From: eregontp@... Date: 2019-09-24T22:14:34+00:00 Subject: [ruby-core:95069] [Ruby master Bug#16178] Numbered parameters: _1 should be the same as |x| and _0 should not exist Issue #16178 has been updated by Eregon (Benoit Daloze). Dan0042 (Daniel DeLorme) wrote: > `proc{ |x,| }.arity` == 1, so `_1` is consistent with that. Which sounds like another bug to me, because that block happily accepts more arguments, and should be identical to `|x,*|`, which `proc {|x,*|}.arity # => -2`. What is your point? My point about arity was that if you add an argument, arity changes, and behavior changes too. Everyone understands adding an extra argument to a block (or lambda) might change semantics (e.g., `[1,[2,3]].map(&-> { _1; _2 })` is ArgumentError), isn't it? That's already the case today with named parameters. How do you argue that `_2` "takes the second parameter" but `_1` "extracts the first element of the first parameter"? How is that consistent? > In order to get the tuples' first value you would need to do `array_of_arrays.map{_2;_1}` because otherwise `_1` would mean the entire tuple. You would need to do `array_of_arrays.map { |x,| x }`. The tiny extra verbosity is warranted for dropping elements. Try adding 1 to each element of a 2 dimensional array (a matrix). `_0` must be used currently, but really taking the element as it is (`|x|`) are the only correct semantics in general if you do not know the specific element types. Why would the correct semantics in general need a different syntax (`_0` and not `_1`)? BTW, about typing, how would you type `enum.map { _1 }` with enum an `Enumerable[T]`? Isn't it impossible, because the behavior is inconsistent at runtime? > This argument is really weird. Is it really so unsufferable to use `_0` instead of `_1`? Yes, it's inconsistent and I'm pretty sure people would use `_1` like `|x|`, without realizing it's just broken when it's passed an Array. Do we want frequent bugs based on this instead of just having to use `|x,|` when wanting to drop elements? ---------------------------------------- Bug #16178: Numbered parameters: _1 should be the same as |x| and _0 should not exist https://bugs.ruby-lang.org/issues/16178#change-81702 * Author: Eregon (Benoit Daloze) * Status: Open * Priority: Normal * Assignee: matz (Yukihiro Matsumoto) * Target version: * ruby -v: ruby 2.7.0dev (2019-09-24T12:57:54Z master 0e84eecc17) [x86_64-linux] * Backport: 2.5: UNKNOWN, 2.6: UNKNOWN ---------------------------------------- Currently on trunk: ```ruby array = ["string", 42, [1, 2]] array.map { |x| x * 2 } # => ["stringstring", 84, [1, 2, 1, 2]] array.map { _1 * 2 } # => ["stringstring", 84, 2] ``` Oops, this trivial code just lost data and completely ignored the element class! This is clearly contrary to intuition and is very dangerous. Using `_0` instead has the correct behavior but it's clear we use 1-based indexing for numbered parameters, and it doesn't solve that `_1` has dangerous behavior. Basically the current behavior is that `_0` is the same as `|x|` and `_1` is the same as `|x,|`. `|x,|` is almost never used in Ruby, and for good reasons, it just throws away data/information/the class of the object. Such a dangerous operation should only be done when it's explicit, and the trailing comma in `|x,|` shows that, but `_1` does not. So let's make `_1` be `|x|` and remove `_0`. I am going to be harsh, but this discussion has gone too long without any serious written argument for the current behavior: I believe it's irrational and irresponsible to have `_1` be `|x,|`, it's just going to lead to nasty bugs. Try to convince me otherwise. If not, in one week I want to apply this change. From the discussion in https://bugs.ruby-lang.org/issues/15723#note-127 and in https://bugs.ruby-lang.org/issues/15708 Some reactions to this behavior in https://twitter.com/eregontp/status/1115318993299083265 -- https://bugs.ruby-lang.org/ Unsubscribe: