Roadmap

go-ruby-stringio/stringio is grown test-first, each capability differential-tested against MRI rather than built in isolation. Ruby's StringIO — the deterministic, interpreter-independent in-memory IO — is complete.

Stage	What	Status
Modes	r / w / a and the r+ / w+ / a+ read-write variants, with w/w+ truncating the seed and append mode writing at the end regardless of the cursor.	Done
Reading	read(n) / read, gets with separator / byte limit / paragraph mode, readline / readlines / each_line, getc / each_char / readchar, getbyte / each_byte / readbyte.	Done
Writing	write / <<, puts / print / printf, putc, and seek-past-end writes that extend and NUL-pad the buffer exactly like MRI.	Done
Positioning	pos / pos= / tell, seek (SEEK_SET / CUR / END), rewind; a negative position raises Errno::EINVAL.	Done
Content & state	string / string=, truncate, size / length, eof?, close / closed?, flush, lineno / lineno=, ungetc / ungetbyte.	Done
Exact MRI raises & coverage	IOError on closed / wrong-mode streams, EOFError on readline/readchar/readbyte at EOF, ArgumentError on a negative read; a StringIO program corpus run by ruby reproduced byte-for-byte. 100% coverage, green across six arches and three OSes.	Done

Documented out-of-scope boundaries

These are deliberate, recorded so the module's surface is unambiguous:

• No interpreter. The library implements the deterministic in-memory IO; it never runs arbitrary Ruby. Wiring $stdout = StringIO.new or routing Kernel#puts through it is the consumer's job — that is why rbgo binds this module rather than the reverse.
• Reference is reference Ruby (MRI). Byte-for-byte conformance targets MRI's StringIO behaviour, pinned by the differential oracle.
• Standalone & reusable. The module has no dependency on the Ruby runtime; the dependency runs the other way.

See Usage & API for the surface and Why pure Go for the deterministic/interpreter split.