[ruby-core:86484] [Ruby trunk Feature#12839] CSV - Give not nil but empty strings for empty fields

Issue #12839 has been updated by kou (Kouhei Sutou).


I added `:nil_value` option as a shortcut:

```ruby
require "csv"

p CSV.parse(',"",a')                # => [[nil, "", "a"]]
p CSV.parse(',"",a', nil_value: "") # => [["", "", "a"]]
```

But it's not so fast:

```ruby
require "csv"

require "benchmark/ips"

csv_text = <<CSV
foo,bar,,baz
hoge,,temo,
roo,goo,por,kosh
CSV

convert_nil = ->(s) {s || ""}

Benchmark.ips do |r|
  r.report "not convert" do
    CSV.parse(csv_text)
  end

  r.report "converter" do
    CSV.parse(csv_text, converters: convert_nil)
  end

  r.report "option" do
    CSV.parse(csv_text, nil_value: "")
  end

  r.compare!
end
```

```text
Warming up --------------------------------------
         not convert   742.000  i/100ms
           converter   620.000  i/100ms
              option   672.000  i/100ms
Calculating -------------------------------------
         not convert      7.480k (賊 1.8%) i/s -     37.842k in   5.061095s
           converter      6.289k (賊 0.5%) i/s -     31.620k in   5.028042s
              option      6.697k (賊 3.7%) i/s -     33.600k in   5.025273s

Comparison:
         not convert:     7479.8 i/s
              option:     6696.8 i/s - 1.12x  slower
           converter:     6288.9 i/s - 1.19x  slower

```

----------------------------------------
Feature #12839: CSV - Give not nil but empty strings for empty fields
https://bugs.ruby-lang.org/issues/12839#change-71431

* Author: 5.5 (5 5)
* Status: Closed
* Priority: Normal
* Assignee: kou (Kouhei Sutou)
* Target version: 
----------------------------------------
The CSV parser gives nil for empty fields.

```rb
require "csv"

CSV.parse(%|,""|) #=> [[nil, ""]]
```

The above behavior maybe be suitable for certain programmers, but I hope to get `[["", ""]]`.

So I had used to write the following code reluctantly till Ruby 2.1:

```rb
require "csv"

CSV.parse(%|,""|, converters: lambda{|v| v || ""})
#=> [["", ""]]
```

It is wasteful, but certainly works for my purpose.

However, because of #11126, the above code does not work from Ruby 2.2.
(Converters are not called for nil)

I merely want an option, which makes the CSV parser give empty strings for empty fields.

Namely,

```rb
require "csv"

CSV.parse(%|,""|, string: true) #=> [["", ""]]
```




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