meta

A metaprogramming library for the Erlang programming language. meta can be used to transform Erlang modules not only at compile time, but also at run time.

Meta (from the Greek preposition and prefix meta- μετά- meaning "after", or "beyond") is a prefix used in English to indicate a concept which is an abstraction from another concept, used to complete or add to the latter.

Metaprogramming is the writing of computer programs with the ability to treat programs as their data. It means that a program could be designed to read, generate, analyse and/or transform other programs, and even modify itself while running.

How to use this library

Most of the functions available in the meta module operate on other Erlang modules. Erlang modules can be fed in to these functions in two flavors: 1) as a list of forms (e.g., result of invoking forms:read(<module-name>), etc.) and 2) as an atom (i.e, module name). For instance, the function meta:is_exported_function/3 can be used as

meta:is_exported_function(new, 0, sets).
% => true

or as

SetsForms = forms:read(sets),
meta:is_exported_function(new, o, SetsForms).
% => true

indistinctively.

Moreover, when operating on modules specified as atoms, most of the functions accept a list of options as the last argument. These options can be used to specify how the transformations will be applied on the target module. At the time of this writing, only two options are supported (i.e., permanent and force). By default, all changes are transient (that is, they do not survive to a node restart) and cannot be applied to protected modules. If one wants to alter this behaviour, one has to set the permanent and/or force options respectively.

Note that when operating on a module represented as a list of forms the transformations will not be effective until they are applied by means of a call to the meta:apply_changes/{1, 2, 3} function. This function accepts the same permanent and force options described above.

Error handling

Most of the functions will throw an exception in case they face an error during their execution.

Function	Exception	When
module_name/1	invalid_module	The provided forms do not contain a -module(<module-name>). attribute
function/3	{function_not_found, {F, A}}	The specified function is not implemented by the provided module
spec/3	{spec_not_found, {F, A}}	The specified function specification is not found in the provided module
type/3	{type_not_found, {T, A}}	The specified type is not defined in the provided module
record/3	{record_not_found, R}	The provided module does not have a definition for the specified record
*	{cannot_load_forms, Module}	The AST of the provided module cannot be loaded. Most likely because it has not been compiled using the +debug_info option
*, apply_changes/3	{protected, Module}	Attempting to apply changes on a protected module without setting the force option
*, apply_changes/3	{compile_error, Module}	There is an error in the AST one is attempting to compile
*, apply_changes/3	Error	When setting the permanent option, if an error occurs when attempting to create the .beam file

Examples

Function injection

The code below illustrates how to add a hello/1 function to an arbitrary module.

HelloFunction = forms:function(hello,
                               fun(Name) ->
                                   io:format("Hello, ~s!~n", [Name])
                               end,
                               []),
meta:add_function(HelloFunction, _Export = true, my_awsome_module).

The example above assumes you have compiled the code using the forms_pt parse transform. If you cannot use the forms_pt parse transform, you can obtain the form of the hello/1 function using forms:to_abstract/1.

HelloFunction = forms:to_abstract("hello(Name) -> io:format(\"Hello, ~s!~n\", [Name]).").

(Temporary) latency measurement in a live system

This example illustrates how to use meta to measure how long does it take to start a virtual node in a live Riak deployment.

%% Wrapper function
WrapperFunction =
    forms:to_abstract(
        "init(X) ->"
        "    {Time, Value} = timer:tc(fun() -> '_init'(X) end),"
        "    io:format(user, \"[meta] init/1 latency = ~p~n\", [Time]),"
        "    Value.").

Forms0 = forms:read(riak_core_vnode).
Forms1 = meta:rename_function(init, 1, '_init', false, Forms0).
Forms2 = meta:add_function(WrapperFunction, false, Forms1).
meta:apply_changes(Forms2).

%% ...
%% [meta] init/1 latency = 51
%% [meta] init/1 latency = 6
%% ...

%% ... after some time we decide to disable the measurements ...

%% Rollback
meta:apply_changes(Forms0).

Note that the code above has been executed on the shell process running on a Riak node and no restart was required.