Statement vs expression in => fat arrow explanation

[suragch]

Page URL

https://dart.dev/language/functions

Page source

https://github.com/dart-lang/site-www/tree/main/src/language/functions.md

Describe the problem

The docs in the linked page state:

Note: Only an expression—not a statement—can appear between the arrow (=>) and the semicolon (;). For example, you can’t put an if statement there, but you can use a conditional expression.

In describing what a statement is in this Stack Overflow answer, I listed some examples of statements, including the following:

• myInt = 1;
• print('hello');
• return null;
• if (name != null) { return name; } else { return 'Guest'; }

One reader commented:

I found example contradictory to myInt = 1; being statement. It's lambda() => myInt = 1;. Why? Look at fat arrow explanation in the docs: dart.dev/guides/language/language-tour#functions. It says "Only an expression—not a statement—can appear between the arrow (=>) and the semicolon (;)". The aforementioned example compiles, linter doesn't complain, so it's an expression, not a statement.

That's true:

void main() {
  int i = 1;
  final function1 = () => i = 3;
  final function2 = () => print(i);
  function1();
  function2(); // 3
}

Am I wrong in my understanding of what a statement is? Is i = 3 an expression rather than a statement? Is print(i) not a statement?

Or is the documentation misleading?

Expected fix

If I'm wrong about what a statement is, then nothing needs to be changed about the documentation note above. However, we should define the terms statement and expression somewhere within the docs since these terms are used liberally without a clear explanation.

If i = 3 and print(i) are in fact statements, then we should refine the => fat arrow note in the docs. I'm not exactly sure what that better wording would look like, though.

Additional context

No response

[lrhn]

I think the examples are fine. They are statements when used as statements.

It says "Only an expression—not a statement—can appear between the arrow (=>) and the semicolon (;)".

Correct, myInt = 1 is an expression, and myInt = 1; (expression + ;) is an expression statement.

Ditto for i = 3 and print(i), which are both expressions, where i = 3; and print(i); are statements.
Where a statement is expected, most expressions can be followed by ; to produce statements.

You're not saying this, but I'll address it anyway: That myInt = 1;, including the semicolon, occurs in lambda() => myInt = 1; doesn't mean that there it is a statement there. It's not parsed as a statement, the ; doesn't bind to the myInt = 1 like it would in an expression statement.

Whether a sequence of characters/tokens is a statement or not also depends on the context they're parsed in, because that decides what they're parsed as.

The examples here are of code that are statments when they're used as statements, if you use them where a statement is expected and allowed. And if you use them anywhere else, one shouldn't be surprised that they're not statements in a place where statements cannot exist to begin with.

Or stated differently, if myVar = 1; is parsed as a unit, with the ; applying only to the myVar = 1, then it is (most likely, unless I've forgotten some corner case) a statement.

If you plug the same sequence of characters into a different context, then it parses differently, and usually the ; applies to something else.

• void main() { myInt = 1; } - myInt = 1 is an expression, myInt = 1; (with the ;) is an expression statement.
• lambda() => myInt = 1; - myInt = 1 is an expression, the following ; ends the function declaration.
• class C { int myInt; C() : myInt = 1; }. The myInt = 1 is a variable initializer in the constructor initializer list. The following ; ends the initializer list and constructor.
• var myInt = 1; - The leading var completely changes the meaning, from an assignment to a declaration. The ; ends the entire declaration.

In the last three cases, the myInt = 1; were not parsed as a unit, the ; applied to more than just myInt = 1, and the myInt = 1;, by itself, is not an statement.

[suragch]

@lrhn Thank you for your thorough explanation!