IO

The IO effect is the most practical effect to teach because it appears in simple programs, shell-facing examples, and compiler onboarding. It is also the easiest place to show what Sounio means by “effects belong in the signature.”

Current contract

• Console output belongs under with IO in public examples.
• Calling an IO-using helper from a pure function should be documented as a type or effect boundary mistake, not as harmless style.
• The checked artifact and the docs both treat IO as the most obvious example of explicit effect tracking.

IO boundary

fn read_name() -> string with IO {
    read_line()
}

fn main() with IO {
    println("Enter your name:")
    let name = read_name()
    println(name)
}

Repo map

• self-hosted/io/stdio.sio, self-hosted/io/file_read.sio, and self-hosted/io/file_write.sio are the obvious places to inspect host-facing I/O work in the self-hosted tree.
• self-hosted/check/effects.sio is where the checker should make sure those operations do not appear in pure contexts by accident.
• examples/io/ and small fixtures under examples/ are the best places to look for current user-facing examples.

Guidance

• Put IO at the program boundary and keep the rest of the pipeline pure where possible.
• If a docs example reads from the console or filesystem, show the effect clause explicitly instead of implying it.
• Use check as the first proof that the boundary is declared correctly.