Input / Output (I/O)

Why This Matters

I/O is the most common runtime boundary. Clear behavior here prevents silent data loss and confusing failures.

Console I/O

Import:

import std.io.*;

print(...): None

Writes scalar value(s) without trailing newline.

println(...): None

Writes scalar value(s) with trailing newline.

Current display support includes:

• Integer, Float, Boolean, String, Char, None
• Option<T> and Result<T, E> when payload types are display-compatible

read_line(): String

Reads one line from stdin.

import std.io.*;

function main(): None {
    print("Enter name: ");
    name: String = read_line();
    println("Hello, " + name);
    return None;
}

Display with Option/Result payloads:

import std.io.*;

function main(): None {
    ok: Result<Integer, String> = Result.ok(7);
    maybe: Option<String> = Option.some("hello");
    println("ok={ok}, maybe={maybe}");
    return None;
}

File APIs (File.*)

Import:

import std.fs.*;

File.read(path: String): String

Reads full text file. Missing/invalid input fails explicitly.

File.write(path: String, content: String): Boolean

Writes full content (overwrite semantics). Returns true on successful write+close.

File.exists(path: String): Boolean

Checks for readable regular file.

File.delete(path: String): Boolean

Deletes regular file.

import std.fs.*;
import std.io.*;

function main(): None {
    if (File.exists("notes.txt")) {
        text: String = File.read("notes.txt");
        println(text);
    }
    return None;
}

Behavioral Notes

• invalid UTF-8 and NUL-containing file contents are rejected
• non-seekable/unsupported paths fail with explicit runtime error
• write/delete return Boolean; check return values in automation code

Effect Note

I/O functions participate in effect checks and require io capability. See Effects.

Common Mistakes

• assuming File.write appends (it overwrites)
• ignoring File.write/File.delete return values
• using unvalidated user-controlled paths directly