[tannevaled]

Closes the templating half of the Sinatra binding (PR #157 landed routing/dispatch and explicitly deferred session-store + template rendering).

What it does

A route/filter action runs against a SinatraCtx (its self). The new erb helper:

1. resolves the template — an inline String or a :symbol naming views/<sym>.erb (read through the File binding; views dir from settings :views, default ./views);
2. compiles it through the already-bound go-ruby-erb compiler (template → Ruby source);
3. evaluates that source in the handler's binding — self = the SinatraCtx, so the action's @ivars and the request helpers (params, request, …) resolve, plus any locals: bound as local variables (positional 3rd arg and options[:locals]) — capturing the _erbout buffer and returning the rendered String, which becomes the Rack response body via sinatraResult.

Trim mode "<>" and eoutvar "_erbout" match Tilt::ERBTemplate's defaults. <%= %> interpolates unescaped, <% %> is control flow.

The eval-in-binding uses the front-end (parser + CompileWithLocals), so it is split open/closed (sinatra_erb_open.go / sinatra_erb_closed.go): a closed-world binary raises NotImplementedError for erb, exactly as Binding#eval / Kernel#eval-on-source already do.

Proof — MRI-identical through the rbgo binary

apps/sinatra_erb_oracle.rb + TestSinatraErbGemOracle dispatch six routes through Sinatra::Base#call(env) and diff byte-for-byte against the real sinatra gem 4.2.1 (MRI 4.0.5): the headline erb "<h1>Hello <%= params['name'] %> (<%= @n %> visits)</h1>" with an @ivar set in a before filter, inline control flow, positional + options[:locals] locals, ERB trim, and a :symbol/file view. All identical.

• GOWORK=off go build ./... && go vet ./... && go test ./... — green
• rbgo real-Ruby conformance — green
• 100% coverage on the new binding code (sinatra_erb*.go); merged coverage gate passes at 100.0%
• prelude_frozen_gen.go untouched (no prelude change)

Flagged follow-ups (not binding gaps)

• Sinatra 4.x renders :erb via Tilt::ErubiTemplate (erubi), whereas go-ruby-erb mirrors MRI's ERB::Compiler. With trim "<>" the two agree byte-for-byte for single-line and idiomatic multiline templates (a code-only <% … %> alone on its line). They diverge only for the unusual line that both opens a code block and ends with an expression tag with trailing content (<% each %>x <%= v %>), where MRI ERB trims the trailing newline and erubi keeps it. Full erubi trim-parity is a go-ruby-erb library follow-up.
• Layouts / partials (stretch).
• :erb escape_html.
• The Sinatra session store (also deferred by PR fix(sinatra): persist handler @ivars across before/route/after + MRI-identity oracle vs sinatra gem 4.2.1 #157).

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