4_a_3: Add solution

2 files changed, 49 insertions, 1 deletions

diff --git a/README.md b/README.md
index 0cc741e..269e404 100644
--- a/README.md
+++ b/README.md
@@ -70,7 +70,7 @@ more visible in the list the first exercise of a group is in bold italics.
 | 3_b_12         | yes, in 3_b_9 |      | |
 | **_4_a_1_**    | yes    | 84   | 4. Functional programming |
 | 4_a_2          | yes    |      | |
-| 4_a_3          |        |      | |
+| 4_a_3          | yes    |      | |
 | 4_a_4          |        |      | |
 | **_4_b_1_**    |        | 97   | |
 | 4_b_2          |        | 98   | |
diff --git a/ch04/4_a_3.hs b/ch04/4_a_3.hs
new file mode 100644
index 0000000..c088ddc
--- /dev/null
+++ b/ch04/4_a_3.hs
@@ -0,0 +1,48 @@
+-- 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