Remove unnecessary uses of head

src/ShellCheck/Analytics.hs

@@ -2293,7 +2293,7 @@ checkWhileReadPitfalls _ (T_WhileExpression id [command] contents)
 
     isStdinReadCommand (T_Pipeline _ _ [T_Redirecting id redirs cmd]) =
         let plaintext = oversimplify cmd
-        in head (plaintext ++ [""]) == "read"
+        in headOrDefault "" plaintext == "read"
             && ("-u" `notElem` plaintext)
             && all (not . stdinRedirect) redirs
     isStdinReadCommand _ = False

src/ShellCheck/AnalyzerLib.hs

@@ -255,7 +255,7 @@ determineShell fallbackShell t = fromMaybe Bash $
 executableFromShebang :: String -> String
 executableFromShebang = shellFor
   where
-    shellFor s | "/env " `isInfixOf` s = head (drop 1 (words s)++[""])
+    shellFor s | "/env " `isInfixOf` s = headOrDefault "" (drop 1 $ words s)
     shellFor s | ' ' `elem` s = shellFor $ takeWhile (/= ' ') s
     shellFor s = reverse . takeWhile (/= '/') . reverse $ s
 
@@ -295,7 +295,7 @@ isQuoteFree = isQuoteFreeNode False
 
 isQuoteFreeNode strict tree t =
     (isQuoteFreeElement t == Just True) ||
-        head (mapMaybe isQuoteFreeContext (drop 1 $ getPath tree t) ++ [False])
+        headOrDefault False (mapMaybe isQuoteFreeContext (drop 1 $ getPath tree t))
   where
     -- Is this node self-quoting in itself?
     isQuoteFreeElement t =
@@ -758,9 +758,8 @@ getReferencedVariables parents t =
         _          -> Nothing
 
     getIfReference context token = maybeToList $ do
-            str <- getLiteralStringExt literalizer token
+            str@(h:_) <- getLiteralStringExt literalizer token
-            guard . not $ null str
+            when (isDigit h) $ fail "is a number"
-            when (isDigit $ head str) $ fail "is a number"
             return (context, token, getBracedReference str)
 
     isDereferencing = (`elem` ["-eq", "-ne", "-lt", "-le", "-gt", "-ge"])

src/ShellCheck/Checks/Commands.hs

@@ -279,10 +279,10 @@ checkGrepRe = CommandCheck (Basename "grep") check where
         grepGlobFlags = ["fixed-strings", "F", "include", "exclude", "exclude-dir", "o", "only-matching"]
 
     wordStartingWith c =
-        head . filter ([c] `isPrefixOf`) $ candidates
+        headOrDefault (c:"test") . filter ([c] `isPrefixOf`) $ candidates
       where
         candidates =
-            sampleWords ++ map (\(x:r) -> toUpper x : r) sampleWords ++ [c:"test"]
+            sampleWords ++ map (\(x:r) -> toUpper x : r) sampleWords
 
     getSuspiciousRegexWildcard str =
         if not $ str `matches` contra

src/ShellCheck/Checks/ShellSupport.hs

@@ -457,8 +457,8 @@ checkEchoSed = ForShell [Bash, Ksh] f
     -- This should have used backreferences, but TDFA doesn't support them
     sedRe = mkRegex "^s(.)([^\n]*)g?$"
     isSimpleSed s = fromMaybe False $ do
-        [first,rest] <- matchRegex sedRe s
+        [h:_,rest] <- matchRegex sedRe s
-        let delimiters = filter (== head first) rest
+        let delimiters = filter (== h) rest
         guard $ length delimiters == 2
         return True
     checkIn id s =

src/ShellCheck/Parser.hs

@@ -186,12 +186,12 @@ getNextIdSpanningTokens startTok endTok = do
 
 -- Get an ID starting from the first token of the list, and ending after the last
 getNextIdSpanningTokenList list =
-    if null list
+    case list of
-    then do
+    [] -> do
         pos <- getPosition
         getNextIdBetween pos pos
-    else
+    (h:_) ->
-        getNextIdSpanningTokens (head list) (last list)
+        getNextIdSpanningTokens h (last list)
 
 -- Get the span covered by an id
 getSpanForId :: Monad m => Id -> SCParser m (SourcePos, SourcePos)
@@ -1826,7 +1826,7 @@ readPendingHereDocs = do
             let thereIsNoTrailer = null trailingSpace && null trailer
             let leaderIsOk = null leadingSpace
                     || dashed == Dashed && leadingSpacesAreTabs
-            let trailerStart = if null trailer then '\0' else head trailer
+            let trailerStart = case trailer of [] -> '\0'; (h:_) -> h
             let hasTrailingSpace = not $ null trailingSpace
             let hasTrailer = not $ null trailer
             let ppt = parseProblemAt trailerPos ErrorC