animal, { n: c.number }).returns(c.string) }))

    When using this better definition of c.animal, the error is caught as it should:

     > var speak = tweetie.speak
     > speak(2)
     ContractError: on `speak()`
     Expected object, but got undefined
     for this `this` argument of the call.

    rho-contracts.js provides three additional pieces of functionality made specifically for object contracts.

    • c.optional() : Contracts marked "optional" by the c.optional() function (as discussed earlier in the Contracts for Optional Arguments section) are also used to specify optional fields of objects. A field is considered missing if is not set, or if it is set to null. All these are OK.
    > = c.object({ carModel: c.string,
                         trunkSize: c.optional(c.number) }) // missing to indicate a sport car with no trunk  
    >{ carModel: "MINI Cooper Coupe",          // OK
                    trunkSize: 9.8 })  
    >{ carModel: "Infiniti IPL G Convertible", // OK
                    trunkSize: null })


    >{ carModel: "Infiniti IPL G Convertible" }) // Also OK

    But not:

    >{ trunkSize: 22.1 })  
    ContractError: Field `carModel` required, got { trunkSize: 9.8 }
    • .strict() : By default, objects are allowed to have additional fields not specified in the contract. Calling .strict() returns a contract that disallows them.
     >{ carModel: "semitruck", towing: true }) // this is fine
     >{ carModel: "semitruck", towing: true }) // but this is not
     ContractError: Found the extra field `towing` in { carModel: 'semitruck', towing: true }
    • .extend :

    • .strict on tuples

    More Functionality

    Additional functionality that's not documented yet:

    • c.pred

    • forwardRef/setRef

    • Contracts on Whole Modules, publish()

    • The partially documented documentation feature:

    • .doc

    • .theDoc

    • documentType

    • documentTable

    • document category

    • document module

    And also

    • anyFunction
    • isA
    • the contract contract
    • implicity promotion to contract
    • toContract: promoting values to a contract explicitely.
    • quacksLike
    • silentAnd
    • c.fn()
    • setErrorMessageInspectionDepth

    Related Work

    • rho-contracts.js is an implementation of the paper Contracts for higher-order functions, by Findler and Felleisen, ICFP 2002.

    • The original and best implementation of the paper's ideas is racket/contract

    • is a dialect of CoffeeScript that like rho-contracts.js also implements Racket's contracts.

    • runs on top of a contract-checking runtime implemented in JavaScript using Proxies, that is currently only implemented in Firefox 4+ and chrome/V8 with the experimental javascript flag enabled.

    • ristretto-js implements a run-time checker for types written in Haskell syntax inside of specification strings. It suffers from the troubles of externally embedded languages, namely that it exists separate from its host language. It support only a limited number of basic type (Int, Num, String, Bool, Object, Array) with no possibility of extensions that's available and its type name space is separate from the JavaScript name space and module machinery.

    • jsContract, cerny.js, are good-old (bad-old?) Eiffel-style contract libraries. True to their Eiffel roots, they require lots of code for little benefit, in particular they cannot check higher-order functions, cannot separate specification from implementation. See Findler and Felleisen for a more thorough comparison.


    This library was created at, originally for internal use. We are releasing it to the open source community under the Mozilla open-source license (MPL).




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