What are the caveats of using source versus parse & eval?

Question

Short version

Can I replace

source(filename, local = TRUE, encoding = \'UTF-8\')

with

eval(parse(filename, encodi         


        

Answer 1

This is not a full answer as it primarily addresses the seq_along part of the question, but too lengthy to include as comments.

One key difference between the seq_along followed by [ vs just using for i in x approach (which I believe is be similar to seq_along followed by [[ instead of [) is that the former preserves the expression. Here is an example to illustrate the difference:

> txt <- "x <- 1 + 1
+ # abnormal expression
+   2 *
+     3
+ "
> x <- parse(text=txt, keep.source=TRUE)
> 
> for(i in x) print(i)
x <- 1 + 1
2 * 3
> for(i in seq_along(x)) print(x[i])
expression(x <- 1 + 1)
expression(2 *
    3)

Alternatively:

> attributes(x[[2]])
NULL
> attributes(x[2])
$srcref
$srcref[[1]]
2 *
    3

Whether this has any practical impact when comparing to eval(parse(..., keep.source=T)), I can only say that it could, but can't imagine a situation where it does.

Note that subsetting expression separately also leads to the srcref business getting subset, which could conceivably be useful (...maybe?).