gocontracts is a tool for design-by-contract in Go.

It generates pre- and post-condition checks from the function descriptions so that the contracts are included in the documentation and automatically reflected in the code.

(If you need invariants, please let us know by commenting on this issue: #25.)

You invoke gocontracts on an individual Go file. Gocontracts will parse the file and examine the descriptions of all the functions for contracts. The existing contract checks will be overwritten to match the contracts in the description.

A usual workflow includes defining the contracts in the function description and invoking gocontracts to automatically update them in the code.

Each contract is defined as a one-line item in a bulleted list. Gocontracts does not validate the correctness of the conditions (e.g. undefined variables, syntax errors etc.). The code is simply inserted as-is in the header of the function body.

Since contracts are logical conditions of the function, failing a contract causes a panic(). If you need to validate the input, rather than check logical pre- and post-conditions of the function, return an error and do not abuse the contracts.

At the time of this writing (2018-08-19), we found only a library that implemented design-by-contract as functions (https://github.com/lpabon/godbc) and a draft implementation based on decorators (https://github.com/ligurio/go-contracts).

None of them allowed us to synchronize the documentation with the code.

Given a function with contracts defined in the description, but no checks previously written in code:

package somepackage

// SomeFunc does something.
//
// SomeFunc requires:
//  * x >= 0
//  * x < 100
//
// SomeFunc ensures:
//  * !strings.HasSuffix(result, "smth")
func SomeFunc(x int) (result string) {
	// ...
}

, gocontracts will generate the following code:

package somepackage

// SomeFunc does something.
//
// SomeFunc requires:
//  * x >= 0
//  * x < 100
//
// SomeFunc ensures:
//  * !strings.HasSuffix(result, "smth")
func SomeFunc(x int) (result string) {
	// Pre-conditions
	switch {
	case !(x >= 0):
		panic("Violated: x >= 0")
	case !(x < 100):
		panic("Violated: x < 100")
	default:
		// Pass
	}

	// Post-condition
	defer func() {
		if strings.HasSuffix(result, "smth") {
			panic("Violated: !strings.HasSuffix(result, \"smth\")")
		}
	}()

	// ...
}

Note that you have to manually import the strings package since goconracts is not smart enough to do that for you.

Additionally, if you want to use go doc, you have to indent conditions with a space in the function description so that go doc renders them correctly as bullet points.

Certain conditions can be hard to understand when the formal definition lacks a textual description. Gocontracts therefore allows you to introduce condition labels to clarify the intent:

package somepkg

// SomeFunc does something.
//
// SomeFunc requires:
//  * positive: x > 0
//  * not too large: x < 100
func SomeFunc(x int, y int) (result string, err error) {
	// Pre-conditions
	switch {
	case !(x > 0):
		panic("Violated: positive: x > 0")
	case !(x < 100):
		panic("Violated: not too large: x < 100")
	default:
		// Pass
	}
	
	// ...
}

Since we need to distinguish the condition labels from the condition code, we had to restrict the labels to strings of characters [a-zA-Z0-9_;.\-=' ]. Otherwise, if we allowed a full character set, there would be ambiguities between the label and the code.

(We decided against clutter in the documentation such as Go string literals. It is our hope that restricted character set should suit 99% use cases out there. Please let us know if this is not the case. See also golang/go#16666 .)

Go allows you to initialize a condition and execute a simple statement before the check. For example, the initialization is common when checking if an item belongs to a map:

if _, ok := someMap[3]; ok {
	...
}

Following Go, Gocontracts also allows you to include the initialization in the condition. The following code snippet shows you how to document the initialization and what Gocontracts generates:

// SomeFunc does something.
//
// SomeFunc requires:
//  * _, ok := someMap[3]; ok
func SomeFunc(someMap map[string]bool) {
	// Pre-condition
	if _, ok := someMap[3]; !ok {
		panic("Violated: _, ok := someMap[3]; ok")
	}

	// ...
}

Usually, functions either return an error if something went wrong or valid results otherwise. To ensure the contracts conditioned on the error, use implication and write:

// Range returns a range of the timestamps available in the database.
//
// Range ensures:
//  * err != nil || (empty || first < last)
func (t *Txn) Range() (first int64, last int64, empty bool, err error) {
	// ...
}

. Gocontracts will generate the following code:

// Range returns a range of the timestamps available in the database.
//
// Range ensures:
//  * err != nil || (empty || first <= last)
func (t *Txn) Range() (first int64, last int64, empty bool, err error) {
	// Post-condition
	defer func() {
	    if !(err != nil || (empty || first < last)) {
	    	panic("Violated: err != nil || (empty || first < last)")
	    }	
	}()
	
	// ...
}

Note that conditioning can be seen as logical implication (A ⇒ B can be written as ¬A ∨ B). In the above example, we chained multiple implications as err == nil ⇒ (¬ empty ⇒ first ≤ last).

When you want to formally define contracts on state transitions you need to capture the state before and after the function execution. However, Gocontract's conventional post-conditions allow you only to access the state afer the execution.

In order to capture the state before the execution, you need to write a preamble. The preamble is a code snippet written in your documentation and automatically synced with the function body by gocontracts. The snippet must follow the Godoc convetion and be indented with a whitespace (or a tab).

The preamble is executed just after the pre-condition(s) have been verified.

Here is a brief (and admittedly a bit constructed) example with the generated code already included:

package somepackage

// increaseFirst increases the first element of the array.
//
// increaseFirst requires:
//  * len(a) > 0
//
// increaseFirst preamble:
//  oldFirst := a[0]
//
// increaseFirst ensures:
//  * a[0] == oldFirst + 1
func increaseFirst(a []int) {
	// Pre-condition
	if !(len(a) > 0) {
		panic("Violated: len(a) > 0")
	}

	// Preamble starts.
	oldFirst := a[0]
	// Preamble ends.

	// Post-condition
	defer func() {
		if !(a[0] == oldFirst + 1) {
			panic("Violated: a[0] == oldFirst + 1")
		}
	}()

	// Implementation
	a[0]++
}

When developing a library, it is important to give your users a possibility to toggle families of contracts so that they can adapt your contracts to their use case. For example, some contracts of your library should be verified in testing and in production, some should be verified only in testing of their modules and others should be verified only in your unit tests, but not in theirs.

To that end, you can use build tags to allow toggling of contracts at compile time (https://golang.org/pkg/go/build/#hdr-Build_Constraints). Define for each of the contract family a separate file which is built dependening on the build tag. In each of these files, define constant booleans (e.g., InTest, InUTest). Depending on the scenario, set these variables appropriately: in production scenario, InTest = false and InUTest = false, in the test scenario for others InTest = true and InUTest = false, while in the scenario of your unit tests InTest = true and InUTest = true.

The examples of the three files follow.

contracts_prod.go:

// +build prod,!test,!utest

package somepackage

const InTest = false
const InUTest = false

contracts_testing.go:

// +build !prod,test,!utest

package somepackage

const InTest = true
const InUTest = false

contracts_utest.go:

// +build !prod,!test,utest

package somepackage

const InTest = true
const InUTest = true

Include each of these boolean constants in your contract conditions and && with the condition. For example, this is how you extend the postcondition of the function Range written in the previous section to be verified only in yours and theirs tests, but not in production:

// Range returns a range of the timestamps available in the database.
//
// Range ensures:
//  * !InTest || (err != nil || (empty || first <= last))
func (t *Txn) Range() (first int64, last int64, empty bool, err error) {
	...
}

Since constant booleans are placed first in the conjunction, the rest of the condition will not be evaluated incurring thus no computational overhead in the production at runtime.

Gocontracts reads the Go file and outputs the modified source code to standard output:

gocontracts /path/to/some/file.go

You can modify the file in-place by supplying the -w argument:

gocontracts -w /path/to/some/file.go

If you want to remove the contract checks from the code, supply the -r argument:

gocontracts -w -r /path/to/some/file.go

The remove argument is particularly useful when you have a build system in place and you want to distinguish between the debug code and the release (production) code.

Before building the release code, run the gocontracts with -r to remove the checks from the code.

We provide x86 Linux binaries in the "Releases" section.

To compile from code, run:

go get -u github.com/Parquery/gocontracts

• Fork the repository to your user on Github.

• Get the original repository:

   go get github.com/Parquery/gocontracts

• Indicate that the local repository is a fork:

   cd gocontracts
   git remote add fork https://github.com/YOUR-GITHUB-USERNAME/gocontracts.git

• Make your changes.

• Push the changes from the local repository to your remote fork:

   git push fork

• Create a pull request on Github and send it for review.

We follow Semantic Versioning. The version X.Y.Z indicates:

• X is the major version (backward-incompatible),
• Y is the minor version (backward-compatible), and
• Z is the patch version (backward-compatible bug fix).