StdIn

The StdIn module provides functions for reading from standard input. This is typically used for interactive programs, command-line tools that accept piped input, and scripts that need user input.

Nullary Functions

All Stdin functions are nullary (zero-argument) and are automatically invoked when referenced. You don't need to call them with (). For example, use StdIn.read-line instead of StdIn.read-line().

Reading Input

StdIn.read-line

Result String IOError

Reads a single line from standard input, returning it as a string without the trailing newline character.

match StdIn.read-line
  | Ok line -> println "You entered: ${line}"
  | Err e -> println "Error reading input: ${e}"

StdIn.read-all

Result String IOError

Reads all content from standard input until EOF (end-of-file), returning the entire contents as a single string.

# Read piped input: echo "hello world" | ./my-program
match StdIn.read-all
  | Ok content -> println "Received: ${content}"
  | Err e -> println "Error: ${e}"

StdIn.read-bytes

Result (List Int) IOError

Reads all content from standard input as raw bytes (integers 0-255). Useful for processing binary data piped to your program.

match StdIn.read-bytes
  | Ok bytes -> println "Read ${length bytes} bytes"
  | Err e -> println "Error: ${e}"

Usage Examples

Interactive Prompt

Create a simple interactive prompt that reads user input:

prompt = fn(message) =>
  print message
  match StdIn.read-line
    | Ok input -> input
    | Err _ -> ""

name = prompt "Enter your name: "
println "Hello, ${name}!"

Reading Multiple Lines

Read lines until the user enters an empty line:

read-lines = fn(acc) =>
  match StdIn.read-line
    | Ok "" -> acc
    | Ok line -> read-lines (acc ++ [line])
    | Err _ -> acc

println "Enter lines (empty line to finish):"
lines = read-lines []
println "You entered ${length lines} lines"

Processing Piped Input

Process input piped from another command:

# Usage: cat data.txt | ./my-program
match StdIn.read-all
  | Ok content ->
      lines = String.split content "\n"
      non-empty = lines |> filter (fn(l) => String.length l > 0)
      println "Processed ${length non-empty} lines"
  | Err e ->
      Stderr.write-line "Failed to read input: ${e}"

Yes/No Confirmation

Ask the user for confirmation:

confirm = fn(question) =>
  print "${question} [y/n]: "
  match StdIn.read-line
    | Ok "y" -> true
    | Ok "Y" -> true
    | Ok "yes" -> true
    | _ -> false

if confirm "Delete file?" then
  println "Deleting..."
else
  println "Cancelled"

Password Input

Read sensitive input (note: input will still be visible):

print "Enter password: "
match StdIn.read-line
  | Ok password ->
      if String.length password < 8 then
        println "Password too short"
      else
        println "Password accepted"
  | Err _ ->
      println "Failed to read password"