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Phase 18. File I/O, JSON, CSV, HTTP

FieldValue
MEPMEP-56 §Phases
StatusLANDED
Started2026-05-29 12:01 (GMT+7)
Landed2026-05-29 12:01 (GMT+7)
Tracking issuenone
Tracking PR#22510
Commit3890f4bd77

Gate​

TestPhase18IO in transpiler3/ruby/build/phase18_test.go: seven subtests (read_file, write_file, append_file, lines, json_decode, load_csv, http_get). Each subtest seeds a temp data file (or stands up a local httptest.Server), compiles a Mochi program that exercises one I/O builtin against the seeded path, runs the emitted .rb under the resolved Ruby toolchain with -I mochi-runtime/lib, and diffs stdout against a recorded expectation (e.g. load_csv reads a,b,c\n1,2,3\n and prints a\n3\n). The http_get subtest uses a real httptest.NewServer so the lowered Ruby is exercised end-to-end against an actual HTTP socket, not a mock.

Lowering decisions​

File I/O lowers to direct File class methods, JSON / CSV / HTTP lower to inline require ...; ... expressions so the relevant stdlib is loaded lazily only when the program actually uses it (transpiler3/ruby/lower/lower.go lines 287 to 317 and 1054 to 1083):

  • aotir.WriteFileStmt to RawStmt rendering File.write(path, content) (lines 287 to 295). File.write opens, writes, and closes in one call, matching Mochi's atomic-write semantics.
  • aotir.AppendFileStmt to RawStmt rendering File.open(path, 'a') { |__f| __f.write(content) } (lines 297 to 305). The block form auto-closes the handle on exit; 'a' is the append mode flag.
  • aotir.SaveCSVStmt to RawStmt rendering (require 'csv'; CSV.open(path, 'w') { |__c| (data).each { |__row| __c << __row } }) (lines 306 to 317); covered more deeply by Phase 21.
  • aotir.ReadFileExpr to RawExpr rendering File.read(path) (lines 1054 to 1059). Returns the file's full content as a single Ruby String.
  • aotir.LinesExpr to RawExpr rendering File.readlines(path, chomp: true) (lines 1060 to 1065). The chomp: true keyword (Ruby 2.4+) strips trailing newlines so each list element is the bare line, matching Mochi's lines() contract.
  • aotir.HttpGetExpr to RawExpr rendering (require 'open-uri'; URI.parse(url).open.read) (lines 1066 to 1071). open-uri patches URI so URI#open follows HTTP redirects and returns an IO; .read then drains the body. This is simpler than Net::HTTP.get for plain GET requests and handles https URLs out of the box via the patched URI.
  • aotir.LoadCSVExpr to RawExpr rendering (require 'csv'; CSV.read(path)) (lines 1072 to 1077). CSV.read returns Array<Array<String>>, matching Mochi's list<list<string>> from loadCSV.
  • aotir.JsonDecodeExpr to RawExpr rendering (require 'json'; JSON.parse(input)) (lines 1078 to 1083). Returns Ruby Hash/Array/scalar trees aligned with Mochi's any-typed JSON decode result.

The inline (require 'x'; expr) pattern is deliberate: it pushes the require cost to the first use of each subsystem at runtime, keeping the static prelude minimal (require "mochi/runtime" only). Ruby's require is idempotent so a hot loop pays the cost once.

Files changed​

FilePurpose
transpiler3/ruby/lower/lower.goFile I/O statements (lines 287 to 317) and ReadFile/Lines/HttpGet/LoadCSV/JsonDecode expressions (lines 1054 to 1083) lower to Ruby stdlib calls
transpiler3/ruby/build/phase18_test.goTestPhase18IO with 7 subtests including a live httptest.Server for the HTTP path

Test set​

  • TestPhase18IO/read_file, write_file, append_file, lines, json_decode, load_csv, http_get.

Closeout notes​

Phase 18 landed on CRuby 4.0 (Homebrew) with all seven subtests green. open-uri (not Net::HTTP.get) is the deliberate choice for fetch URL: open-uri handles redirects, HTTPS, and basic auth headers without extra glue and is in the stdlib on every supported runtime. chomp: true in File.readlines is the one place where Mochi diverges from Ruby's default (which keeps trailing \n); without it, lines("first\nsecond\n") would return ["first\n", "second\n"] instead of the expected ["first", "second"].