IO & the World

Milang uses a capability-based IO model. Side effects are not global — they flow through an explicit world record that the runtime passes to main. If a function never receives world (or a sub-record of it), it cannot perform IO.

Hello World

main world =
  world.io.println "Hello, world!"

Hello, world!

main is the program entry point. It receives world and its return value becomes the process exit code.

The World Record

world is a record containing sub-records for different capabilities:

Console IO

world.io.println msg          -- print with trailing newline
world.io.print msg            -- print without newline
line = world.io.readLine      -- read one line from stdin

File IO

File operations are split by capability: world.fs.read for reading and world.fs.write for writing. This enables fine-grained trait annotations.

content = world.fs.read.file "data.txt"
world.fs.write.file "out.txt" content
world.fs.write.append "log.txt" "new line\n"
exists = world.fs.read.exists "data.txt"     -- returns 1 or 0
world.fs.write.remove "tmp.txt"

Process

output = world.process.exec "ls -la"      -- run shell command, return output
world.process.exit 1                       -- exit immediately with status code

Command-Line Arguments and Environment

world.argv is an inert list — it does not perform IO, so it is always available:

main world =
  world.io.println (len world.argv)

world.getEnv reads an environment variable by name:

home = world.getEnv "HOME"

Capability Restriction

Because capabilities are just record fields, you can restrict what a helper function can do by passing only the sub-record it needs:

greet io = io.println "hello from restricted IO"

main world =
  greet world.io

hello from restricted IO

greet receives world.io and can print, but it structurally cannot access world.process — there is no way for it to execute shell commands or exit the process.

Exit Code

The return value of main is used as the process exit code. An integer is used directly; any non-integer value (record, string, etc.) defaults to exit code 0.

main world =
  world.io.println "exiting with code 0"

exiting with code 0

Script Mode

When a file has no main binding that takes a parameter, milang runs in script mode: every top-level binding is evaluated and printed.

x = 6 * 7
y = x + 1
greeting = "hello"

build =  {target = c, os = linux, arch = x86_64}
x = 42
y = 43
greeting = hello

This is useful for quick calculations and exploring the language without writing a full main function.

Auto-Monad Spine

You do not need monads or do-notation in milang. The compiler automatically tracks which expressions are world-tainted (they transitively reference world). Impure expressions are guaranteed to execute in the order they appear in the source. Pure expressions can float freely, opening the door for future optimizations. The result is imperative-looking code that is safe and predictable.