Strict (non-coercing) expressions

[T.J. Crowder]

In the thread on [strict relational operators][1], felix [suggested][2] an
expression mode affecting all operators:

> Maybe every operator can have a non-coercing variant?
>
> One possible syntax is to have a modifier on operators
>
>    x = a (<) b (+) c (&&) (!)d;
>
>    if (x (!=) y) ...
>
>
> Another possible syntax is to have a modifier on expressions
>
>    x = #(a < b + c && !d)
>
>    if #(x != y) ...

I though the idea deserved its own thread. It's much broader in scope than
strict relational operators, and not exclusive with them.

I quite this idea of an expression mode making all operators within it
non-coercing, and having them throw a `TypeError` when presented with mixed
types. Using flagged parentheses (I didn't want to say "decorated") seems
like a great way to do it, too:

```js
if #(a > b) { /*...*/ }
while #(a > b) { /*...*/ }
for #(let i = 0; i < a.length && a > b; ++i) { /*...*/ }
if (answer == 42 && #(a > b))  { /*...*/ }
```

(Note the intentially-loose `answer == 42` in that last one.)

Which raises the question of possibly extending that concept to the block
level, where all expressions in the block are strict:

```js
#{
    if (a > b) { /* ... */ }
    //    ^-- Throws if a and b are not the same type
}
```

If you like strict comparisons, a `#{ /*...*/ }` wrapper around all the
code in your module is all you need...

Or for just a function:

```js
function foo() #{ // <== Note the #
}
const foo = () => #{
}
```

I wonder if people would then ask for an explicitly-loose version of `!`
(or, indeed, other operators)... :-)

-- T.J. Crowder

[1]: https://esdiscuss.org/topic/strict-relational-operators
[2]: https://esdiscuss.org/topic/strict-relational-operators#content-5
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