Use JuliaCall as Julia Engine in R Markdown

To use JuliaCall package for julia engine in R Markdown document. Just set the engine for julia to JuliaCall::eng_juliacall like this:

knitr::knit_engines$set(julia = JuliaCall::eng_juliacall)
## This is a julia language chunk.
## In julia, the command without ending semicolon will trigger the display
## so is JuliaCall package. 
## The julia display will follow immediately after the corresponding command
## just as the R code in R Markdown.
a = sqrt(2)
## 1.4142135623730951
a = sqrt(2);

## And lots of different types of display are supported in JuliaCall.
## Like markdown.
@doc sqrt
## ```
## sqrt(x)
## ```
## 
## Return $\sqrt{x}$. Throws [`DomainError`](@ref) for negative [`Real`](@ref) arguments. Use complex negative arguments instead. The prefix operator `√` is equivalent to `sqrt`.

Get Access to Julia in R Chunk

And you can also get access to julia variables in R code chunk quite easily using JuliaCall, for example:

## This is a R language chunk.
## In the previous julia chunk, we define variable a, 
## we can use functions in JuliaCall to get access to it.
JuliaCall::julia_eval("a")
## [1] 1.414214