What Is the Difference Between a Block, a Proc, and a Lambda in Ruby? (2013)
blog.awaxman.com86 points by Tomte a year ago
86 points by Tomte a year ago
For those unaware, Ruby blocks (and procs) are more flexible than anonymous functions as most language implement them. The article briefly goes over that, mentioning that lambda (regular anonymous functions) and procs (blocks) don't treat execute `return` the same way. There are also particularities with regard to next (continue) and break.
I made a post about the niceties of blocks: https://maxlap.dev/blog/2022/02/10/what-makes-ruby-blocks-gr...
Flexible in a way, sure. But non-locality is generally a bad property, not good. Adding it is a workaround for all the enumeration methods using blocks in a way that makes people think they're weird looping syntax instead of a fundamentally different idea.
People want to do early returns from looping over a collection, so take the easy solution of adding more messy language semantics instead of finding a semantically simple solution instead. (For that matter, have fun working out the semantics of break and next when used in a block that isn't an argument to an enumeration method. How do you as a method author opt in to distinguishing between the two after yielding to a block?)
This is generally the case with Ruby everywhere. Why does that thing have an edge case with weird semantics? To work around the edge case with weird semantics somewhere else. It's all fine if you want to just try things until something works. But if you want to really understand what's going on and write code that's a first-class participant in the language features, it gets really frustrating to try to deal with it all.
It's really not that complex.
For my (buggy, unfinished, languishing without updates) prototype Ruby compiler, lambda and proc (and blocks) are all implemented nearly the same way, with the exception that for proc/blocks, return/break/next will act as if having unwound the stack to the scope where the proc was defined first (or throw an error if escaped from there).
The distinction is very obvious when you think of it in that way - a proc acts as if called in the context/scope it was defined, while a lambda acts as if called in the scope it is called in.
> How do you as a method author opt in to distinguishing between the two after yielding to a block?
You don't. A block acts as a proc, not a lambda. If you want lambda semantics, then take a lambda as an argument - don't take a block/proc and be surprised it doesn't act like something it isn't.
Given that you completely ignored what I said I wanted to do and gave an answer for some other question, I'm pretty sure it's more complicated than you think.
I want to write a method that takes a block and distinguishes between next and break, exactly like the methods of enumeration do. It's obviously possible because a super common interface does it.
Last time I looked, that interface does it by being written in native code that interfaces with the interpreter. That is, it's not part of the language semantics. It's a special case with weird rules unlike what anything else gets to do.
Or at least it was. Maybe the language has actually made it accessible since then, but I'm not optimistic. That's not the ruby way.
Your sentence structure did not at all make it clear that "the two" referred to next and break rather than proc and lambda.
Assuming you don't care whether a "next" was actually called, but only whether you've exited the block and whether or not you exited it via a break, you can do this check in a number of ways, but it I will agree it's a bit dirty that sometimes you do need to rely on standard library functionality rather than language constructs if you want to do these things.
Here's one way of doing it, since "break" within a Fiber triggers a LocalJumpError:
def next_or_break(&block)
Fiber.new(&block).resume
:next_or_end_of_block_reached
rescue LocalJumpError
:break
end
p(next_or_break do next end)
p(next_or_break do break end)> but it I will agree it's a bit dirty that sometimes you do need to rely on standard library functionality rather than language constructs if you want to do these things
You don't need to introduce a Fiber or rely on the standard library, you can just use the core language, leveraging the fact that code in ensure sections gets called even when you are returning "past" the calling method.
(You may need a dirty catch-all rescue so you can set a flag before reraising that lets you distinguish "bypassing direct return by exception" from "bypassing direct return by break or proc-semantics return", but that's still the core language, not standard library.)
I put example code in another comment: https://news.ycombinator.com/item?id=44065226
Ah, of course. I'm embarassed I didn't think of that now... That's absolutely better than abusing Fiber. Great suggestion.
I think you’re seeing the base language.
Effectively blocks are self-contained chunks of code. You can change things around them, but you can’t change how keywords work inside them. Because you’re crossing a method boundary when you call a block you’re not able to access next and break. (Or capture return.)
Ruby is defining scope here and C methods are not limited by the language they define.
Exactly. You can leverage some of the standard library methods to figure out if a block did a next or exit normally vs. did a break, by e.g. abusing Fiber's (there are probably other ways too), and that might feel a bit dirty, but those are also part of the language.
I think it'd be nice if Ruby language constructs offered a few of the building blocks used by the standard library so that you could implement more of the standard library without dipping down into some low level mechanism - for my prototype Ruby compiler a priority was to implement as much as possible in Ruby, and there are certainly more things in the core classes that can't be done in pure Ruby than I'd like.
But it's not that much, and while there are plenty of warts, the inconsistencies are often smaller than people think.
It seems you're pretty upset about your experience with Ruby. I'm sorry that's been the case for you.
However, in Ruby blocks aren't just about flexibility, more importantly they're about generality. They're not there to resolve an edge case at all (Ruby also has keywords for loops). They're a generalization of control flow that is just slightly less general than continuations. In practical use they provide an alterative to Lisp macros for many use cases.
These were some of the problems Matz was trying to sort out with his design of Ruby--creating a language that was fun and easy to use for day-to-day programming, with as much of the meta-programming power of Lisp and Smalltalk as possible. Blocks are one of his true innovatations that came from trying to balance that tension.
> Blocks are one of his true innovatations …
How do they differ from Smallalk blocks? (I don't know.)
Blocks in Smalltalk (to my understanding) are closures. Blocks in Ruby are closures that also bring the call stack they were created in with them.
One way to think of about it is this: anonymous functions as originally implemented in early Lisps are code as an object, closures are code with its lexical environment as an object. You can think of a Ruby block as code with its lexical environment and its call stack as an object.
So they don't just handle return differently than closures, they have access to the call stack of the site where they're created like a continuation. This is why they handle return differently, but this is just one of the things that falls out from that. It also comes with other control flow features like "redo", "retry", "next", "rescue", "finally", and others. These are all call stack control (control flow) conveniences, just like return is. All of them can be thought of as being abstractions built on top of continuations (just ask a Scheme hacker).
Originally Ruby was basically a Lisp without macros, but with continuations, a Smalltalk like object system and a lot of syntactic affordances inspired by Perl, and other languages. Blocks are one of the conveniences built on top of the Lispy semantics.
Note that I'm explaining how blocks work as an abstraction (vidarh below explains how they work as a concretion, as implemented in MRI).
> … other control flow features like "redo", "retry", "next", "rescue", "finally", and others.
At-a-glance afaict Smalltalk provides those features too, so I would guess Smalltalk blocks may have access to the call stack too?
Yes Smalltalk has continuations. So it can do all of those things as well. But I don't think they're explicitly tied to blocks like they are in Ruby. This really isn't a problem for Smalltalk since it's not as syntax oriented as Ruby.
The invovation is to have those features tied to convenient syntax.
So is the innovation to make something that was available in Lisps / Smalltalks, available within the different constraints of Ruby.
(I should check how Smalltalk blocks behave.)
I would say more broadly the innovation was two fold: 1) to make these features available in a syntactic form that would seem more familiar to programmers and 2) the powerful insight that when combined with Smalltalk style meta programming you can have a language that on the surface seems very conventional but underneath is just as powerful as Smalltalk or Lisp.
Although I would say he didn’t get 100% there although that this point Ruby isn’t too far from that.
These are ideas that I think are worth trying to take even further. In fact, I’ve been experimenting with that.
fwiw "Efficient Implementation of Smalltalk Block Returns"
https://wirfs-brock.com/allen/things/smalltalk-things/effici...