17 Jun 2018

# I do love Julia

Fluent interface in Julia.

### What is fluent interface?

Fluent interface is a style on method chaining that possesses good readability It is extensively used in languages such as C#. One example is in data processing:

var query = db
.Where(i => i.Name.Contains("zz"))
.OrderBy(i => i.Name.Length)
.Select(i => i.Name.ToUpper());


Another example is in configuration:

var host = new WebHostBuilder()
.UseKestrel()
.UseUrls("http://*:5000")
.UseContentRoot(Directory.GetCurrentDirectory())
.UseIISIntegration()
.UseStartup<Startup>()
.Build();


These two examples demonstrates that fluent interface is handy in some areas.

### How about fluent interface in Julia?

Unfortunately, Julia does not support fluent design well. Partly, this is because Julia is not OO by design. There are some features that approximately does something similar. One is the pipe operator |> that allows for easy function chaining. For example:

[1:5;] |> x->x.^2 |> sum |> inv


There are nothing to blame in its functionality However, the operator |> itself is very ugly, and not friendly to typing Besides, except for the first one, each part requires a function, which requires lots of typing (the lambda protocol) if the function is not pre-defined. For the first blame, it is a personal taste thing. While for the second one, the trouble is genuine. There is a pending pull request that aim to make creating anonymous functions easier. It treats underscore _ as a special signal. f(_, y) turns into x -> f(x, y), and _[i] turns into x -> x[i]. This greatly reduces the characters we need to type for creating a function. So the second problem is solved to some extent. Let’s turn to the first one.

### do

The do syntax is a special one in Julia. Let’s look at an example.

map(1:10) do x
x^2
end


This is equivalent to map(x -> x^2, 1:10). When you use do, it assumes that the first argument of the previous function accepts a function. And the definition of that function is defined right after the keyword do.

How about we use another word, say dont, and when dont appears, it assumes the function after it takes an object as the first argument, and feed the output of previous functions to it? And if it is okay, how about not calling it dont, but reuse the term do? For example,

love(a, b) = print("$a love$b")

"I" do love("Julia")


The last line would translate into love(I, "everyone"). With this, the fluent interface is easy to implement. And here is an example,

[1:5;] do (x->x.^2)() do sum() do inv()


### broadcast

This do not play nicely with half of the functions. Because their first argument accepts a function. One example is map. map is used to do some element-wise transformation. With the new do syntax, element-wise transformation is not easy to accomplish. broadcast is our life saver. consider this example:

[1:5;] .do (x->x^2)() do sum() do inv()


## Implementation

### Try 1: Macro

First I try to implement the idea using macros. However, the first trouble is, the proposed do syntax is invalid. They cannot pass the Julia parser, so the codes are blocked by errors before they arrived to macros. Okay, how about use another word, say dont?

function rearrange(expr)
if length(expr) > 2 && expr[2] == :dont
insert!(expr[3].args, 2, expr[1])
return rearrange(expr[3:end])
else
return expr
end
end

macro dont(something...)
esc(rearrange(something)[1])
end


Then the following code works as expected.

love(a, b) = print("$a love$b")

@dont "I" dont love("Julia")

@dont [1:5;] dont (x->x.^2)() dont sum() dont inv()


This is not satisfying because of the use of dont, and because the use of @dont. To get rid of these annoyances, we need hack the Julia parser.

### Try 2: Parser

Let’s do something to the parser.