Software Tools in Haskell: echo

write arguments to stdout

This page is part of a series on Software Tools in Haskell.

This post is literate Haskell; you can load the source into GHCi and play along.

As usual, we start with some imports.

-- sth-echo: write arguments to stdout
module Main where

import System.Exit (exitSuccess)
import System.Environment (getArgs)

All the programs we’ve written so far are strictly filters: they read data from stdin and write data to stdout. The metaphor here is that small programs are chained together in a larger “pipeline”, and data flows from one end to the other; along the way, each filter changes the data in some way. By reading and writing from stdin and stdout, individual programs do not need to worry about where their data comes from and goes.

echo is the first program we’ve written that produces data without needing to take any from stdin; it is a source. (The converse, a program which consumes data without producing any, is a sink). echo simply takes a list of arguments at the command line and writes them to stdout. Although this is simple, like copy, we have to decide how to interpret the arguments to echo. By default, echo will treat its arguments as lines and write one per line. We use the --char argument to write raw characters.

main :: IO ()
main = do
  args <- getArgs

  case args of
    ("--char":xs) -> putStr (concat xs)
    xs            -> sequence_ $ map putStrLn xs

  exitSuccess

We can now use echo to test our other programs. For instance, using the shell to run

sth-echo "hello" | sth-count --char

prints 6 to the terminal (including the trailing newline). Woo! But if we run

sth-echo --char "hello" | sth-count --char

we get 5.