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:

macroinline(ex)esc(isa(ex,Expr)?pushmeta!(ex,:inline):ex)end

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

@inlinefunction myfunction(x)x*(x+3)end

gets turned into an expression like this:

quotefunction myfunction(x)Expr(:meta,:inline)x*(x+3)endend

Base.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, Base.popmeta! is very handy: Base.popmeta!(body,:symbol) will scan a function body expression (one without the function signature) for the first :meta expression containing :symbol, 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++.