`Let` struct first field contract should be a `Var`

[iambrj]

Hello! I would first like to thank you for making such a wonderful resource available for free.

While I was going through the code, I noticed that the contract for the Let struct restricts the variable binding to any arbitrary symbol instead of just Var structs. Ins't the latter choice more appropriate here?

[jsiek]

Dear Bharathi,

Thank you for your interest in Essentials of Compilation!

Consider the following example:

(let ([x 5]) (+ x 2))

The Var struct represents uses of variables, such as x in (+ x 2).

The x in the (let ([x 5) ...) is different, it is a binding site for x, or put another way, it is the definition of x. In that context, the symbol x should be viewed as part of the Let AST and not a separate AST node.

Hope that helps! Jeremy