Papercraft update: IRB Support, Bug Fixes, More Speed

12·10·2025

This week I was away on a little trip to Paris to attend a Paris.rb meetup and meet some friends, so I was less productive, but still got some stuff done, and still managed to do some work on Papercraft. Here’s what’s changed:

Using Papercraft in IRB

Up until now, perhaps the biggest limitation of Papercraft was that you couldn’t use it in an IRB session. That was because Papercraft always compiles your templates, and for that it needs access to the templates’ source code. But if you’re defining a template in IRB, where is that source code?

Then, while taking the train to Paris, it occurred to me that maybe IRB keeps the lines of code you input into it somewhere, and maybe it would be possible to access those lines of code. It took a bit of digging, but finally I’ve found it.

As a result, you can now define ad-hoc Papercraft templates right in your IRB session:

sharon@nf1:~$ irb -rpapercraft
irb(main):001> ->{ h1 "Hello, IRB!" }.render
=> "<h1>Hello, IRB!</h1>"
irb(main):002>

It’s fun to be able to explore Papercraft templates in IRB, enjoy!

Some Bug Fixes

I’ve also had the time to deal with some edge cases I discovered while using Papercraft:

Fix compilation of an empty template:
```
-> {}.render #=> ""
```

Raise error on a void element with child nodes or inner text:

-> { input 'foo' }.render #=> !!! Papercraft::Error
-> { hr { span } }.render #=> !!! Papercraft::Error

Fix apply parameter handling when called with a block:

a = proc { |*a, **b| body { render_yield(*a, **b) } }
b = a.apply(:foo, p: 42) { |*c, **d| article { render_yield(*c, **d) } }
b.render(:bar, q: 43) { |*a, **b|
  h1 a.first, class: b[:p]
  h2 a.last, class: b[:q]
}
#=> "<body><article><h1 class=\"42\">foo</h1><h2 class=\"43\">bar</h2></article></body>"

HTML Escaping Gets a Speed Boost

Last but not least, I was looking at the source code to ERB::Escape.html_escape, which is the method Papercraft uses to escape all HTML content (in order to prevent HTML injection). I figured the implementation could be improved, and opened a PR with the proposed change:

The existing html_escape implementation always allocates buffer space (6 times the length of the input string), even when the input string does not contain any character that needs to be escaped.

This PR modifies the implementation of optimized_escape_html to not pre-allocate an output buffer, but instead allocate it on the first occurence of a character that needs escaping. In addition, instead of copying non-escaped characters one by one to the output buffer, continuous non-escaped segments of characters are copied using memcpy.

A synthetic benchmark employing the input strings used in the test_html_escape method in test/test_erb.rb shows the modified implementation to be about 35% faster than the original…

The PR was merged and a new version of ERB is already released. And the best part - this speed up is available to the entire Ruby ecosystem, not just Papercraft. Everybody that uses ERB (or its html_escape method) will benefit from this!