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+-- Using the command framework from the earlier section "A simple command line
+-- framework" on page 71, write a program that prints the first word of each
+-- line of its input.
+
+{-- From examples/examples/ch04/InteractWith.hs --}
+import System.Environment (getArgs)
+
+interactWith function inputFile outputFile = do
+  input <- readFile inputFile
+  writeFile outputFile (function input)
+
+main = mainWith myFunction
+  where mainWith function = do
+          args <- getArgs
+          case args of
+            [input,output] -> interactWith function input output
+            _ -> putStrLn "error: exactly two arguments needed"
+
+        -- replace "id" with the name of our function below
+        myFunction = firstWords
+{-- End of code from examples --}
+
+firstWords' :: [String] -> [String]
+firstWords' [] = []
+firstWords' (x:xs) = first:(firstWords' xs)
+  where lineWords = words x
+        first = if not (null lineWords)
+                then head lineWords
+                else ""
+
+firstWords input = unlines (firstWords' (lines input))
+
+-- $ stack ghc 4_a_3.hs
+-- [1 of 1] Compiling Main             ( 4_a_3.hs, 4_a_3.o )
+-- Linking 4_a_3 ...
+-- $ cat input.txt
+-- ab cde fghi
+--
+--   j   k
+--
+-- lm
+-- $ ./4_a_3 input.txt output.txt
+-- $ cat output.txt
+-- ab
+--
+-- j
+--
+-- lm