Fix aff main example in chapter 9

Author: Zelenya

exercises/chapter9/test/Copy.purs

@@ -3,23 +3,27 @@ module Test.Copy where
 -- ANCHOR: copyFile
 import Prelude
 import Data.Either (Either(..))
-import Effect.Aff (Aff, attempt, message)
+import Effect.Aff (Aff, attempt, message, launchAff_)
+import Effect (Effect)
 import Effect.Class.Console (log)
 import Node.Encoding (Encoding(..))
 import Node.FS.Aff (readTextFile, writeTextFile)
 import Node.Path (FilePath)
 
-copyFile :: FilePath -> FilePath -> Aff Unit
-copyFile file1 file2 = do
-  my_data <- readTextFile UTF8 file1
-  writeTextFile UTF8 file2 my_data
+main :: Effect Unit
+main = launchAff_ program
 
-main :: Aff Unit
-main = do
+program :: Aff Unit
+program = do
   result <- attempt $ copyFile "file1.txt" "file2.txt"
   case result of
     Left e -> log $ "There was a problem with copyFile: " <> message e
     _ -> pure unit
+
+copyFile :: FilePath -> FilePath -> Aff Unit
+copyFile file1 file2 = do
+  my_data <- readTextFile UTF8 file1
+  writeTextFile UTF8 file2 my_data
 -- ANCHOR_END: copyFile
 
 -- Main is unused, and is only linked to in text

text/chapter9.md

@@ -52,6 +52,8 @@ The `Aff` monad in PureScript offers similar ergonomics of JavaScript's `async`/
 {{#include ../exercises/chapter9/test/Copy.purs:copyFile}}
 ```
 
+Note that we have to use `launchAff_` to convert the `Aff` to `Effect` because `main` must be `Effect Unit`.
+
 It is also possible to re-write the above snippet using callbacks or synchronous functions (for example with `Node.FS.Async` and `Node.FS.Sync` respectively), but those share the same downsides as discussed earlier with JavaScript, and so that coding style is not recommended.
 
 The syntax for working with `Aff` is very similar to working with `Effect`. They are both monads, and can therefore be written with do notation.