scope of let

[Claus Möbus]

Dear all,

happy and successful New Year to you all. 

Now, to WebPPL. To my astonishment I saw two lets in ch.07 of the ebook Cognition models. More precisely it was in a code box explaining the MH-algorithm.

I always saw that "var" was used for the purpose of definitions. "let" was not in the WebPPL docs so I looked how "let" was defined in JS. 

I read that "var" has no block scope but "let" has since ES2015. I tried an example from W3School. This revealed that "let" in WebPPL has **no** block scope:

Here's the code snippet:

var x = 10;

// Here x is 10

print("x1 = " + x)

{

  let x = 2;

  // Here x is 2

  print("x2 = " + x)

}

// Here x is 10

print("x3 = " + x)

WebPPL's output is:

x1 = 10

x2 = 2

x3 = 2

it should be:

x1 = 10
x2 = 2
x3 = 10
Best, Claus

[null-a]

Hello. Happy new year to you too.

The way I see it, this is a bug, in the sense that (IIUC) the language was never intended to include `let`, so using it should be a syntax error.

http://docs.webppl.org/en/master/language.html#syntax

(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.)

Thanks,

Paul