Add Haskell greeter.

examples/haskell/README.md

@@ -9,3 +9,11 @@ $ websocketd --port=8080 --devconsole --passenv PATH ./count.hs
 ```
 
 The passing of `PATH` was required for me because a typical Haskell installation of `runhaskell` does not go into `/usr/bin` but more like `/usr/local/bin`.
+
+### Greeter
+
+The greeter server waits for a line of text to be sent, then sends back a greeting in response, and continues to wait for more lines to come.
+
+```
+$ websocketd --port=8080 --devconsole --passenv PATH ./greeter.hs
+```

examples/haskell/greeter.hs

@@ -0,0 +1,13 @@
+#!/usr/bin/env runhaskell
+
+import Control.Monad (unless)
+import System.IO (stdin, hIsEOF)
+
+-- | For each line FOO received on STDIN, respond with "Hello FOO!".
+main :: IO ()
+main = do
+  eof <- hIsEOF stdin
+  unless eof $ do
+    line <- getLine
+    putStrLn $ "Hello " ++ line ++ "!"
+    main