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| -rw-r--r-- | README.md | 2 | ||||
| -rw-r--r-- | ch04/4_a_3.hs | 48 |
2 files changed, 49 insertions, 1 deletions
@@ -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 |