multifor

`for` <- function(it, seq, expr) {
  if(!identical(substitute(it), quote(.))) {
    tryCatch(
      eval.parent(substitute(base::`for`(it, seq, expr))),
      error = function(e) {stop(c("In `for`: ", e$message), call. = FALSE)})
    return(invisible())
  }
  l <- length(seq[[1]])
  nms1 <- names(seq)
  nms2 <- paste0("*", names(seq), "*")
  names(seq) <- nms2
  e <- new.env(parent = parent.frame())
  list2env(seq, e)
  expr0 <- as.call(c(quote(`{`), parse(text=sprintf("%s <- `%s`[[`*i*`]]", nms1, nms2))))
  code <- bquote(splice = TRUE, {
    on.exit({
      rm(list = ls(pattern = "^\\*.*\\*$"))
      list2env(as.list(environment()), parent.env(environment()))
    })
    for (`*i*` in 1:.(l)) {
      .(expr0)
      .(substitute(expr))
    }
  })
  tryCatch(
    eval(envir = e, code), 
    error = function(e) {stop(c("In `for`: ", e$message), call. = FALSE)})
}

# works as before
for(i in 1:3) print(i^2)

# use dot for multiple iterators
for(. in list(a=1:3, b=3:5)) {
  print(a*b)
}

# just like original for, we keep values defined in  loop, errors are slightly different
for(. in list(a=1:3, b=3:5)) {
  y <- a*b
  print(y)
  if(y==8) stop("!!!")
}

# defined values are in current environment
y