在R中实现ifLet功能

代码如下：

setGeneric("ifLet",
        function(sym, x, body1, body2, ...) {
            standardGeneric("ifLet")
        })

setMethod("ifLet",
        signature(sym = "name", x = "ANY", body1 = "ANY", body2 = "ANY"),
        function(sym, x, body1, body2 = {}) {
            e = new.env()
            sym_str = deparse(substitute(sym))
            ifLet(sym_str, x, body1, body2)
        })

setMethod("ifLet",
        signature(sym = "name", x = "ANY", body1 = "ANY", body2 = "ANY"),
        function(sym, x, body1, body2 = {}) {
            stopifnot(length(sym) == 1)
            e = new.env()
            assign(sym, x, envir = e)
            if(e[[sym]]) {
                eval(substitute(body1), e)
            } else {
                eval(substitute(body2), e)
            }
        })

ifLet("..temp..", TRUE, {print(paste("true.", as.character(..temp..)))}, 
        {print(paste("false.", as.character(..temp..)))})
ifLet(..temp.., TRUE, {print(paste("true.", as.character(..temp..)))}, 
        {print(paste("false.", as.character(..temp..)))})

“.temp..”版本工作正常，但

.temp..

版本产生错误：

  error in evaluating the argument 'sym' in selecting a method for function 'ifLet': Error: object '..temp..' not found

为什么会出现此错误？我如何解决此问题