From: "alexeymuranov (Alexey Muranov)" <redmine@...>
Date: 2013-11-06T23:32:43+09:00
Subject: [ruby-core:58203] [ruby-trunk - Feature #9076] New one-argument	block syntax: &.


Issue #9076 has been updated by alexeymuranov (Alexey Muranov).


asterite (Ary Borenszweig) wrote:
> Did you know that you can't do `&:to_s` wherever you want?
> 
> irb(main):001:0> &:to_s
> SyntaxError: (irb):1: syntax error, unexpected tAMPER
> &:to_s
>  ^

Yes, this is because blocks do not exist as object in ruby, they appear and are evaluated or captured in procs during method calls.  So the result of `&` cannot be stored in a variable, it has to be run or converted to a proc.
 
> When you do `map &:to_s` you are skipping the curly braces.

I am not skipping curly braces, i am just not defining any block, it is obtained by the `&` operator.

> > 
> > The performance penalty of the `&` operator probably can be worked around by compiling the code.
> 
> What do you mean?
> 
> I think this can be done by Ruby, yes: just make `a.map &:to_s` be the same as `a.map { |x| x.to_s }` by the parser... I think they are discussing similar things to do related to frozen strings.

Yes, this is what i mean.

----------------------------------------
Feature #9076: New one-argument block syntax: &.
https://bugs.ruby-lang.org/issues/9076#change-42784

Author: asterite (Ary Borenszweig)
Status: Feedback
Priority: Low
Assignee: 
Category: core
Target version: Next Major


Hello,

I'd like to introduce a new syntax for blocks that have one argument.

Currently you can do this:

[1, 2, 3].map &:to_s

With the proposed syntax this will be written as:

[1, 2, 3].map &.to_s

Instead of ":" we use a ".".

The idea is that this new syntax is just syntax sugar that is expanded by the parser to this:

[1, 2, 3].map { |arg| arg.to_s }

This new syntax allows passing arguments:

[1, 2, 3, 4].map &.to_s(2) #=> ["1", "10", "11", "100"]

It also allows chaining calls:

[1, 10, 100].map &.to_s.length #=> [1, 2, 3]

You can also use another block:

[[1, -2], [-3, -4]].map &.map &.abs #=> [[1, 2], [3, 4]]

Pros:
- Doesn't conflict with any existing syntax, because that now gives a syntax error, so it is available.
- Allows passing arguments and chaining calls
- It's *fast*: it's just syntax sugar. The "&:to_s" is slower because the to_proc method is invoked, you have a cache of procs, etc.
- It looks ok (in my opinion) and allows very nice functional code (like the last example).

Cons:
- Only supports one (implicit) argument. But this is the same limitation of "&:to_s". If you want more than one argument, use the traditional block syntax.
- It's a new syntax, so users need to learn it. But to defend this point, users right now need to understand the &:to_s syntax, which is hard to explain (this calls the "to_proc" method of Symbol, which creates a block... vs. "it's just syntax sugar for")

What do you think?

We are using this syntax in a new language we are doing, Crystal, which has a syntax very similar to Ruby, and so far we think it's nice, simple and powerful. You can read more about it here: http://crystal-lang.org/2013/09/15/to-proc.html


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