Explicitly specifying argument when chaining operations in Julia

Using DataFramesMeta.jl module, operations can be linked in Julia using the following approach

using DataFrames, DataFramesMeta
df = DataFrame(a = collect(1:5), b = ["a","b","c","d","e"])
@> begin
    df
    @where(:a .> 2)
    @select(:a, :b, c = :a*2)
end
# or:
print(
    @linq df |>
    @where(:a .> 2) |>
    @select(:a,:b, c = :a*2)
)

3x3 DataFrames.DataFrame
| Row | a | b   | c  |
|-----|---|-----|----|
| 1   | 3 | "c" | 6  |
| 2   | 4 | "d" | 8  |
| 3   | 5 | "e" | 10 |

The idea is that df is the first argument of macro @where and the whole @where statement feeds as the first argument into the @select macro.

However we might want the top line to become the second argument or we might want to use it in several places. For R users (in R operations can be chained using %>%), dplyr package enables this with a dot (.) notation, so the following would work:

library(dplyr)
df = data.frame(a = 1:5, b = c("a","b","c","d","e"))
df %>% filter(a > 2) %>% mutate(c = nrow(.):1) %>% select(b,c) 

   b c
 1 c 3
 2 d 2
 3 e 1

I was looking for a way to mimic R's dot notation, but unfortunately, the dot notation does not work in Julia and I couldn't find anything about this in the package documentations.

If anyone know how this can be achieved, please let us know.

Solution

When using Lazy ,@as macro lets you name the threaded argument:

@as _ x f(_, y) g(z, _) == g(z, f(x, y))

With @as macro the mentioned task could be done like this:

julia> import Lazy.@as

julia> using DataFrames, DataFramesMeta

julia> df = DataFrame(a = collect(1:5), b = ["a","b","c","d","e"])
5x2 DataFrames.DataFrame
| Row | a | b   |
|-----|---|-----|
| 1   | 1 | "a" |
| 2   | 2 | "b" |
| 3   | 3 | "c" |
| 4   | 4 | "d" |
| 5   | 5 | "e" |

julia> @as _ df @where(_, :a .> 2) @select(_,:a, :b, c = :a*2)
3x3 DataFrames.DataFrame
| Row | a | b   | c  |
|-----|---|-----|----|
| 1   | 3 | "c" | 6  |
| 2   | 4 | "d" | 8  |
| 3   | 5 | "e" | 10 |