erb.cr

Rails-compatible ERB for Crystal, compiled at build time. Templates keep ERB syntax and are compiled into native render methods via a run() template processor (the same trick as the stdlib's ECR) — no runtime parsing, no runtime interpolation.

Extracted from action_view.cr, where parity with the real ActionView 8.1.4 is verified with oracle specs (151 examples, 0 failures).

⚠️ EXPERIMENTAL FRAMEWORK, DO NOT USE IN PRODUCTION YET.

Rails semantics

• <%= expr %> auto-escapes String; a SafeString (the SafeBuffer analog — Crystal's String cannot be subclassed) is appended raw
• <% stmt %> is control code; <%# ... %> comments are dropped
• Erubi trim: a newline directly after a <% %> code tag is removed
• Block helpers work: <%= helper do %> ... <% end %> keeps the whole call as the escaped expression, the block body appends to the shared output buffer (the capture/content_for machinery)
• Layouts use <%= yield %>; partials get typed locals and collection rendering with a 0-based <name>_counter

The expression language inside the tags is Crystal, not Ruby.

Installation

dependencies:
  erb_cr:
    github: OrelSokolov/erb.cr

shards install

Requires Crystal >= 1.21.0; no other dependencies.

Usage

Include ERB::Rendering and compile templates into methods:

require "erb_cr"

class View
  include ERB::Rendering

  getter title : String

  def initialize(@title = "Hello & welcome")
  end

  compile_template "page", "app/views/page.html.erb"
end

View.new.page.to_s # template compiled into #page at build time

Layouts (the compiled method takes a block for <%= yield %>):

compile_layout "application", "app/views/layouts/application.html.erb"

def render_in_layout(content : String) : String
  application { content }.to_s
end

Partials — locals are declared with types at compile time and become real local variables inside the compiled method:

compile_partials({
  "post" => {path: "app/views/_post.html.erb", locals: {"post" => Post}},
})

render("post", {"post" => post})                  # render partial:, locals:
render("post", collection: posts)                 # + post_counter, 0-based
render("post", collection: posts, spacer_template: "gap")

Output safety:

"<b>x</b>".html_safe?          # => false
"<b>x</b>".html_safe           # => ERB::SafeString, appended raw by <%= %>
ERB::SafeString.new("<b>") + " plain" # SafeString#+ keeps the wrapper

Why not stdlib ECR?

ECR does not escape <%= %>, has no safe-string contract, no block helpers, no trim/partials machinery. erb.cr reconstructs the ERB slice of Rails ActionView — including capture/content_for — behind a compile-time pipeline.

License

Part of the rails.cr monorepo experiments; see the repository.