Avoid -Wx-partial warnings

These were uncovered by GHC 9.8, where -Wx-partial is included in -Wall.

data-reify.cabal

@@ -121,7 +121,8 @@ Executable data-reify-test3
 Executable data-reify-test4
   Build-Depends:  base, data-reify
   Main-Is:        Test4.hs
-  Hs-Source-Dirs: test
+  other-modules:  Common
+  Hs-Source-Dirs: test, test-common
   ghc-options:    -Wall
   default-language: Haskell2010
   if !flag(tests)
@@ -130,7 +131,8 @@ Executable data-reify-test4
 Executable data-reify-test5
   Build-Depends:  base, data-reify
   Main-Is:        Test5.hs
-  Hs-Source-Dirs: test
+  other-modules:  Common
+  Hs-Source-Dirs: test, test-common
   ghc-options:    -Wall
   default-language: Haskell2010
   if !flag(tests)
@@ -139,7 +141,8 @@ Executable data-reify-test5
 Executable data-reify-test6
   Build-Depends:  base, data-reify
   Main-Is:        Test6.hs
-  Hs-Source-Dirs: test
+  other-modules:  Common
+  Hs-Source-Dirs: test, test-common
   ghc-options:    -Wall
   default-language: Haskell2010
   if !flag(tests)

test-common/Common.hs

@@ -0,0 +1,15 @@
+module Common (head_, tail_) where
+
+-- | Like 'head', but with a more specific error message in case the argument is
+-- empty. This is primarily defined to avoid incurring @-Wx-partial@ warnings
+-- whenever 'head' is used.
+head_ :: [a] -> a
+head_ (x:_) = x
+head_ []    = error "head_: Empty list"
+
+-- | Like 'tail', but with a more specific error message in case the argument is
+-- empty. This is primarily defined to avoid incurring @-Wx-partial@ warnings
+-- whenever 'tail' is used.
+tail_ :: [a] -> [a]
+tail_ (_:xs) = xs
+tail_ []     = error "tail_: Empty list"

test/Test4.hs

@@ -2,6 +2,7 @@
 {-# OPTIONS_GHC -fno-warn-orphans #-}
 module Main (main) where
 
+import Common
 import Control.Applicative hiding (Const)
 import Data.Reify
 import System.CPUTime
@@ -12,11 +13,11 @@ data List a b = Nil | Cons a b
 
 
 instance MuRef [a] where
-  type DeRef [a] = List a 
+  type DeRef [a] = List a
 
   mapDeRef f (x:xs) = Cons x <$> f xs
   mapDeRef _ []     = pure Nil
-  
+
 instance Functor (List a) where
    fmap _ Nil = Nil
    fmap f (Cons a b) = Cons a (f b)
@@ -32,7 +33,7 @@ main = do
 
         -- now, some timings.
         ns <- sequence [ timeme n | n <- take 8 (iterate (*2) 1024) ]
-        print $ reverse $ take 4 $ reverse [ n2 / n1 | (n1,n2) <- zip ns (tail ns) ]
+        print $ reverse $ take 4 $ reverse [ n2 / n1 | (n1,n2) <- zip ns (tail_ ns) ]
 
 timeme :: Int -> IO Float
 timeme n = do
@@ -42,5 +43,5 @@ timeme n = do
         j <- getCPUTime
         let n' :: Float
             n' = fromIntegral ((j - i) `div` 1000000000)
-        putStrLn $ " ==> " ++ show (n' / 1000)   
+        putStrLn $ " ==> " ++ show (n' / 1000)
         return n'

test/Test5.hs

@@ -2,6 +2,8 @@
 {-# OPTIONS_GHC -fno-warn-orphans #-}
 module Main (main) where
 
+import Common
+
 import Control.Applicative hiding (Const)
 
 import Data.Dynamic
@@ -15,7 +17,7 @@ data List a b = Nil | Cons a b
   deriving Show
 
 instance Typeable a => MuRef [a] where
-  type DeRef [a] = List a 
+  type DeRef [a] = List a
 
   mapDeRef f (x:xs) = Cons x <$> f xs
   mapDeRef _ []     = pure Nil
@@ -34,7 +36,7 @@ main = do
 
         -- now, some timings.
         ns <- sequence [ timeme n | n <- take 8 (iterate (*2) 1024) ]
-        print $ reverse $ take 4 $ reverse [ n2 / n1 | (n1,n2) <- zip ns (tail ns) ]
+        print $ reverse $ take 4 $ reverse [ n2 / n1 | (n1,n2) <- zip ns (tail_ ns) ]
 
 timeme :: Int -> IO Float
 timeme n = do
@@ -44,5 +46,5 @@ timeme n = do
         j <- getCPUTime
         let n' :: Float
             n' = fromIntegral ((j - i) `div` 1000000000)
-        putStrLn $ " ==> " ++ show (n' / 1000)   
+        putStrLn $ " ==> " ++ show (n' / 1000)
         return n'

test/Test6.hs

@@ -3,6 +3,8 @@
 {-# OPTIONS_GHC -fno-warn-orphans #-}
 module Main (main) where
 
+import Common
+
 import Control.Applicative hiding (Const)
 
 import Data.Dynamic
@@ -76,12 +78,12 @@ main = do
             g2 = [1..10] ++ g2
         reifyGraph g2 >>= print
 
-        let g3 = [\ x -> x :: Exp, \ y -> y + head g3 2] ++ g3
+        let g3 = [\ x -> x :: Exp, \ y -> y + head_ g3 2] ++ g3
         reifyGraph g3 >>= print
 
         -- now, some timings.
         ns <- sequence [ timeme n | n <- take 8 (iterate (*2) 1024) ]
-        print $ reverse $ take 4 $ reverse [ n2 / n1 | (n1,n2) <- zip ns (tail ns) ]
+        print $ reverse $ take 4 $ reverse [ n2 / n1 | (n1,n2) <- zip ns (tail_ ns) ]
 
 -- zz :: [[Int]]
 -- zz = let xs = [1..3]