shake: refactor; use only the first argument as build target

Shake.hs

@@ -116,6 +116,8 @@ towebmd = "-t markdown-smart-fenced_divs-fenced_code_attributes-simple_tables-mu
 
 main = do
 
+  -- 1. gather some IO values used by rules
+
   -- hledger manual also includes the markdown files from here:
   let commandsdir = "hledger/Hledger/Cli/Commands"
   commandmds <-
@@ -126,13 +128,14 @@ main = do
   let sitedir = "site"
   pages <- map takeBaseName . filter (".md" `isSuffixOf`) <$> S.getDirectoryContents sitedir
 
-  shakeArgs
-    shakeOptions
-      {
-       shakeVerbosity=Quiet
-      -- ,shakeReport=[".shake.html"]
-      }
-      $ do
+  args <- getArgs
+  let args2 = drop 1 args
+
+  -- 2. define the shake rules
+
+  let
+    rules :: Rules ()
+    rules = do
 
       want ["help"]
 
@@ -678,6 +681,24 @@ main = do
         putNormal "Cleaning shake build cache"
         removeFilesAfter ".shake" ["//*"]
 
+  -- 3. run the shake rule selected by the first command line argument,
+  -- leaving the other args for the rule to use
+
+  let
+    opts = shakeOptions{
+       shakeVerbosity=Quiet
+      -- ,shakeReport=[".shake.html"]
+      }
+
+    runWithArgs :: Rules () -> [a] -> [String] -> IO (Maybe (Rules ()))
+    runWithArgs rules _flags args = pure $ Just $
+      case args of
+        []    -> rules
+        (a:_) -> want [a] >> withoutActions rules
+
+  -- shakeArgsWith :: ShakeOptions -> [OptDescr (Either String a)] -> ([a] -> [String] -> IO (Maybe (Rules ()))) -> IO () 
+  shakeArgsWith opts [] (runWithArgs rules)
+
 
 -- Convert numbered man page names to manual names.
 -- hledger.1 -> hledger, hledger_journal.5 -> hledger_journal