Method Closures: You Can't Do That In Go




Neugram is a scripting language that sticks very close to Go. Go statements are Neugram statements, you can import Go packages, scripts can be compiled to Go programs, and types look just like the equivalent Go types at run time (which means packages built on reflection, like fmt, work as expected). These requirements put a lot of restrictions on the design of Neugram. This post is about one such restriction on methods that I did not discover until I tried to use it without thinking.

Background: Go without declarations

When designing a language for use in a REPL (a read-eval-print-loop like your shell), you want to be able to dive right in and have code executed as quickly as possible. That is, a scripting language should be able to say "Hello, World!" in one reasonable line.

Popular scripting languages like Perl and Python use statements as the topmost grammatical construction. A simple statement can consist of a single expression, like the command to print a string.

Go is different. The topmost grammatical construction in Go is a declaration. Declarations consist of package-wide constants, variables, functions, types and methods. Inside declarations are statements. The statement is in charge of program control flow, and contain some number of expressions. An expression is an actual computation, where we do the work of programming.

The concept of having a layer of declarations above statements is common in programming languages. Both C and Java have declarations. Declarations are useful. The order of top-level declarations in Go does not affect the order of execution of the program. This makes it possible to depend on names defined later in the file (or in an entirely different file in the package) without developing a system of forward declarations or header files.

One of the key changes that makes Neugram a different language from Go is we do not have top-level declarations. Neugram starts with statements. We lose the advantages of declarations in exchange for executing statements quickly

Without declarations, packages are restricted to a single file (to avoid thinking about order of file execution) and referring to names not yet defined is tricky, but the feel of many programs stays the same because in Go most declarations also work as statements. For example:

var V = 4
type T int

Method grammar

The one top-level declaration that we miss in Neugram is the method declaration. In Go you declare a method by writing:

func (t T) String() string {
	return fmt.Sprintf("%d", int(t))
}

Critically, this declaration does not stand on its own. You need another declaration somewhere in your package defining the type T. While type declarations can be made as statements, method declarations cannot. There are several possible arguments for why not, but given the current syntax one is that it would introduce the notion of incomplete types to the run time phase of Go programs. Imagine:

func main() {
	type T int
	var t interface{} = T{}

	_, isReader := t.(io.Reader)
	fmt.Println(isReader) // prints false

	if rand {
		func (t T) Read([]byte) (int, error) {
			return 0, io.EOF
		}
	}

	_, isReader = t.(io.Reader)
	fmt.Println(isReader) // prints ... what?
}

Method declarations in Go break the complete definition of a type out over many top-level declarations. This works in Go because there is no concept of time for declarations, they all happen simultaneously before a program is run. This won’t work in Neugram where all declarations have to be made inside statements that happen during program execution.

Methodik

To resolve this, Neugram introduces a new keyword to define types with all of its methods in a single statement, methodik.

methodik T int {
	func (t) Read([]byte) (int, error) {
		return 0, io.EOF
	}
}

This statement is evaluated in one step. The type T does not exist beforehand, and after the statement is evaluated it exists with all of its methods.

So far so good.

Method closures: You can’t do that in Go

While testing out method declarations, I attempted to reimplement io.LimitReader. The version I came up with didn’t work:

func limit(r io.Reader, n int) io.Reader {
	methodik lr struct{} {
		func (*l) Read(p []byte) (int, error) {
			if n <= 0 {
				return 0, io.EOF
			}
			if len(p) > n {
				p = p[:n]
			}
			rn, err := r.Read(p)
			n -= rn
			return rn, err
		}
	}
	return &lr{}
}

Why not? Using the values r and n in a closure is normal Go programming, but this is something unusual: I am trying to construct a method closure.

An implication of methods only being definable by top-level declaration in Go is that there is no closure equivalent form. There is also no way (presently, issue #16522 may make it possible) to create a method using reflection which would allow closing over variables.

This is not a particularly problematic limitation, we can move the free variables of the closure explicitly into the type being defined to get the same effect:

func limit(r io.Reader, n int) io.Reader {
	methodik lr struct{
		R io.Reader
		N int
	} {
		func (*l) Read(p []byte) (int, error) {
			if l.N <= 0 {
				return 0, io.EOF
			}
			if len(p) > n {
				p = p[:l.N]
			}
			rn, err := l.R.Read(p)
			l.N -= rn
			return rn, err
		}
	}
	return &lr{r, n}
}

Avoiding method closures also avoids some reflection surprises: two different lr types, defined as closing over different values, would probably have to be different types. That means run time creation of new types without the use of the reflect package, which is a category of possibilities I’m glad I don’t have to imagine.

The restriction itself however could be confusing for someone new to Neugram who doesn’t know about the limits of Go underlying it. In particular, consider the interaction with global variables. It is fine for a method defined in Go to refer to globals, and so too in Neugram:

var x = "hello" // a global

methodik obj struct{} {
	func (o) String() string {
		return x // this is fine
	}
}

However if we take this code and try to indent it into a block, the type checker will now have to produce an error, because x is no longer a global variable. This is unfortunate. In Go there is a clear distinction between global (defined by top-level declarations) and non-global variables (defined by statements). In Neugram they look similar, so this is one more thing the programmer has to track themselves.

Surprising expressivity

Accidentally introducing syntax for method closures is a good example of the kind of problem I have spent a lot of time trying to avoid in Neugram. Even the smallest changes to Go result in unexpected ways to write programs. I did not find this particular problem until months after creating the methodik syntax.

comments powered by Disqus