Throwing errors on mutating immutable bindings

Mark S. Miller erights at
Wed Oct 1 09:05:40 PDT 2014

Good point. If we can require all such assignments to be rejected
statically, why is a runtime assignment to a const variable even possible?
Can't we just assert that this cannot occur?

On Wed, Oct 1, 2014 at 8:59 AM, Allen Wirfs-Brock <allen at>

> On Oct 1, 2014, at 8:39 AM, Mark S. Miller wrote:
> ...
> I was with you until you got to the following point
> >
> > If there is an intervening "with" or sloppy direct eval, then there is
> not a statically apparent assignment to a const variable. Since this can
> only occur in sloppy code anyway, it seems more consistent with the rest of
> sloppy mode for this failed assignment to be silent, rather than
> dynamically throwing an error.
> >
> const is a new kind of declaration unlike any declaration form that
> previous existed in ES, so Ii don't think its handling introduces any
> legacy consistency issues.  If somebody is using const, regard less of
> mode, they pretty clearly expect assignments to any const bindings to be
> illegal.  And, I don't think any body wants new silent failure errors, even
> in sloppy mode.  The most consistent thing is for runtime detected
> assignments to const bindings to always be noisy errors.  Early where
> possible, at runtime in the rest of the cases.
> Allen

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