Fix let (requires prelude to be merged first) (BREAKING CHANGE)

[openorclose]

Problem

Program:

let a = 1;
function f() {
  return a;
}

roughly becomes:

let a = 1;
function f() {
  return a;
}
variables.a = {value: a, kind: 'let'};
variables.f = {value: f, kind: 'const'};

---

In the same context, if you do:

a = 2;
f();

it gets roughly transformed into

{
// put back the old values
let a = variables.a.value;
const f = variables.f.value;
{// new scope
  a = 2;
  f();
}
variables.a.value = a;
}

Problem the a referenced in f is still unchanged!

Fix

Program:

let a = 1;
function f() {
  return a;
}

roughly becomes:

let a = 1;
function f() {
  return a;
}
variables.a = {value: a, kind: 'let', assignNewValue: function (newValue) {
  return a = this.value = newValue;
}
};
variables.f = {value: f, kind: 'const'};

---

In the same context, if you do:

a = 2;
f();

it gets roughly transformed into

{
// put back the old values
let a = variables.a.value;
const f = variables.f.value;
{// new scope
  a = variables.a.assignNewValue(2);
  f();
}
}

so that the actual a referenced by f does indeed get updated correctly.
🎉

[coveralls]

Pull Request Test Coverage Report for Build 1525

• 113 of 120 (94.17%) changed or added relevant lines in 8 files are covered.
• 3 unchanged lines in 2 files lost coverage.
• Overall coverage decreased (-0.06%) to 91.623%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/stdlib/stream.ts	10	12	83.33%
src/transpiler.ts	56	61	91.8%

Files with Coverage Reduction	New Missed Lines	%
src/interop.ts	1	94.85%
src/interpreter.ts	2	91.73%

Totals
Change from base Build 1517:	-0.06%
Covered Lines:	1709
Relevant Lines:	1844

---

💛 - Coveralls

[martin-henz]

This works now. Took a glance at the implementation. Makes sense. I trust it all works.

[openorclose]

Note that this includes breaking changes:

const f = () => x;

is now now longer a valid program, since x is undefined. Previously this was valid, and only threw an error when f was called.

[martin-henz]

Note that this includes breaking changes:

const f = () => x;

is now now longer a valid program, since x is undefined. Previously this was valid, and only threw an error when f was called.

Will take note of this for the staff briefing next week.