alt.hn

5/18/2025 at 6:34:12 PM

What Is the Difference Between a Block, a Proc, and a Lambda in Ruby? (2013)

 https://blog.awaxman.com/what-is-the-difference-between-a-block-a-proc-and-a-lambda-in-ruby

by Tomte

5/22/2025 at 6:15:39 AM

The fact that there was a book written on this tells me that it's more complicated than it needs to be:

https://www.goodreads.com/book/show/35980970-mastering-ruby-... (not sure why the book was removed from the Pragmatic Bookshelf website).

by digianarchist

5/22/2025 at 8:34:53 PM

I suspect that it is connected to what appears to be the in-progress new edition the author is doing outside of pragprog, though what the exact connection is I couldn't guess.

https://leanpub.com/therubyclosuresbook

by dragonwriter

5/22/2025 at 9:07:18 PM

That was published before. I think they “upgraded” to publishing via Pragmatic Bookshelf

by digianarchist

5/22/2025 at 9:15:55 PM

Oh, weird then. The errata and samples are still up at Pragprog, but not the book page, which suggests something quite unusual.

by dragonwriter

5/21/2025 at 5:46:36 PM

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...

by maxlapdev

5/21/2025 at 9:30:41 PM

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.

by chowells

5/22/2025 at 6:36:50 PM

> have fun working out the semantics of break and next when used in a block that isn't an argument to an enumeration method.

The semantics are:

* next returns from the block, optionally taking a return value that will be returned as the result of the function (this is identical to the behavior of "return" from a Proc with lambda semantics)

* break returns from the function that yielded to the block, optionally taking a return value that will be returned as the result of the function (EDIT: deleted comparison to returns from a Proc with proc rather than lambda semantics here, because it wasn't quite accurate.)

This is, incidentally, also exactly the semantics when they are called from a block passed to an enumeration method, there is no special case there.

> How do you as a method author opt in to distinguishing between the two after yielding to a block?

If control returns to your method, it was a return from the block, either via return (from a block with lambda semantics), next, or running through the end of the block. If, OTOH, break is called (or return from a block with proc semantics), control won't return to the calling method normally, but code in any "ensure" sections applicable will be run, which can be used to identify that this has occurred, and even to override the return value provided by the break.

The simplest possible function illustrating this:

  def how_exited
    yield
    direct = true
    return "next, completion, or lambda return"
  rescue
    direct = true
    return "exception"
  ensure
    return "break" if not direct
  end

by dragonwriter

5/21/2025 at 10:07:15 PM

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.

by vidarh

5/21/2025 at 10:31:59 PM

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.

by chowells

5/22/2025 at 6:11:59 PM

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)

by vidarh

5/22/2025 at 6:52:31 PM

> 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

by dragonwriter

5/22/2025 at 7:25:51 PM

Ah, of course. I'm embarassed I didn't think of that now... That's absolutely better than abusing Fiber. Great suggestion.

by vidarh

5/21/2025 at 11:18:21 PM

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.

by kayodelycaon

5/22/2025 at 6:16:01 PM

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.

by vidarh

5/21/2025 at 10:57:22 PM

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.

by drnewman

5/21/2025 at 11:48:15 PM

> Blocks are one of his true innovatations …

How do they differ from Smallalk blocks? (I don't know.)

https://drcuis.github.io/TheCuisBook/Block-syntax.html

by igouy

5/22/2025 at 1:28:30 AM

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).

by drnewman

5/22/2025 at 1:57:50 AM

> … 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?

https://drcuis.github.io/TheCuisBook/The-Debugger.html

by igouy

5/22/2025 at 2:05:33 AM

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.

by drnewman

5/22/2025 at 2:22:05 AM

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.)

by igouy

5/22/2025 at 5:52:07 PM

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.

by drnewman

5/22/2025 at 3:59:26 AM

I don't believe this is correct. Blocks are closures.

Continuations were introduced in Ruby 1.8, via a callcc method in a Kernel module or something like that.

Since 1.9 they are in some sort of deprecated status.

I don't think that even the yield() stuff is implemented using continuations.

by kazinator

5/22/2025 at 5:28:24 PM

I agree blocks are closures. But they’re call stack aware closures. Which gives them _some_ of the power of continuations.

I’m also sure you’re right that they’re not implemented using continuations. However, my understanding is that Ruby was originally conceived as a language with continuations. I’ll see if I can find a reference for that. But from what I recall reading in a blog post from someone who was at a programming language conference in 1997 when Matz introduced the language that’s how he described it.

by drnewman

5/22/2025 at 6:12:18 PM

> they’re call stack aware closures

What does that even mean?

If you create a block deeply within a cascade of nested function calls, nineteen activation levels deep, and return that block out of all those nestings, is it still aware of the nineteen levels that have terminated, and to what purpose/benefit?

What example Ruby code would break without continuing access to the dynamic scope that has terminated, rather than just the lexical scope?

by kazinator

5/22/2025 at 7:25:18 PM

> If you create a block deeply within a cascade of nested function calls, nineteen activation levels deep, and return that block out of all those nestings, is it still aware of the nineteen levels that have terminated, and to what purpose/benefit?

A block cannot be returned, because it is not an object; it has to be used to create a Proc object with either proc or lambda semantics to be returned.

With lambda semantics, its just a closure, doesn't care about the dynamic scope, and returns immediately to whatever calls it with return/next, or returns from the calling method with break.

With proc semantics, it does retain the connection, and return or break will result in an error when those scopes have terminated, but next will still return to the caller. (You don't generally want to return a proc for that reason, the use for procs is passing down a call chain, not returning them up.)

by dragonwriter

5/23/2025 at 1:26:25 AM

Nice explanation

by drnewman

5/22/2025 at 12:53:45 AM

No, return exiting the enclosing function scope from a block is the special case that's there to resolve an edge case.

Blocks should be nothing special. They're anonymous functions that capture the environment mutably. The only new part is all the special bits added to handle the weird edge cases that they're trying to pretend don't exist.

by chowells

5/22/2025 at 4:52:07 AM

I disagree that it's just there to handle edge cases. It's a useful generalization.

I think the "Building an intuition" section of my blog post[1] makes a good case for that.

  When dealing with loops, you have 3 nested constructs interacting: a wrapping function, a loop statement and the loop's body; and you have 3 keywords to choose where the flow of the code goes.

    return returns from the wrapping function
    break leaves the loop statement
    next / continue leaves loop's body

  When dealing with blocks or anonymous functions, it's instead 3 nested "functions" that are interacting: a wrapping function, a called function and an anonymous functions (or block).

    Ruby's blocks, let you use the same 3 keywords to choose where the flow of the code goes.

    return returns from the wrapping function (ex: my_func)
    break returns from the called function (ex: each, map)
    next returns from the block

  Quite consistent. But since we are talking about functions instead of statements (loop), return values are also involved. Allowing both break and next to provide a return value fits well in that model and is quite useful. The 3 keywords are basically return, but they have different targets.
[1] https://maxlap.dev/blog/2022/02/10/what-makes-ruby-blocks-gr...

by HellzStormer

5/22/2025 at 7:09:47 PM

> No, return exiting the enclosing function scope from a block is the special case that's there to resolve an edge case.

What is the edge case? It seems to be there so that Ruby enumeration methods can provide the behavior expected of looping statements (which is kind of necessary if you want the looping statements to just be semantic sugar for enumeration methods so that they can work correctly with any enumerable.)

> Blocks should be nothing special.

"Special" compared to what?

by dragonwriter

5/22/2025 at 12:02:28 AM

Can you explain further? I'm not sure I follow.

> How do you as a method author opt in to distinguishing between [break and next] after yielding to a block?

I don't use Ruby much lately, but if I yield to a block which calls break, control will pass to the code following the block (and not back to my method). If the block calls next or simply finishes, control passes back to me though I cannot know if next was called or not (but do I care? I can go ahead and yield the next element of a collection either way)

by pmahoney

5/22/2025 at 12:55:26 AM

Ah, you're right. They do have observably different behavior. But I can't even close an open file handle after a break. That's still pretty miserable.

by chowells

5/22/2025 at 7:12:18 PM

> But I can't even close an open file handle after a break.

Yes, you can. That's what ensure (the Ruby equivalent of "finally") is for. Or, better using File.open with a block where you do the work, which uses ensure under the hood.

by dragonwriter

5/22/2025 at 4:26:03 AM

Yes you can, you use a finally clause around your call to yield, as you would already do to handle possible exceptions happening in the block. So no extra consideration is needed.

by HellzStormer

5/21/2025 at 9:27:57 PM

Which is exactly what I don’t like about them. They should’ve used a different keyword or something.

This runs you into problems similar to missing a break in such statements with many language

by jbverschoor

5/22/2025 at 4:32:47 AM

I don't understand what problem you are referring to? A different keyword for what? for the return? for the break?

Care to share an example problem?

by HellzStormer

5/22/2025 at 12:45:25 PM

Return for example would be return_caller or something (naming is hard)

Return in lambda/proc is error-prone

by jbverschoor

5/18/2025 at 10:45:16 PM

Separating out blocks this way is pretty meaningless. Blocks are a syntactical construct that can be optimized a certain way, and is in MRI.

You can obtain a value of a block by naming it, and when you do, what you obtain is a proc.

A Ruby implementation could if it chooses make any block a proc, because you shouldn't be able to tell the difference without extensive contortions (e.g. you could iterate over ObjectSpace and show that a block causes the creation - or not - of an object of the Proc class). And in-fact my (woefully buggy and incomplete) Ruby compiler prototype does just that.

by vidarh

5/21/2025 at 11:47:09 PM

I think this is the important point. Something I've mentioned about blocks in Ruby before: if you’re prototyping a new interpreted language and don’t want to have to build a JIT that can optimize code, but still want some semblance of good performance, the block approach Ruby picks can be pretty useful.

Ruby doesn’t haven to allocate a full-on, garbage-collected, closure object every time a function accepts a block: it only has to do this if the block gets stored to a variable. If the block is only ever yield’d to, the allocation can be skipped.

And when your language’s primary looping mechanism is done with blocks, the difference adds up:

    xs.each do |ys|
      # with normal closures and no fancy JIT,
      # the VM has to allocate a closure once per loop:
      ys.each do |y|
      end
    end
Ruby was able to get away with its closure-heavy standard library APIs without a JIT for almost 3 decades because of the affordances that blocks provide over procs/lambdas.

by jez

5/22/2025 at 1:39:57 AM

That's insightful

by drnewman

5/21/2025 at 7:58:49 PM

> Lamdas check the number of arguments, while procs do not

FWIW, Matz himself called this difference "a design mistake".

by endorphine

5/22/2025 at 1:40:27 AM

Yeah, I think I've heard that too.

by drnewman

5/21/2025 at 11:45:12 PM

Do you have a source for this?

by jez

5/21/2025 at 8:16:53 PM

which way would he have liked to be the "correct way"

by weaksauce

5/22/2025 at 1:44:02 AM

I'd think the preferred way would be to implement them as a separate class. That'd be more in line with the design of other core classes (like TrueClass & FalseClass). It'd also have created the opportunity to create a "Callable" protocol early in the design which would have been great, and would harmonize with the design of Ruby's many other protocols Enumerable, Comparable, Range-compatible values, etc.

by drnewman

5/21/2025 at 8:31:12 PM

What happens when you return from a proc when the enclosing function has already returned?

by kubb

5/21/2025 at 9:03:50 PM

You can't call return from inside a Proc inside a method; you will encounter a LocalJumpError.

by lcnPylGDnU4H9OF

5/21/2025 at 10:12:09 PM

You can call return from inside a proc all you want. You can't do so if you return the proc to an outside scope without getting LocalJumpError.

E.g, given:

    def foo = proc do 42 end
    def bar = proc do return 42 end
Then `foo.call` is fine, but `bar.call` will indeed give a LocalJumpError as you say.

But a return in a proc that hasn't escaped its defining scope is fine:

    def baz = proc do return 42 end.call
Calling `baz` here will just return 42 to the surrounding scope.

by vidarh

5/21/2025 at 9:50:35 PM

This example:

    def foo
      proc do
        return 42
      end.call
    end

    def bar
      proc do
        return 42
      end
    end
    
    p foo
    p bar.call
Will produce:

    42
    test.rb:11:in `block in bar': unexpected return (LocalJumpError)
from test.rb:16:in `<main>'

So in other words, it's handled.

by vidarh

5/21/2025 at 7:06:37 PM

tl;dr

A block is a part of the AST. Like a pair of braces.

A proc is a function.

A lambda is a proc that treats args and returns differently.

by paulddraper

5/21/2025 at 10:16:10 PM

This is the biggest wart on the Ruby language by far.

by whalesalad