Dafny compilation to Go

This documentation is intended primarily to help with writing Go code that makes use of code generated from Dafny. The emphasis is therefore on features visible to the user of the module.

Note: Identifiers

As much as possible, identifiers in generated code include underscores in their names. Therefore, to avoid namespace collisions, avoid using underscores in names in Dafny code if you anticipate compiling to Go.

Top-Level Structure

Unlike C# and JavaScript, Go imposes strict requirements on modules and file structure. As it is not possible to put a multi-package Go program into one file, a whole directory structure is produced alongside the original .dfy file. For a Dafny source file Example.dfy which doesn’t define any modules, the structure will look like so:

  • Example-go: The top level, to which GOPATH is set in order to run the program.
    • src: All source files are placed here.
      • Example.go: A stub which calls the Main method in module_/module_go.
      • module_/module_.go: The main module.
      • dafny/dafny.go: The Dafny run-time library.
      • System_/System_.go: Additional definitions for built-in types.

Each Dafny module will be placed in a package named after the module, in a file such as Module/Module.go. If the module’s name begins with an underscore (such as in the _module and _System modules), the filename will move the underscore to the end. (Go ignores any file whose name starts with an underscore.)

Anything declared outside of a module will end up in the default module, called _module.

Predefined Types

Dafny Go  
bool bool  
int _dafny.Int Immutable wrapper for *big.Int
bv _dafny.BV Synonym of _dafny.Int
real _dafny.Real Immutable wrapper for *big.Real
char _dafny.Char Defined as rune
string _dafny.Seq  
object *interface{}  
array<X> *_dafny.Array  
A -> B func(A) B  
seq<X> _dafny.Seq  
set<X> _dafny.Set  
multiset<X> _dafny.MultiSet  
map<X, Y> _dafny.Map  

Here big refers to the Go built-in bignum library "math/big".

Note that nullable Dafny types (object and array<X>) are modelled as pointer types in Go so that they have the distinguished value nil (to which null translates). In Go, each pointer type has its own nil value; that is, nil is typed to a specific pointer type (see also discussion of nil in the section on Traits below).


Instance members of classes are described in docs/Compilation/ReferenceTypes.md.

Basic class functionality is mapped onto Go structs. An instance field becomes a field in the struct, and an instance method becomes a method of that struct.

class Class {
    var x: int
    constructor(x: int) {
        this.x := x;
    method Frob(z: string, c: Class) returns (a: int, b: char) {
type Class struct {
    X _dafny.Int

func (_this *Class) Ctor__(x: _dafny.Int) {
    _this.X = x

func (_this *Class) Frob(z dafny.Seq, c *Class) (_dafny.Int, _dafny.Char) {

Caution: Constant fields are represented as normal fields in generated Go code. There is no enforcement mechanism. Be careful!

Note that the method has a pointer receiver, and the parameter type Class is translated as the pointer type *Class. Objects are always translated as pointers, including in receiver types, since (a) they’re mutable in general, (b) they may be nullable, and (c) it’s necessary for our implementation of inheritance (see Traits).

Note also that the constructor becomes a regular method called Ctor__ taking an already-allocated value as its receiver.

Finally, all field and method names are capitalized so that they are exported from the defining module.


In addition to any constructors, each class also has an initializer which allocates an object, with all fields given the default values for their types. The initializer will be called New_*Class*_:

func New_Class_() *Class {
    _this := Class{}

    _this.X = _dafny.Zero
    _this.Y = _dafny.ZeroReal

    return &_this

Note that the initializer is not a constructor. Dafny constructors translate to instance methods which are passed the output of the initializer.

(The translation currently uses Class{} and then a separate assignment statement for each field, rather than putting the field values in the braces, for internal reasons involving type parameters.)

Static Members

Go doesn’t have static members per se. Typical Go code uses the module, not the type, as the organizing structure, with supporting functions and variables exported from the same module as the type itself. Thus it’s tempting simply to translate static fields as global variables and static methods as non-method functions. Unfortunately, this invites namespace collisions, as two classes in the same module can happily use the same name for a static member. Therefore (borrowing from Scala terminology) we declare a companion object called Companion_Class_ for each class:

class Class {
    var x: int
    static const y = 42.0;
    static method Frob(z: string, c: Class) returns (a: int, b: char) {
type Class struct {
    X _dafny.Int

type CompanionStruct_Class_ struct {
    Y _dafny.Real
var Companion_Class_ = CompanionStruct_Class_ {
    Y: _dafny.RealOfString("42.0")

func (_this *CompanionStruct_Class_) Frob(z _dafny.Seq, c *Class) (_dafny.Int, _dafny.Char) {

The Default Class

All methods are represented as being members of some class. Top-level methods, then, are static methods of the default class, called Default__.

method Main() {
    print "Hello world!\n";
type Default__ struct {

type CompanionStruct_Default___ {
var Companion_Default___ = CompanionStruct_Default___{}

func (_this *CompanionStruct_Default___) Main() {
    _dafny.Print(_dafny.SeqOfString("Hello world!"))


Instance members of traits are described in docs/Compilation/ReferenceTypes.md.


A class or array type is compiled into a pointer type in Go. This means it includes the Go value nil. A trait is compiled into a Go interface. Abstractly, an interface value is either nil or a (value, type) pair. This means that the Dafny null value for a trait may be represented either as the Go interface value nil or a pair (nil, class pointer type).

For instance, consider the following program:

trait Trait { }
class Class extends Trait { }
method TestNil() {
  var c: Class? := null;
  var t: Trait? := null;
  var u: Trait? := c;
  var w := c == c;
  var x := t == c;
  var y := t == t;
  var z := t == u;

This Dafny program sets all of c, t, and u to null, and therefore also sets all four boolean variables to true. A simplified version of the target code in Go for this program is:

type Trait interface {
type Class struct {
  dummy byte
func TestNil() {
  var c *MyClass
  c = (*MyClass)(nil)  // c becomes nil of the pointer type *MyClass
  var t MyTrait
  t = (MyTrait)(nil)   // t becomes nil of interface type MyTrait
  var u MyTrait
  u = c                // u becomes (nil, *MyClass)

  var w bool
  w = c == c
  var x bool
  x = _dafny.AreEqual(t, c)
  var y bool
  y = _dafny.AreEqual(t, t)
  var z bool
  z = _dafny.AreEqual(t, u)

As illustrated in this example, values of Dafny class types can be compared directly with == in Go, but values of other Dafny reference types need to be compared by the runtime function _dafny.AreEqual, which handles the two representations of null.



Each inductive datatype is implemented as a struct that embeds an interface:

datatype List = Nil | Cons(head: int, tail: List)
type List struct {

type Data_List_ interface {

We could simply declare List as an interface type and be rid of Data_List_, but then we could not give List concrete methods. The interface’s isList() method is simply a marker to make sure no-one tries to pass off a type from another module as a List.

Each constructor is a struct that implements the interface, along with a constructor function:

type List_Nil struct {}
func (List_Nil) isList() {}
func Create_Nil_() List {
    return List{List_Nil{}}

type List_Cons struct{
    head _dafny.Int
    tail List
func (List_Cons) isList() {}
func Create_Cons_(head _dafny.Int, tail List) List {
    return List{List_Cons{head, tail}}

Constructor-check predicates operate using type assertions:

func (_this List) Is_Nil() bool {
    _, ok := _this.(List_Nil)
    return ok

func (_this List) Is_Cons() bool {
    _, ok := _this.(List_Cons)
    return ok

A destructor corresponding to only one constructor is implemented using a type assertion:

func (_this List) Dtor_head() _dafny.Int {
    return _this.(List_Cons).head

func (_this List) Dtor_tail() List {
    return _this.(List_Cons).tail

If multiple constructors share a destructor, its implementation will use a switch on the data struct’s type.


A coinductive datatype has a data struct like that of an inductive datatype, but the datatype itself is implemented as yet another interface:

codatatype Stream = Next(shead: int, stail: Stream)
type Stream struct {

type Iface_Stream_ {
    Get() Data_Stream_

Then, in addition to the usual constructors, a lazy constructor is provided:

func (CompanionStruct_Stream_) Lazy_Stream_(f func () Stream) Stream {

The implementation allocates a value of the non-exported lazyStream type, which implements Get() by calling f once then caching the returned value for subsequent Get()s.

Type Parameters

Go doesn’t have parameteric polymorphism, so parameterized types are implemented by erasure: the Go type of a value whose type is a parameter is always interface{}. The compiler takes care of inserting the necessary type assertions.

For example:

function method Head<A>(xs: seq<A>): A requires |xs| > 0 {
func Head(xs _dafny.Seq) interface{} { ... }

(Here Head is actually a method of the default class’s companion object, so it should have a receiver of type CompanionStruct_Default___; we’ve omitted this and other details for clarity throughout this section.)

Any sequence has the same type _dafny.Seq, and the Head function’s signature says it takes any sequence and may return any value. Calls therefore often require type assertions:

var xs: seq<int> := ...;
var x: int := Head(xs);
xs := ...
x := Head(xs).(_dafny.Int)

In more complex situations, it is necessary to retain type information that would otherwise be erased. For example:

method GetDefault<A(0)>() returns (a: A) { }

Here we cannot simply compile Default with type func() interface{}—what would it return? Thus the compiled method takes a run-time type descriptor (RTD) as a parameter:

func GetDefault(A _dafny.Type) interface{} {
    var a interface{} = A.Default()
    return a

The _dafny.Type type is a simple interface; currently its only purpose is to allow for zero initialization in precisely such cases as these:

type Type interface {
    Default() interface{}

Each compiled class or datatype comes with a function called Type_*Class*_ that takes a _dafny.Type for each type parameter and returns the _dafny.Type for the class or datatype.


An iterator is translated as a class with a MoveNext() method, as in Dafny. The constructor fires off a goroutine for the body of the iterator. As goroutines are lightweight, this should not impose too much overhead. One caveat as that we have to use a finalizer to end the goroutine early if the iterator is garbage-collected, as otherwise the goroutine stays live indefinitely, leaking any memory it holds onto. Because of the way finalizers are invoked, however, the memory retained by the iterator cannot be reclaimed until the GC after the iterator itself is collected. In the worst case, several iterators could be chained together, therefore taking n GC cycles to clean up n iterators.


Dafny code may freely interoperate with existing Go code using {:extern} declarations. This may include both pre-existing Go modules and modules written specifically to interact with compiled Dafny code.

Go modules to be included as part of the compiled code should be passed as additional arguments on the Dafny command line.

An {:extern} declaration on a module indicates that the module is external—either one that was passed on the command line or one from a pre-existing package.

module {:extern "os"} OS { }
import "os"

Import statements are automatically inserted in the default module and in subsequent (non-extern) modules (this may produce problems; see this TODO item).

As a special case, types and values in the built-in top-level scope in Go can be accessed by passing the empty string as the package name (see the next example). Such a declaration does not generate an import statement.

An interface can be imported in Dafny code by declaring it as a trait with an {:extern} annotation:

module {:extern ""} Builtins {
    trait {:extern} error {
        method {:extern} Error() returns (s : string)

Similarly, a class with methods can be imported, and a top-level function can be imported as a module-level method:

module {:extern "bufio"} BufIO {
    class {:extern} Scanner {
        method {:extern} Scan() returns (ok: bool)
        method {:extern} Text() returns (text: string)

    method {:extern} NewScanner(r: OS.Reader) returns (s: Scanner)

The alert reader may notice here that the Reader interface belongs to the io package, not os. Unfortunately, extern modules must abide by Dafny’s restriction that a class can only extend a trait in its own module. In Go terms, then, we can’t directly express that a struct in one module implements an interface from another one.

Fortunately, there is a “cheat”:

module {:extern "io"} IO { }

module {:extern "os"} OS {
    import Builtins

    trait {:extern "io", "Reader"} Reader { }
    class {:extern} File extends Reader { }
    method {:extern} Open(name: string) returns (file:File, error: Builtins.error?)

Here we declare an empty module for io just to be sure that io gets imported. Then we use a special two-argument {:extern} form that specifies that Reader actually lives in the io namespace. Dafny will understand that we can call Open and use the File returned as a Reader.


  • There isn’t always enough run-time type information to determine whether a sequence is a string (that is, a seq<char>). In particular, a value created as a seq<A> will always be output as a sequence of individual A values rather than as a string, even if A is char for a particular invocation.

  • Currently it is assumed that, once an extern module is declared, every subsequent Dafny module (plus the default module) imports it. If a module does not, the Go compiler will complain about an unused import. To avoid this, it suffices to declare a dummy variable of a type exported by that module or a dummy function that calls a function exported by it.

  • All symbols are exported from all modules (by capitalizing their names). There isn’t yet a way to hide internal fields, methods, etc.

  • Coercion to and from native datatypes currently only happens for method calls, not field accesses.

  • Go implements a switch on a type as a linear search. There are a few places where we switch on the type of a datatype value’s data struct. It would probably be better to replace these by separate implementations of functions.