let unexpectedly parsed as variable declaration

[null-a]

This was reported on the mailing list.

I suspect this happens because the parser is doing something like the following: Both var and let are represented as similar "variable declaration" nodes in the AST, and are only distinguished by a flag on the node, which WebPPL is ignoring.

As mentioned in the original report, a small number of code boxes on probmods.org currently rely on this behavior, so we ought to ensure those are updated if we fix this.