Std\IO

Std\IO — non-blocking console and handle-based byte I/O.

Every operation that can block on a slow device submits through io_uring (on Linux) or the thread pool (for reads), returns a Promise, and auto-awaits at the use site. The only synchronous calls are pure syscalls that never block — isTty, flush.

Trivial programs stay trivial: println "hello" is still one line. The non-blocking machinery is invisible until you put several I/O calls in a let block, at which point the structured-concurrency grouping runs them concurrently.

import println, readLine from Std\IO in
do
println "What is your name?"
case readLine of
Some name -> println "Hello, {name}"
None      -> println "Goodbye."
end
end

Functions

stdinFd : Int = 0

File descriptor numbers, exposed as Int so any Std\File handle call that expects FileHandle can be wrapped — FileHandle stdoutFd builds the Linear-compatible handle — and the raw int is also useful for passing to write without constructing the ADT.

stdoutFd : Int = 1

stderrFd : Int = 2

print : String -> ()

Write s to stdout. Returns a Promise that resolves when the kernel has accepted the write. Non-blocking.

println : String -> ()

Write s followed by a newline to stdout. Non-blocking.

eprint : String -> ()

Write s to stderr. Non-blocking.

eprintln : String -> ()

Write s followed by a newline to stderr. Non-blocking.

putStr : Int -> String -> ()

Write s to an arbitrary fd. Non-blocking.

putStrLn : Int -> String -> ()

Write s followed by a newline to an arbitrary fd. Non-blocking.

write : Int -> String -> ()

write fd s is an alias for putStr fd s. Kept for when you want the “I’m emitting bytes, not printing text” shape at the call site.

readLine : Option String

Read one line from stdin, stripping trailing ‘\n’ (and any ‘\r’). Returns Some line or None at EOF. Non-blocking — the read runs on a thread-pool worker.

readLineFrom : Int -> Option String

Read one line from an arbitrary fd. Same semantics as readLine.

flush : Int -> Bool

Force pending writes on fd to disk / device (fsync). Most io_uring writes are durable on completion so this is rarely needed.

isTty : Int -> Bool

True if fd is attached to a terminal (as opposed to a pipe or file). Useful for turning off colored output or prompting prefixes.

isatty : Int -> Bool

Alias for isTty, following the libc spelling.