Disallow programs where function declarations do not appear at the top of their block scope

[ning-y]

const f = g();

function g() {
    return 1;
}

Throws reference error in source, but should work according to ES6.

[martin-henz]

Yes, this will require quite some effort. There are two cases:

1. Local functions

It's tempting to restrict Source such that all local functions need to appear first within a function body. This will require a small note in the Source docs, and a simple check during interpretation. A nice error message will remind the students to do the right thing:
"local functions must appear before any other statements in the function body"

2. Toplevel

Here, I think hoisting is both feasible and desirable, and should be implemented as a preprocessing step.

To be discussed are weird cases like this (at toplevel):

f(1);
if (true) { function f(x) { } }

Allowed in Source 2019 or not? Hoisted in Source 2019 or not?

[openorclose]

@martin-henz Up till ES5 the example code was invalid per the specs as functions are not allowed inside a block statement, but browsers being browsers ignored this and each had a different way of interpreting the code.

Since ES2015 however, functions are now block scoped and so the example code would also be invalid but because the function will not be accessible outside the block scope created by if. To be clear, the function would still be hoisted, but only to the top of the if statement and not outside of it.

[martin-henz]

See discussion in:
#71

I think we should stay away from hoisting in CS1101S.

We will say in the specs: "Local functions within an outer function must precede all other statements in body of the outer function."

In 2018, we just live with whatever the implementation gives us, and if there are questions, we say "undocumented feature; don't do this". In 2019, we should aim for a nice error message, to enforce the specs.

There is a small cost: The following programming pattern will not work any longer:

function f1(...) {...}

// test f1
f1(...);

function f2(...) {...}

// test f2
f2(...);

[ning-y]

Sorry, I don't quite understand why the example program would not run. Each function is defined before being called, aren't they?

[martin-henz]

Sorry, I don't quite understand why the example program would not run. Each function is defined before being called, aren't they?

We would simply reject this program, because the function declaration for f2 appears after a statement that is not a function declaration.

The reasoning here is to disallow programs where the "hoisting" of a function declaration would happen in ES. This way, we can remain a sublanguage of ES and claim that all legal Source programs are legal ES programs and produce the same result.

(We are deviating from ES in the result of loops, which we declare a "trivial" difference, if anyone asks.)

-martin

[thomastanck]

If we're keeping this, then the issue should be renamed to something like: "2019: Disallow programs where function declarations do not appear at the top of their block scope."

Or a new issue should be made.

[thomastanck]

Closing as this is now a duplicate of #136