# Why I Use Julia

## Multiple Dispatch + JIT (Dynamic and Compiled)

When I am asked why I use Julia, my immediate response is “multiple dispatch”. Julia is well-known for performance, but that is only a part of what keeps me using it every day. Multiple dispatch is a feature where different code is called by a function depending on the types of the arguments. Combined with the JIT (Just-in-time compiler), Julia will automatically compile specialized code for each set of argument types the function is called with.

### Simple Example

f(x) = x + x

I have not given Julia any hint as to what the type of the argument is, but as long as it’s a type that supports addition, Julia will compile an optimized method for it. You can peek at the LLVM compiler code with the @code_llvm macro.

julia> @code_llvm f(1)

define i64 @julia_f_62945(i64) #0 !dbg !5 {
top:
%1 = shl i64 %0, 1
ret i64 %1
}

julia> @code_llvm f(1.0)

define double @julia_f_62949(double) #0 !dbg !5 {
top:
%1 = fadd double %0, %0
ret double %1
}

Ignoring some of the details, notice that these functions do not call the same code: one method is specific to 64-bit integers and the other for double precision floats!

### What About More Complicated Types?

Here we will use the Distributions package to implement a naive quantile finder using Newton's Method:

$x_{n+1} = x_n - \frac{f(x_n)}{f'(x_n)}$

For quantiles, we are trying to find the number x, for a given number q (between 0 and 1), such that

cdf(dist, x) - q = 0

where cdf is the cumulative distribution function for distribution dist. We also need the derivative of the cdf, which is the probability density function, or pdf.

using Distributions

function myquantile(d, q)
out = mean(d)
for i in 1:10
out -= (cdf(d, out) - q) / pdf(d, out)
end
out
end

Again, I have not told Julia anything about what d or q is, but when I provide arguments such as Distributions.Normal(0, 1) and 0.5, Julia will compile specialized code to run the algorithm and then return the median for a standard normal distribution (which is 0).

julia> myquantile(Normal(0,1), .5)
0.0

Right out of the box, myquantile will also work with other distributions! In fact, as long as the function arguments have methods for mean, cdf, and pdf, it will just work! If you were to implement this quantile algorithm in R, you would need to rewrite it for each distribution using the dnorm/pnorm family of functions.

julia> myquantile(Gamma(5,1), .7)
5.890361313697006

julia> myquantile(Beta(2, 4), .1)
0.11223495854585855

## Takeaway

The language you use has a tremendous effect on how you approach problems (see linguistic relativity). I have a background in statistics, so naturally R was one of the first languages I learned. I don’t mean to bash R (language wars are boring) as it is a fantastic tool for data analysis, but I often find myself asking “how do I solve this without a for loop?” since loops are slow in R. In Julia, I have fewer performance obstacles, so my questions are more along the lines of “what are the methods I’m trying to accomplish this task with?”. If I can reduce a task to the operations that need to be performed, it becomes easy to write abstract yet performant code that works with any types I throw at it.

Multiple dispatch has become invaluable to how I code, and with Julia you get it along with stellar performance. If you want to see how I use multiple dispatch to get a lot done with very little code, check out my package OnlineStats.jl for calculating statistics/models on data streams with single-pass algorithms.

I hope you try Julia for yourself and have the same experience I had.

Come for the speed. Stay for the productivity.