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(isa(ex, Expr) ? pushmeta!(ex, :inline) : ex)
end
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
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++.