How to construct the local variables for Block automatically (somewhere in the code)

Question

I am working on the fancy abstraction for Block function. The idea is to use rules to specify which local variable will be used, even if it already defined in the global context

like this

str = "something in the global context"; 
localEval[str, "str"->"Hello"]

and the output will be

"Hello"

where the global definition of str remains untouched.

I came up with the following solution, that works well

localEval[exp_, vars___] := With[
 {args = With[{val = #[[2]]},
                  ToExpression[#[[1]], InputForm, Function[name, Hold[name = val], HoldAll]]
             ] & /@ List[vars]
 },
   
   Block[args,
     exp
     ] // ReleaseHold
   ];
SetAttributes[localEval, HoldFirst];

but for me it is not fancy, because firstly it creates an error message, which doesn't affect anything

Block::lvsym: Local variable specification {Hold[str=hello]} contains Hold[str=hello], which is not a symbol or an assignment to a symbol.

I assume that ReleaseHold firstly tries to evaluate Block with the first argument contained Hold expressions and only after that goes to release them.

How could it be improved? Also I am worrying about the possible performance issues due to the error messages generated.

Answer 1

The community helped me to realize the better way for my application. Hah ;)

localEval[exp_, vars_:{}] := Block[vars, exp];
SetAttributes[localEval, HoldAll];

that't it