Feature #6869
openDo not treat `_` parameter exceptionally
Description
I started by commenting on #6693, but i have realized that this is a slightly different request.
I propose to not treat the variable name "_
" exceptionally. Current behavior:
{0=>1}.each_with_index { |_,_| p _ } # [0, 1]
prints "[0, 1]", but
{1=>2}.each_with_index { |x,x| p x } # SyntaxError: (eval):2: duplicated argument name
raises "SyntaxError: (eval):2: duplicated argument name".
Similarly for methods:
def f(_, _)
_
end
f(0, 1) # => 0
def f(x, x)
x
end # => SyntaxError: (eval):2: duplicated argument name
Observe also that the use of repeated _
parameter is not consistent between methods and blocks: for methods the value is the first assigned value, and for blocks it is the array of all the assigned values.
- I propose to use the same rule for all variables, without distinguishing
_
specially.
In particular i propose to allow to repeat any variable, not only _
, in block or method arguments without raising an error.
There may be several solutions what the repeated argument will hold: it may hold the array of all assigned values, the first assigned value, the last assigned value, the first non-nil assigned value, or the last non-nil assigned value.
-
I propose to treat repeated arguments in methods and in blocks the same way (do not know which one).
-
For unused variables i propose to introduce a special placeholder, for example "
-
" not followed by anything other than a delimiter (comma or bracket):
each_with_index { |-, value| puts value }
-, -, suffix = parse(name)
Updated by drbrain (Eric Hodel) about 12 years ago
- Category set to core
- Assignee set to matz (Yukihiro Matsumoto)
Seems to be part of variable shadowing checks. The check was added before r8857 (which was a refactor of the feature) and checking for '_' was removed in r14186.
Since it was committed by matz I think your chances at acceptance are low.
Updated by marcandre (Marc-Andre Lafortune) about 12 years ago
Hi,
alexeymuranov (Alexey Muranov) wrote:
I propose to not treat the variable name "
_
" exceptionally.
Sorry for the naive question, but why? What are you trying to achieve? What real world problem do you want to fix?
- For unused variables i propose to introduce a special placeholder
I feel that unused variables do not warrant a change to the already complex Ruby syntax.
Updated by alexeymuranov (Alexey Muranov) about 12 years ago
marcandre (Marc-Andre Lafortune) wrote:
Hi,
alexeymuranov (Alexey Muranov) wrote:
I propose to not treat the variable name "
_
" exceptionally.Sorry for the naive question, but why? What are you trying to achieve? What real world problem do you want to fix?
I do not like exceptions. When i was first learning Ruby, i thought that the underscore is a letter like any other, but sometimes it behaves like any other, and sometimes not.
It also seems to me more natural to use a placeholder for a discarded value than to assign it to a variable first and then discard.
- For unused variables i propose to introduce a special placeholder
I feel that unused variables do not warrant a change to the already complex Ruby syntax.
In my opinion, treating variables differently based on their names is also a part of syntax, and in my opinion such rules are harder to follow than a rule for a single placeholder. As there is no dedicated placeholder in Ruby now, this one may be adapted later to other situations as well, i think.
Update: The most important real world problem this addresses is reading the code! With a placeholder, it is immediately clear that the value is discarded, but with a special variable one needs to look through the code to be sure it is not used somewhere.
Plus one needs to remember currently what a repeated variable is holding in different situations.
Updated by alexeymuranov (Alexey Muranov) over 10 years ago
It looks like the use of the underscore _
as a "placeholder" is quite common in other languages ("black hole" register in Vim, "whatever" pattern that matches everything in Haskell), but there it is really a placeholder and not a variable: values "assigned" to _
cannot be retrieved.
With this in view, maybe, instead of this my proposal, the underscore can be "downgraded" to a "placeholder" (or "black hole" peudo-variable)?
Updated by nobu (Nobuyoshi Nakada) over 10 years ago
- Description updated (diff)
Alexey Muranov wrote:
Observe also that the use of repeated
_
parameter is not consistent between methods and blocks: for methods the value is the first assigned value, and for blocks it is the array of all the assigned values.
It is unrelated to _
, but because of Enumerable#each_with_index
.
Try:
{0=>1}.each_with_index {|x,y| p x} # [0, 1]
Alexey Muranov wrote:
It looks like the use of the underscore
_
as a "placeholder" is quite common in other languages ("black hole" register in Vim, "whatever" pattern that matches everything in Haskell), but there it is really a placeholder and not a variable: values "assigned" to_
cannot be retrieved.
Isn't it more exceptional?
Updated by alexeymuranov (Alexey Muranov) over 10 years ago
Nobuyoshi Nakada wrote:
Alexey Muranov wrote:
Observe also that the use of repeated
_
parameter is not consistent between methods and blocks: for methods the value is the first assigned value, and for blocks it is the array of all the assigned values.It is unrelated to
_
, but because ofEnumerable#each_with_index
.
Try:{0=>1}.each_with_index {|x,y| p x} # [0, 1]
Thanks, i do not know what i was thinking.
Alexey Muranov wrote:
It looks like the use of the underscore
_
as a "placeholder" is quite common in other languages ("black hole" register in Vim, "whatever" pattern that matches everything in Haskell), but there it is really a placeholder and not a variable: values "assigned" to_
cannot be retrieved.Isn't it more exceptional?
Yes, so this proposal would need to be closed, and i would need to open a new one. When i opened this one, i did not know that the underscore was a common "placeholder" in other languages and i thought that Ruby documentation presents the underscore in identifiers roughly as equivalent to a lowercase letter (doesn't it?).
Here is a sentence from the online version of Programming Ruby:
In these descriptions, lowercase letter means the characters ''a'' though ''z'', as well as ''_'', the underscore.
In any case, in Ruby the following works perfectly, and in my opinion this all is confusing:
_ = 1
p _
So, yes, my new proposal would be to downgrade the underscore to a placeholder, so that in something like this
foo do |_,x|
# 10 lines of code
end
or
_, _, suffix = parse something
it would be immediately clear the values "assigned" to _
are discarded.
Updated by docx (Lukas Dolezal) over 4 years ago
Hi. This is interesting and I can see that the inconsistent treatment of _
can be confusing (I never noticed tho because I never tried to access _
).
I wonder however, I always thought that _
is exactly explicitly part of syntax as "unused parameter". Am I wrong?
So if that is the case, what about going the other direction to remove the inconsistency of what value it takes, and just make it explicitly "unusable" - can we raise SyntaxError
when any _
is being accessed inside of method/block?
Updated by docx (Lukas Dolezal) over 4 years ago
Re
In these descriptions, lowercase letter means the characters ''a'' though ''z'', as well as ''_'', the underscore.
I think what they mean here is that you can use it inside of a variable. Probably just did not realized the single underscore case :) But that is my interpretation.
Updated by hsbt (Hiroshi SHIBATA) 7 months ago
- Status changed from Open to Assigned