CODEWITHSUNDEEP
syntaxjulia
Quinten
Quinten

Reputation: 41601

What is @ in Julia?

Recently I started learning Julia and have studied a lot of examples. I noticed the @ sign/syntax a couple of times. Here is an example:

using DataFrames
using Statistics
df = DataFrame(x = rand(10), y = rand(10))

@df df scatter(:x, :y)

This will simply create a scatterplot. You could also use scatter(df[!, :x], df[!, :y]) without the @ and get the same result. I can't find any documentation about this syntax. So I was wondering what this syntax is and when you should use this in Julia?

Upvotes: 4

Views: 396

Answers (2)

Przemyslaw Szufel
Przemyslaw Szufel

Reputation: 42264

When you do not know how something works try typing ? followed by what you want to know in Julia REPL.

For an example typing ?@ and pressing ENTER yields:

The at sign followed by a macro name marks a macro call. Macros provide the ability to include generated code in the final body of a program. A macro maps a tuple of arguments, expressed as space-separated expressions or a function-call-like argument list, to a returned expression. The resulting expression is compiled directly into the surrounding code. See Metaprogramming for more details and examples.

Macros are a very advanced language concept. They generally take code as an argument and generate new code that gets compiled. Consider this macro:

macro myshow(expr)
  es = string(expr)
  quote
    println($es," = ",$expr)
  end
  end

Which can be used as:

julia> @myshow 2+2
2 + 2 = 4

To understand what is really going try @macroexpand:

julia> @macroexpand @myshow 2+2
quote
    Main.println("2 + 2", " = ", 2 + 2)
end

You can see that one Julia command (2+2) has been packed around with additional julia code. You can try @macroexpand with other macros that you are using.

For more information see the Metaprogramming section of Julia manual.

Upvotes: 5

Shayan
Shayan

Reputation: 6395

What is @ in Julia?

Macros have a dedicated character in Julia's syntax: the @ (at-sign), followed by the unique name declared in a macro NAME ... end block.

So in the example you noted, the @df is a macro, and the df is its name. Read here about macros. This concept belongs to the meta-programming feature of Julia. I guess you used the StatsPlots.jl package since @df is one of its prominent tools; using the @macroexpand, you can investigate the functionality of the given macro:

julia> using StatsPlots

julia> @macroexpand @df df scatter(:x, :y)
:(((var"##312"->begin
          ((var"##x#313", var"##y#314"), var"##315") = (StatsPlots).extract_columns_and_names(var"##312", :x, :y)
          (StatsPlots).add_label(["x", "y"], scatter, var"##x#313", var"##y#314")
      end))(df))

Upvotes: 5

Related Questions