Talking to the compiler (the :meta mechanism)

In some circumstances, one might wish to provide hints or instructions that a given block of code has special properties: you might always want to inline it, or you might want to turn on special compiler optimization passes. Starting with version 0.4, Julia has a convention that these instructions can be placed inside a :meta expression, which is typically (but not necessarily) the first expression in the body of a function.

:meta expressions are created with macros. As an example, consider the implementation of the @inline macro:

macro inline(ex)
    esc(_inline(ex))
end

_inline(ex::Expr) = pushmeta!(ex, :inline)
_inline(arg) = arg

Here, ex is expected to be an expression defining a function. A statement like this:

@inline function myfunction(x)
    x*(x+3)
end

gets turned into an expression like this:

quote
    function myfunction(x)
        Expr(:meta, :inline)
        x*(x+3)
    end
end

pushmeta!(ex, :symbol, args...) appends :symbol to the end of the :meta expression, creating a new :meta expression if necessary. If args is specified, a nested expression containing :symbol and these arguments is appended instead, which can be used to specify additional information.

To use the metadata, you have to parse these :meta expressions. If your implementation can be performed within Julia, popmeta! is very handy: popmeta!(body, :symbol) will scan a function body expression (one without the function signature) for a :meta expression, extract any arguments, and return a tuple (found::Bool, args::Array{Any}). If the metadata did not have any arguments, or :symbol was not found, the args array will be empty.

Not yet provided is a convenient infrastructure for parsing :meta expressions from C++.