3 ms·
Why does `let*` need to have mutation? It can be nested `let`s.
by amake 3y ago
Why does `let*` need to have mutation? It can be nested `let`s.
- convolvatron 3y agolet* permits expressions on the right refer to arbitrary other symbols bound by the let*. in particular it allows for construction of recursive lambdas that may not be linearlizable.
- amake 3y ago> let* permits expressions on the right refer to arbitrary other symbols bound by the let* In what language? I just checked Elisp, SBCL, and Guile, and they all error out if you refer to a variable not previously defined by a left-to-right traversal of the varlist: (let* ((a (+ b 1)) (b 1)) a) Edit: This doesn't work either: (let* ((a (lambda () (+ b 1))) (b 1)) (funcall a)) ; (funcall a) -> (a) for Schemes
- amake 3y agoAnyway, as far as I'm seeing it's perfectly possible to implement let* consistent with the above behavior as a macro without mutation as such: (define-macro (let* bindings &rest body) (if (null? bindings) `(progn ,@body) `(let (,(car bindings)) (let* ,(cdr bindings) ,@body))))
- tonyg 3y agoYou're thinking of letrec.
- amake 3y agoAh, that does look to be the case. I didn't know about that one. Signature (letrec BINDERS &rest BODY) Documentation Bind variables according to BINDERS then eval BODY. The value of the last form in BODY is returned. Each element of BINDERS is a list (SYMBOL VALUEFORM) that binds SYMBOL to the value of VALUEFORM. The main difference between this macro and let/let* is that all symbols are bound before any of the VALUEFORMs are evalled.