Is there a Julia function similar to the R function `with`?

General Usage
1

Hello,

In R, when you have a list like mylist <- list(a = 2, b = 3), then instead of doing mylist$a + mylist$b you can do with(mylist, a + b). Is there something similar in Julia for a named tuple like (a = 2, b = 3)?

2

Can you clarify what exactly you are trying to do? We have similar syntax with tables, but maybe you don’t even need the syntax and there is a better way to achieve your goal with idiomatic Julia.

3

Not exactly what you ask for, but you have the let-statement where you can do a small destructure

julia> mytup = (a=3, b=4)
(a = 3, b = 4)

julia> let (; a, b) = mytup
       a + b
       end
7
4

@juliohm I have a large tuple params = (a = 1, b = 2, ..., j = 10) and a function with arguments a, b, …, j, I don’t want to type f(params.a, params.b, ..., params.j).

Hmm… but I just found a solution for this situation: f(params...) works.

5

But well, what if params is not ordered as the arguments of the function?

6

This has been discussed before, for example here and here.

My impulse is to accomplish something like this with function keywords, e.g.

julia> t = (a=2, b=3)
(a = 2, b = 3)

julia> foo(;a, b) = a + b
foo (generic function with 1 method)

julia> foo(;t...)
5
7

If you don’t need different methods for f you could define it with keyword arguments and call it by splatting those, i.e., f(; params...) as also suggested by @brendaisy.
In general, to replicate the functionality of R requires non-standard evaluation and could be done with a macro in Julia. The package StatsPlots has a similar macro for tables which allows you to write: @df mytable :a + :b. You could check their implementation and build something similar for named tuples. Yet, given the short syntax for unpacking names tuples as shown by @albheim a macro might not be necessary here.

8

For fun.

julia> macro with(container, expressions...)
           esc(:( let (; $(propertynames(getfield(__module__, container))...))=$container
               $(expressions...)
           end ))
       end
@with (macro with 1 method)

julia> let t=(a=2, b=3)
           @with t a+b
       end
5

julia> tup = NamedTuple(Symbol(c)=>c for c ∈ 'a':'j')
(a = 'a', b = 'b', c = 'c', d = 'd', e = 'e', f = 'f', g = 'g', h = 'h', i = 'i', j = 'j')

julia> @with tup (j, i, h, g, f, e, d, c, b, a)
('j', 'i', 'h', 'g', 'f', 'e', 'd', 'c', 'b', 'a')

julia> tup = (a=1, b=2, c=3)
(a = 1, b = 2, c = 3)

julia> @with tup begin
           local temp=a+b
           temp+c
       end
6

You can compare this against this macro. But you should also consider whether such a thing is even worthwhile, considering this.

Edit:

Oops, I polluted my global namespace. The above code does not, infact, work for locally-declared named tuples; it only works for globally-visible objects.

Looks like this works though. It’s quite clever.