Category Archives: Erlang

Python style decorators in Erlang

Alan Perlis said that “A language that doesnt change how you think about programming isnt worth knowing”.
My favourite languages are Erlang, Python and C++ (and Lua, but these days Python is taking more of that market share). I like them all for very different things, and they all have vastly different strengths,weaknesses and feature sets.
I also like thinking about how/if you can apply idioms that are common in one language to another language, and if theres any benefit in doing so. Today Im going to look at Python style decorators implemented in Erlang.

The example code is here

Decorators overview

Firstly: Python decorators are unrelated to the GoF Decorator pattern.
Decorators are syntactic sugar in python for applying a function to another.

def some_expensive_func(n):
   return some_expensive_computation(n)

is the same as

def some_expensive_func(n):
   return some_expensive_computation(n)
some_expensive_func = memoize(some_expensive_func)

This works because Pythons functions are first class values.

Decorators are a common idiom in Python and one that I really like.
I personally think that when used appropriately they make functions easier to read, by moving small details out of the main logic.

You can read more about the origin of the decorator syntax here.
You can see some of the neat things people do with decorators here.
Decorators are also very common in one of my favourite web frameworks; Django.

Decorators in Erlang

Following the goals laid down in the PEP I decided to set the following goals:

* Allow decorators to run code before and after a call to the function they are decorating. Eg to commit db transactions, cache values, trace or time execution etc.
* Allow decorators to alter the argument list that gets passed to the function they are decorating.
* Have a simple, non intrusive syntax. (Within the confines of erlang syntax rules.)
* Support multiple decorators on a single function
* Support passing custom arguments to the decorator.

Erlang does not support reassignment of variables, so no monkey business (monkey patching) here folks.
It also doesnt support executing code at module scope. Though on_load can often achieve similar results to module scope.
This complicates the implementation a little.

Id been intending to play with parse transforms for a while, and thought they could help.

The Plan

transform something like

-decorate( {decorator_module, decorator_function} ).
foo(N) -> N.


foo(N) -> foo_arity0_1([N]).
foo_arity0_1(Args) ->
   F = decorator_module:decorator_function( fun foo_arity0_0/1, Args),
foo_arity0_0([N]) -> foo_original___(N).
foo_original___(N) -> N.

This would let each decorator do whatever it wanted. It could call a completely different function or charge the arguments.

Our simple first test case

As a fan of TDD I wrote a very basic first test that I could use to start development

-compile([{parse_transform, decorators}]).
% example decorator that replaces the return value with the atom 'replaced'
	fun() -> 
		_R = apply(F, [Args] ),
-decorate({ ?MODULE, replace_return_value_decorator }).
replace_ret_val_decorated() -> ok.
	?assertEqual(replaced, replace_ret_val_decorated() ).

Getting the abstract forms

I did the simplest thing possible and printed the forms that were passed to the parse transform.

	io:format("~p~n=======~n",[ Ast ]),

We get the following info out.
You can read a bit about the format here

                        {string,20,"replace_ret_val_decorated ( )"}]},

Pretty Printing

I think the abstract forms are quite easy to follow, but it would be nicer to be able to see what we are producing as if it was real Erlang code.
Luckily erl_pp has this.
If you are writing your own parse transforms this will be very useful for debugging.

pretty_print(Ast) -> lists:flatten([erl_pp:form(N) || N<-Ast]).

for the untransformed code we see

-file("test/decorator_test.erl", 1).
-file("c:/PROGRA~2/ERL57~1.5/lib/eunit-2.1.5/include/eunit.hrl", 1).
-file("test/decorator_test.erl", 3).
replace_return_value_decorator(F, Args) ->
    fun() ->
           _R = apply(F, [Args]),
replace_ret_val_decorated() ->
replace_ret_value_test() ->
    fun(__X) ->
           case replace_ret_val_decorated() of
               __X ->
               __V ->
                                    "replace_ret_val_decorated ( )"},
test() ->

The transform

We need to walk the forms collecting any decorators, and apply collected decorators to a function when we meet it.
For the application step I figured it would be easy to output a nested list for when 1 form (the original function) expands to many, and clean it up later.
We also need to remove the decorator attributes, as you really are only meant to have attributes before all functions.

	{ExtendedAst2, RogueDecorators} = lists:mapfoldl(fun transform_node/2, [], Ast),
	Ast2 = lists:flatten(lists:filter(fun(Node)-> Node =/= nil end, ExtendedAst2)),
% transforms module level nodes
% see
% outputs nil (to swallow the node), a single node, or a list of nodes.
% nil nodes are removed in a subsequent pass and the lists flattened
transform_node(Node={attribute, _Line, decorate, _Decorator}, DecoratorList) ->
	% keep a list of decorators but dont emit them in the code.
	% this is important as you arent meant to have attributes after functions in a module
	{nil, [Node|DecoratorList]};
transform_node(Node={function, _Line, _FuncName, _Arity, _Clauses}, []) ->
	% pass through decoratorless functions
	{Node, []};
transform_node(Node={function, _Line, _FuncName, _Arity, _Clauses}, DecoratorList) ->
	% apply decorators to this function and reset decorator list
	{apply_decorators(Node,DecoratorList), []};
transform_node(Node, DecoratorList) ->
	% some other form. (the only other valid forms are other attributes)
	{Node, DecoratorList}.
apply_decorators(Node={function, Line, FuncName, Arity, _Clauses}, DecoratorList) when length(DecoratorList) > 0 ->
		% output the original function renamed
		% output a trampoline into our decorator chain
		function_form_trampoline(Line, FuncName, Arity, DecoratorList),
		% output the funname_arityn_0 function to unpack the arg list and forward to the original 
		% output our decorator chain
		| function_forms_decorator_chain(Line, FuncName, Arity, DecoratorList)

I wont go to much into the details of what each of these do, because frankly its pretty simple.
They just fill in various absform templates.. very basic code gen..

function_form_decorator_chain(Line,FuncName,Arity, {DecMod, DecFun}, DecoratorIndex) ->
	NextFuncName = generated_func_name({decorator_wrapper, FuncName, Arity, DecoratorIndex-1}),
	{function, Line, 
		generated_func_name({decorator_wrapper, FuncName,Arity, DecoratorIndex}), % name
		1, % arity
		[{ clause, Line,
			emit_arguments(Line, ['ArgList'] ),
			emit_guards(Line, []),
				% F = DecMod:DecFun( fun NextFun/1, ArgList),
				emit_decorated_fun(Line, 'F', {DecMod, DecFun},	NextFuncName, 'ArgList'),
				% call 'F'
				{call, Line,{var,Line,'F'},[]}

Sure theres some emit_* funcs in there to make it easier to read, but anyone that even glances as the absform docs and the absform for some sample functions could piece this together in short time.

I often find myself thinking “I cant believe how nice Erlang is”. Python too.

custom warnings and errors

Oh yeah, I also wanted to warn if there was any decorators at the end of the file that werent associated with a function.
To do that I just add warning nodes to the end of the file for each element in RogueDecorators.

	{ExtendedAst2, RogueDecorators} = lists:mapfoldl(fun transform_node/2, [], Ast),
	Ast2 = lists:flatten(lists:filter(fun(Node)-> Node =/= nil end, ExtendedAst2))
		++ emit_errors_for_rogue_decorators(RogueDecorators),
	[{error,{Line,erl_parse,["rogue decorator ", io_lib:format("~p",[D]) ]}} || {attribute, Line, decorate, D} <- DecoratorList].

We also need to do it when we hit the eof node. Otherwise the decorators would get applied to the function that eunit generates after the eof.
So I added the following clause to transform_node

transform_node(Node={eof,_Line}, DecoratorList) ->
	{[Node| emit_errors_for_rogue_decorators(DecoratorList) ], []};



to the end of the file makes it emit the following

Ender@PROSPERO /j/git/erlang_decorators
$ ./rebar compile eunit
==> erlang_decorators (compile)
==> erlang_decorators (eunit)
test/decorator_test.erl:22: rogue decorator {f,f}

the result

The code is here.
It could be developed more and cleaned up a little.

Heres the code after transformation (ie pretty_print(Ast2))

-file("test/decorator_test.erl", 1).
-file("c:/PROGRA~2/ERL57~1.5/lib/eunit-2.1.5/include/eunit.hrl", 1).
-file("test/decorator_test.erl", 3).
replace_return_value_decorator(F, Args) ->
    fun() ->
           _R = apply(F, [Args]),
replace_ret_val_decorated_original___() ->
replace_ret_val_decorated() ->
replace_ret_val_decorated_arity0_0([]) ->
replace_ret_val_decorated_arity0_1(ArgList) ->
    F = decorator_test:replace_return_value_decorator(fun replace_ret_val_decora
replace_ret_value_test() ->
    fun(__X) ->
           case replace_ret_val_decorated() of
               __X ->
               __V ->
                                    "replace_ret_val_decorated ( )"},
test() ->

As we can see it did the transform that we planned.

and the result

$ ./rebar compile eunit
==> erlang_decorators (compile)
Compiled src/decorators.erl
Compiled src/decorators_app.erl
Compiled src/decorators_sup.erl
==> erlang_decorators (eunit)
Compiled src/decorators.erl
Compiled src/decorators_app.erl
Compiled src/decorators_sup.erl
Compiled test/decorator_test.erl
  All 4 tests passed.

Thats enough for now.

Is it useful?

Its just syntactic sugar (The same is true in python).
If you are wondering where decorators are useful take a look at django.

But It was interesting to play with parse transforms, and it does provide an interesting syntax option.
I personally like this form of decorator, but beauty (especially in programming languages) is in the eye of the beholder.
I feel that the things that decorators *feel right* for is things that are a bit of a cross cutting concern, so its nicer to not need to pollute the code in the function.

After this Im thinking of doing a more general purpose attribute system.
I think it could be neat to eg automatically expose certain functions as RPC methods based on this.
Pretty atypical Erlang, but its interesting to see how you can apply constructs and idioms from other programming communities to achieve goals and save time.

Limitations and future extensions

I didnt end up implementing passing arguments to the decorator, but its a relatively simple extension.

Erlang is a functional language, with single assignment. Unless you use something like meck, you cannot reassign a function.
So these decorators don’t execute any code when the module is loaded, so we cant use this to eg track test coverage of certain annotated functions.
We could extend this in future to support generating on_loaded function, or to allow us to query a module for its absforms at runtime without requiring debug_info compiler option. Both of these approaches would go someway to addressing this limitation.

We are still bound by Erlang syntax rules. So eg you cant put decorations on individual clauses

bar(1) -> ok;
bar(N) -> bar(N-1).

If I was serious about using this Id also like to extend it to make it easier/prettier to define custom decorators in a header, which could use a simplified syntax. And of course to support passing extra information to the decorator.
something like

-log_entry_exit( [{logger, logger_proc }] ).
foo() -> 1.

Erlang R14B03 out

See the readme here

Things Im excited about:

  • lots of halfword vm fixes and improvements
  • Open ssl now staticly linked to crypto on Windows.
  • Majority option for mnesia tables.
    This is one convenient way to handle network splits. I have to give it a go with a large distributed+fragmented table

        OTP-9304  Add {majority, boolean()} per-table option.
    	      With {majority, true} set for a table, write transactions
    	      will abort if they cannot commit to a majority of the nodes
    	      that have a copy of the table. Currently, the implementation
    	      hooks into the prepare_commit, and forces an asymmetric
    	      transaction if the commit set affects any table with the
    	      majority flag set. In the commit itself, the transaction will
    	      abort if it cannot satisfy the majority requirement for all
    	      tables involved in the transaction.(Thanks to Ulf Wiger)
  • better cover support for tests
    OTP-9204  add user specified compiler options on form reloading
    	      In order to be able to test non-exported functions from
    	      another (test) module it is necessary to compile the specific
    	      module (at least during the test phase) with the export_all
    	      compiler option. This allows complete separation of testing
    	      and productive code. At the moment it is not possible to
    	      combine this with a test code coverage using the cover
    	      module. The problem is that when cover compiling a module
    	      using cover:compile_* the code is reloaded into the emulator
    	      omitting/filtering the passed user options. In my example
    	      above the export_all option would be removed and the
    	      non-exported functions cannot be called any more. (Thanks to
    	      Tobias Schlager)
  • Table viewer can now display small binaries!
    I tend to use binaries for strings so this bugged me on a number of occasions (obviously not enough for me to fix it myself).

        OTP-9153  tv: Allow table viewer to display refs, ports and small
    	      Table viewer displayed #Port, #Ref, or #Bin as place holders
    	      for their respective object types in ets and mnesia tables.
    	      This can make table viewer difficult to use when viewing
    	      tables containing those data types. It doesn't make sense to
    	      render large binaries so #Bin will still be used for binaries
    	      that exceed 100 bytes. (Thanks to Blaine whittle)
  • Fixed a display problem with the Event Tracer on Windows.
    OTP-9238  The popup window 'contents viewer' did not display properly on Windows.

Using Zlib in Erlang

The simplest way to compress and decompress binaries in Erlang is zlib:compress and zlib:decompress.
This works great for large data and on unreliable media.
However for small data the headers and checksum take a relatively large size, which is often unneeded when the media is reliable.
The zlib library supports removing the header and checksum information.
Erlang exposes this through the zlib:zip and zlib:unzip functions.

But if you’re use case is a lot of small documents you can do better using the API:

compress(Data,Dict) ->
	zlib:deflateInit(Z, best_compression,deflated, -15, 9, default),
	zlib:deflateSetDictionary(Z, Dictionary),
	B1 = zlib:deflate(Z, Data),
	B2 = zlib:deflate(Z, <<>>, finish),
	list_to_binary([B1, B2]).
uncompress(Data,Dict) ->
	zlib:inflateInit(Z, -15),
	zlib:inflateSetDictionary(Z, Dict),
	B1 = zlib:inflate(Z, Data),

For long documents; specifying a dictionary doesn’t make a huge difference, but if you are storing/transmitting a lot of short XML/JSON snippets it can make a difference.
If you don’t know what dictionary to use just specify an empty binary to get default behavior.

The zlib docs say this about specifying a dictionary:

The dictionary should consist of strings (byte sequences) that are likely to be encountered later in the data to be compressed, with the most commonly used strings preferably put towards the end of the dictionary. Using a dictionary is most useful when the data to be compressed is short and can be predicted with good accuracy; the data can then be compressed better than with the default empty dictionary.