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“Ineffective Ruby sugar”: Argument forwarding

“Ineffective Ruby sugar”: Argument forwarding

2023-11-24 08:41:51

This is part of a weblog submit sequence about “ineffective” (or: controversial) syntax components that emerged in current Ruby model. The aim of the sequence is to not defend (or criticize) the options, however to share a “thought framework” for evaluation of their causes, design, and impact the brand new syntax has on a code that makes use of it. See additionally intro/ToC post.

Right now’s submit covers the function that, in contrast to many of the others within the sequence, precipitated little or no pushback: a set of shortcuts for argument forwarding.


Since Ruby 2.7, that is potential (please notice that ... within the code beneath is the precise legitimate syntax, not “code omitted for a weblog submit”):

def foo(...)
  # bar receives all positional, named and block arguments
  # that foo obtained

Since Ruby 3.1, an nameless block argument will be “forwarded” individually:

def iterate_through_data(&)

Since Ruby 3.2, one also can “ahead” positional and named (key phrase) arguments individually:

def split_arguments(*, **)
  pass_positional(*) # passes all positional arguments
  pass_keywords(**) # passes all key phrase arguments

split_arguments(1, 2, 3, a: 4, b: 5)
# after this name, these strategies can be known as:
#   pass_positional(1, 2, 3)
#   pass_keywords(a: 4, b: 5)

Why & How

…are inseparable right here!

The introduction of ... was not one thing mentioned for years (at the very least, I’m unaware of such discussions). It was slightly an impromptu invention on the wave of the Huge Adjustments in Ruby 2.7.

Model 2.7 was the final preparatory model earlier than 3.0, and it launched a number of the “large oh” modifications so all people had time to organize. One in every of such modifications was a last separation of positional and key phrase arguments: making some guidelines of treating arguments stricter and fewer cumbersome than they traditionally had been.

It is a sophisticated subject, effectively coated by the official explanation. Within the context of the shorthand invention, we have to know that one of many uncomfortable side effects of the separation was that writing a technique that simply passes all of its arguments to a different technique turned sophisticated. The pre-2.7 method of doing so was this:

def reader(title, mode:)
  # does some studying

def author(title, content material)
  # does some writing

def wrap(technique, *args)
  # a typical wrapper technique, which could, say, log execution,
  # catch additional errors, do a extra sophisticated dispatching and so forth.
  ship(technique, *args)

wrap(:reader, "file.txt", mode: 'wb')
wrap(:author, "file.txt", 'some content material')

*args within the technique signature and the strategy name was sufficient to move all positional and key phrase arguments round. In Ruby 2.7, the separation would turn into extra formal, so to deal with each type of technique, one wanted to jot down:

def wrap(technique, *args, **kwargs)
  ship(technique, *args, **kwargs)

In a generic scenario, the third kind of the argument ought to’ve been thought-about: a block (Ruby’s particular “tail lambda”). So for a really common delegator, one wanted to jot down this:

def wrap(technique, *args, **kwargs, &block)
  ship(technique, *args, **kwargs, &block)

That’s a complete lot of syntax and naming to do a factor that’s so easy by its concept!

So the sensible answer for a shortcut “simply move no matter arguments you might have” was proposed and rapidly accepted within the type of ....

A number of technicalities and edge circumstances had been mentioned and settled, some instantly, some within the subsequent model. Say, support for passing a leading argument before forwarding syntax was launched in 3.0 however thought-about so helpful it was then backported to the two.7 department and has been obtainable there since round 2.7.3.

It was type of an enormous deal as a result of “delegate all the things” is a really widespread phrasing in Ruby, used for all types of results:

# conceal the true name sequence complexity:
def log(textual content, stage:, **payload)
  Loggers::Registry.fetch(:http_logger).log(textual content, stage: stage, **payload)

# dynamically select an implementation
def make_event(kind, sender, **particulars)
  EVENT_CLASSES[type].new(sender, **particulars)

# make a pleasant DSL with dynamically outlined strategies:
class HTML
  def method_missing(title, content material, **attributes)
    # assemble tag string from tag title, content material and attrs
finish'Ruby', href: '')
#=> "<a href="">Ruby</a>"

All of these circumstances will be made a lot shorter with ..., with out dropping the actual which means of “simply move all the things”!

This syntax change was a uncommon case when many teams with regularly conflicting views instantly noticed a direct acquire:

  • those that are often interested by syntax modifications and shortcuts discovered it fairly;
  • these extra cautious and regularly asking “what’s the use case”, on this case, instantly knew loads of them (particularly contemplating that after ruby 2.7, a variety of delegating code ought to’ve been rewritten anyway, both to ... or to *args, **kwargs, so making peace with a shortcut appeared acceptable);
  • lastly, these with an emphasis on language as a realistic engineering instrument noticed a acquire of the improved efficiency.

The latter has a easy clarification: what you don’t title, you don’t must put within the object. E.g., this allocates an intermediate array and hash to place positional and key phrase arguments into:

def delegator(*args, **kwargs)
  # `args` is Array, and `kwargs` is Hash right here
  # ...however we would have liked them solely to right away unpack
  delegatee(*args, **kwargs)

Whereas this code doesn’t make further native variables obtainable, and subsequently no must allocate an array/hash:

def delegator(...)
  # no additional native vars right here

So, it wasn’t a shock or a scandal when, a few variations later, separate shortcuts to move solely positional and solely key phrase args had been proposed.

Furthermore, the change was small; these signatures had been already legitimate syntax:

def ignore_my_args(*)

def ignore_keyword_args(some, positional, **)

…to say, “the strategy accepts any numbers of positional or key phrase args (possibly for compatibility with the identical technique in neighbor lessons), however ignores them.” So the one change in Ruby 3.2 was to moreover permit to say “…and passes them additional”:

def pass_my_args(*)

def pass_keyword_args(some, positional, **)

Contemplating the intuitive feeling of “no new syntax” and that ... was already there, and with the identical “no pointless allocations” argument, the change was rapidly accepted. The truth that it was proposed (and the high-quality implementation equipped) by Jeremy Evans, writer of outstanding libraries like Sequel and Roda and a member of Ruby core helped, too.

Curiously sufficient, the brand new syntax is appropriate not just for delegation to a different technique however virtually all over the place the place unpacking of named variables was supported:

def with_anonymous_args(*, **)
  ary = [*]
  hash = {**}
  p ary, hash

with_anonymous_args(1, 2, 3, a: 4, b: 5)
# Prints:
#   [1, 2, 3]
#   {:a=>4, :b=>5}

This may be thought-about extra of a curiosity (at the very least the “don’t instantiate an array/hash” acquire is misplaced right here), however could be at the very least helpful for short-term debugging statements within the pass-everything strategies:

def make_event(kind, **)
  places "DEBUG!" if {**}.key?(:password)  # temp

Or, because the slightly-over-the-top instance on the finish of the “Pattern Matching / Taking it further” exhibits, the “we don’t care in regards to the title” will be repurposed to additional sample match the argument listing as a number of potential signatures with completely different meanings (and, subsequently, names) for components of the argument listing.

The story with & for nameless block forwarding is a little more sophisticated.

In contrast to * or **, there was no & for “simply ignore this block” as a result of block arguments in Ruby are all the time elective, and there’s no method neither to exhibit within the technique signature “we require it” nor that we don’t. (It’s generally an issue when blocks are erroneously handed—and ignored—to strategies that by no means anticipated them, however it’s fairly hard to solve.)

So, when the standalone forwarding with & was proposed—lengthy earlier than the two.7’s “argument forwarding” work—primarily as an optimization for block allocation, it was met with nice warning. At the moment, the optimization half was implemented by itself as simply optimization of passing the block round even when it was named. Later, although, when the essential argument forwarding with ... was already within the language, the six-year-long dialogue in regards to the acceptability and readability of & was ended with its introduction in Ruby 3.1.

That’s the identical impact we noticed while discussing key phrase argument omission (which turned acceptable after we obtained used to different circumstances of value-less key: syntax). As soon as a “larger” function takes its mindshare, the smaller ones would possibly comply with extra simply.

Irks and quirks

A small irk round ellipsis-based delegation is expounded to parentheses. The character of the issue is just like what we noticed within the key phrase omission case:

def my_method(...)
  p ...

Is definitely

def my_meth(...)
  (p()...) # empty technique name + a spread from its outcome to infinity

This impacts solely ellipsis (not different types of nameless forwarding) and is remedied, as ordinary, by including parentheses:

def my_method(...)

The more serious drawback is that nameless forwarding just isn’t supported in blocks/procs. That is particularly complicated contemplating that an outdated syntax of “nameless splat” is supported, and subsequently one would possibly doubtlessly write code with a really complicated impact:

def course of(*)
  # ...a variety of code...
  ['test', 'me'].every  places(*) 

course of('enter')

This seems to be as if it should print “check” and “me” (inside proc, * accepts its arguments and passes them to places), however truly, it prints:

What occurs right here is:

  • every { |*| is handled in outdated logic “settle for all arguments and discard them;”
  • places(*) is handled within the new logic “see if the context has nameless forwardable arguments”—and take into account the strategy’s arguments as such.

That is an open discussion on the matter, with a complicated (for me, at the very least) final result: the Ruby builders’ assembly appears to be leaning towards an concept of simply prohibiting the case like above (*-forwarding inside a block with * arguments) whereas permitting extra unambiguous circumstances.


What occurs on a (conscious) utilization of argument forwarding shortcuts is the onset of explicitness.

This would possibly sound complicated as a result of the “explicitness” is regularly related to including extra names to the code or extra steps to assemble the worth. Like splitting the system into a number of named native variables or, as a substitute of passing the results of some technique to a different, first attaching it to some title. (In pathological circumstances, it’s “each non-trivial name/examine/calculation must be its personal technique with the title explaining its utilization.”)

However right here, I’m speaking about the explicitness of the intention of some sizeable chunk of code, a “web page” or a “chapter” of it. (In the identical method within the intro article I underlined we’ll be speaking in regards to the reader’s consolation in comprehension of the narrative as a substitute of the “readability” of a single line.)

Think about a code like this:

def occasion(kind, sender:, content material:, particulars:)
  EventBus::Registry.occasion.push_event(kind, sender: sender, content material: content material, particulars: particulars)

Such “intermediaries” are frequent in layered programs: the params are already checked and defaults assigned by the layer above; the dealing with itself can be carried out by the layer beneath; and this present technique is only a shortcut within the present module (that most likely invokes it many instances, so repeating the verbose name of the underlying layer is tiresome).

There are a number of methods to make this definition shorter, like utilizing values omission

def occasion(kind, sender:, content material:, particulars:)
  EventBus::Registry.occasion.push_event(kind, sender:, content material:, particulars:)

…or “keyword-rest” splatting:

def occasion(kind, **event_data)
  EventBus::Registry.occasion.push_event(kind, **event_data)

However the true intention of this technique is simply to “move all the things additional,” what the writer thinks about its argument names is nearer to “like, you understand, all the things” or “yeah, no matter, simply move it” (and regularly would simply name the remainder arguments **kwrest, or, **choices—the latter is a protracted but regularly deceptive custom of naming the final hash/key phrase argument).

So, ultimately, we will be simply express in expressing these “you understand” or “no matter”:

def occasion(...)

Or, with one-line technique definitions (a subject of the subsequent article), simply

def occasion(...) = EventBus::Registry.occasion.push_event(...)

Such simplifications would possibly seem in numerous levels of design. Generally, “simply move all the things by means of” is an early-prototype model that permits to rapidly assemble the cheap stack of layers; and later make clear on every step what are the actual obligations moreover pass-through.

Different instances, after a protracted interval of design and clarification, it turns into apparent {that a} little bit of significant realignment of layers with one another’s capabilities and expectations permits dripping the trivial issues in favor of the literal embodiment of “you understand”, .... The intention to take action generally would uncover an unjustified signature change by means of the layers and, as a consequence, would possibly result in helpful cleanups.

In any case, a capability to designate “what’s apparent/doesn’t matter right here” permits the reader to focus higher on the opposite components: those who are non-trivial and vital. Or: if all the things is vital (and underlined by language means like lengthy explanatory names), then nothing is.

That’s why I’m speaking about explicitness: akin to the case of numbered block parameters, generally giving a reputation to a price is simply pretending to clarify one thing. In these circumstances, it’s good to have a syntax that permits to be clear and express about “nothing extra to clarify right here.”

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How others do it

Like final time (with “worth omission syntax”), I truly struggled to seek out the precise correspondence of the “move each argument” syntax: partially, possibly as a result of there isn’t a different languages to reach to (*args, **kwargs, &block) because the shortest normative strategy to say “all the things that may be handed to a perform.”

The design area right here is expounded to accepting to the perform a variable variety of arguments—the idea is regularly known as “varargs” (regularly demonstrated by formatted-printing features comparable to printf). Evidently languages which have them (some, like Rust, consciously don’t; Zig had them within the early variations however then determined against), there are a number of teams:

Group 1, “The old-fashioned”: make a declaration “variable variety of arguments is accepted right here” within the perform signature, and supply some particular title (of variable/perform/macro) to entry them. Say, in C:

int sum(int depend, ...) {
  va_list args;
  va_start(args, depend);
  // deal with `depend` of `args`, calling `va_arg(args, int);` every time

Group 2, “The old-fashioned, however dynamic”: within the outdated JS and outdated PHP any perform, no matter its signature, may settle for any variety of arguments and uncovered them with arguments (JS) or func_get_args() (PHP):

perform variadic() { console.log(arguments) }

variadic(1, 2, 3, {foo: 'bar'})
// Prints
//   Arguments(4) [1, 2, 3, {foo: 'bar'}]

In a radical variant of this, Perl’s sub doesn’t have a syntax for arguments declaration in any respect, and all arguments are accessible contained in the subroutine in an inventory variable @_, and the best way to designate their names is to assign them to native variables:

sub check {
  my($x, $y) = @_;
  print "x=$x y=$yn"

check(1, 2)
# prints x=1, y=2

Group 3, “New college”: Many languages have a particular syntax for a perform signature to declare a named parameter that may “catch” the variable listing of parameters. Like in trendy JS:

See Also

perform new_variadic(...myargs) { console.log(myargs) }

new_variadic(1, 2, 3, {foo: 'bar'})
// Prints
//   [1, 2, 3, {foo: 'bar'}] -- notice no particular "Arguments" object wrapper

The designation used is regularly ... or *, although C# makes use of param key phrase for such circumstances, and Kotlin makes use of vararg.

It appears to be a typically agreed-upon observe these days.

The symmetric query can be kind of agreed upon: if in case you have an inventory/array/tuple of values and need to move them as separate arguments to some perform, there may be an operator for that (often trying the identical as “relaxation arguments” declaration, and regularly known as “splat” or “unfold”):

perform function_with_3_args(arg1, arg2, arg3) {
  console.log({arg1, arg2, arg3})
args_in_array = [1, 2, 3]
// Prints: {arg1: 1, arg2: 2, arg3: 3}

…which offers a strategy to carry out pass-through (settle for in a single perform “no matter arguments handed” and move them to a different perform).

It wasn’t a given in outdated instances! In arguments days of JS, “move all arguments additional” was as cumbersome as:

perform b(){
    console.log(arguments); //arguments[0] = 1, and so on
perform a(){
    b.apply(null, arguments); // move them by means of, first `null` is for `this`

So in the present day’s scenario (with, often, only one named “splat” argument to be “splat” additional down the layers) is sufficient for many pass-through conditions.

Although, wait!

The attention-grabbing factor occurred to Lua (as I found whereas writing the article).

As of version 5.0, it belonged to “group 1”: ... in signature to designate “accepts any variety of arguments,” a particular variable arg with a desk of all arguments inside a perform.

perform f(...)

f(1, 2, 3)
-- prints  1  2  3

And but, in Lua 5.1, ... replaced the arg, so now it seems to be the closest to Ruby’s shortcuts:

perform f(...)

f(1, 2, 3)
-- prints  1  2  3

-- even can be utilized as a easy variable:
perform f(...)

f(1) -- prints 6

The world of programming languages evolution is filled with wonders!

An apart notice: the temptation to make use of the ... as a syntax assemble alongside the strains of “and so forth” is tempting not just for Ruby and Lua: it’s current at the very least in Python, the place naked ... produces an object Ellipsis, which will be handed round as an everyday worth and has numerous makes use of:

def technique():
  ... # do-nothing technique

# specify kind of enjoyable as a callable with any enter arguments and str outcome
enjoyable: Callable[..., str]

matrix = np.matrix([[1, 2], [3, 4]])
# take all information by all dimensions, however solely 0th column
matrix[..., 0] #=> matrix([[1], [3]])

Taking it additional

There’s one concept that’s unlikely to be applied but nonetheless tempting: can we generally shift the place the place “effectively, you understand” will be stated?

Think about this code: a frequent idiom for a “callable class” (which could encapsulate a sophisticated multi-step algorithm that’s break up into a number of non-public strategies):

class MyOperation
  def initialize(some, arguments, of:, numerous: "varieties")
    # arguments task

  def name
    # ... implementation ...

  # public interface:
  def = new(...).name

Its meant utilization is easy:, some, of: :arguments)

…which simply creates an occasion with all arguments handed and instantly invokes its #name (the meat of the implementation) technique.

The issue right here is that the one public technique of this class doesn’t give any data in its signature: neither to render into autogenerated documentation nor to extract by way of introspection:

m = MyOperation.technique(:name)
#=> #<Technique:> -- not informative!
#=> [[:rest, :*], [:keyrest, :**], [:block, :&]] -- not informative both!

The “callable wrapper” just isn’t the one scenario demonstrating this drawback: a typical HTTP consumer implementation may need one thing like this (whether it is written in a contemporary Ruby):

def get(...)
  make_request(technique: :get, ...)

Right here, once more, “public interface” technique get doesn’t exhibit any details about its signature (which “non-public implementation” technique make_request is aware of).

So, possibly it will be potential to nonetheless help the “move all the things” syntax within the presence of express arguments declaration? As a compromise between “say it the shortest method potential” and “spell all the things like a newbie train”:

def get(endpoint, params: {}, headers: {}, redirect: false) # spell it right here
  make_request(technique: :get, ...) # "simply move it additional" right here

The ticket, describing the concept, exists, nevertheless it by no means obtained a lot consideration.

BTW, there may be place in Ruby the place one thing like this works! Calling tremendous (“mother or father class’ model of this technique”) implicitly passes all arguments of the present technique, which permits for some very good shortcuts. Say, right here is all the things it is advisable to do so as to add some defaults to a Knowledge initialization:

class Measurement < Knowledge.outline(:quantity, :unit)
  def initialize(quantity:, unit: '-none-') = tremendous
finish, 'm') #=> #<information Measurement quantity=100, unit="m">      #=> #<information Measurement quantity=100, unit="-none-">

So, the concept of “settle for params declared explicitly, then simply move all the things” just isn’t unimaginable, at the very least!


So, listed here are some issues to spherical up in the present day’s entry:

  1. Not all issues must be named—we already talked about this whereas discussing numbered block parameters.
  2. Being express in regards to the absence of further which means is a helpful type of explicitness that must be thought-about to underline the meaningfulness of vital components.
  3. Generally, efficiency optimization and readability optimization align like stars, producing a function that (whereas nonetheless assembly its haters) could be defined and defended from a number of factors of view without delay!

The following half might be devoted to the one-line (limitless) strategies, the syntax function that was born out of April Fools’ joke.

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