---
title: "Software Tools in Haskell: echo"
subtitle: write arguments to stdout
date: 2016-02-19
author: nbloomf
tags: software-tools-in-haskell, literate-haskell
---

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``.