>
> On 7 Oct 2019, at 13:47, Duncan Murdoch <murdoch.dun...@gmail.com> wrote:
>
> On 07/10/2019 4:22 a.m., Lionel Henry wrote:
>> Hi Gabe,
>>> There is another way the pipe could go into base R that could not be
>>> done in package space and has the potential to mitigate some pretty
>>> serious downsides to the pipes relating to debugging
>> I assume you're thinking about the large stack trace of the magrittr
>> pipe? You don't need a parser transformation to solve this problem
>> though, the pipe could be implemented as a regular function with a
>> very limited impact on the stack. And if implemented as a SPECIALSXP,
>> it would be completely invisible. We've been planning to rewrite %>%
>> to fix the performance and the stack print, it's just low priority.
>
> I don't know what Gabe had in mind, but the downside to pipes that I see is
> that they are single statements. I'd like the debugger to be able to single
> step through one stage at a time. I'd like to be able to set a breakpoint on
> line 3 in
>
> a %>%
> b %>%
> c %>%
> d
>
> and be able to examine the intermediate result of evaluating b before piping
> it into c. (Or maybe that's off by one: maybe I'd prefer to examine the
> inputs to d if I put a breakpoint there. I'd have to try it to find out
> which feels more natural.)
In order to place a breakpoint on line 3, I think you'll need to wrap
`c()` in curly braces and insert a `browser()` call. And at that point
you're changing the semantics of `c()` and you'll need to manually
write the placeholder for the input:
a() |>
b() |>
{ browser(); c(.) } |>
d()
I don't see any way around this. I guess it could be done behind the
scenes by the IDE when a breakpoint is set though. Note that this
doesn't require any changes to the parser and already works with the
magrittr pipe.
Then there's the issue of continuing to step-debug through the
pipeline. This could be achieved by parsing `a |> b()` as `{a} |>
{b()}`. so that each sub-expression carries source references. In
general, there are metaprogramming patterns that would be made easier
if calls to `function` or `if` always had a body wrapped in `{`. It is
too late to change historical operators but maybe it makes sense for
newer ones?
Lionel
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