Extendable syntaxes

Created by: bopjesvla

Extendable syntaxes would allow for:

• the usage of independently written macros with the same name, provided that they're used in different contexts
• the inline extension of an imported macro without forking a module
• anything that requires backtracking, such as the macro syntax

The most natural implementation I can think of would be to allow syntax functions to return null, after which the reader tracks back and tries the previous macro with the same name:

syntax each = ctx => {
    let variable = ctx.next("expr").value,
        OF = ctx.next().value,
        iterable = ctx.next("expr").value,
        body = ctx.next().value

    if (variable && iterable && OF.isIdentifier("of") && body.isBraces()) {
        return #`for (${variable} ${OF} ${iterable}) ${body}`
    }

}
syntax each = ctx => {
    let iterable = ctx.next("expr").value,
        body = ctx.next().value

    if (iterable && body.isBraces()) {
        return #`for ($_ of ${iterable}) ${body}` // yeah this wouldn't work because of hygiene
    }
}

each x of y {
    x()
}

each y {
    $_()
}

This would cover a lot of the same ground as readtables.