Fix cabal layout: move Main.hs to app/, derive Eq/Show on lexer types.

The previous layout had the library and the executable sharing
`hs-source-dirs: ./`, which let GHC's home-module rule treat every
library module as a home source of the executable.  It then tried to
recompile them against the executable's (much smaller) dependency list
and failed on clock/array/hscurses.

Putting Main.hs under `app/` gives the executable a non-overlapping
source directory and lets the library do its job: compiled once, linked
into both the executable and the test-suite.

Lexer-relevant types in `Syntax` now derive `Eq` and `Show`, which lets
`LexerSpec` compare whole `Msg` values directly instead of pattern-
matching out individual fields.  The tests get noticeably shorter.

Author: claude

Syntax.hs

@@ -66,17 +66,20 @@ instance Pretty Quit where
 
 -- mpg123 tagline. Output at startup.
 data Tag = Tag
+    deriving stock (Eq, Show)
 
 -- Track info if ID fields are in the file, otherwise file name.
 newtype File = File (Either ByteString Id3)
+    deriving stock (Eq, Show)
 
--- ID3 info 
+-- ID3 info
 data Id3 = Id3
         { id3title  :: !ByteString
         , id3artist :: !ByteString
         , id3album  :: !ByteString
         , id3str    :: !ByteString
         }
+    deriving stock (Eq, Show)
 
 --      , year   :: Maybe ByteString
 --      , genre  :: Maybe ByteString }
@@ -111,8 +114,9 @@ newtype Info = Info {
            --   checksummed   :: !Bool,
            --   emphasis      :: !Int,
            --   bitrate       :: !Int,
-           --   extension     :: !Int 
+           --   extension     :: !Int
             }
+    deriving stock (Eq, Show)
 
 -- @F <current-frame> <frames-remaining> <current-time> <time-remaining>
 -- Frame decoding status updates (once per frame).
@@ -128,6 +132,7 @@ data Frame = Frame {
                 currentTime    :: !(Fixed E2),
                 timeLeft       :: !(Fixed E2)
              }
+    deriving stock (Eq, Show)
 
 -- @P {0, 1, 2}
 -- Stop/pause status.
@@ -158,3 +163,4 @@ data Msg = T {-# UNPACK #-} !Tag
          | I {-# UNPACK #-} !Info
          | R {-# UNPACK #-} !Frame
          | S                !Status
+    deriving stock (Eq, Show)

Main.hs app/Main.hsMain.hs renamed to app/Main.hs

@@ -80,7 +80,7 @@ library
 executable hmp3
   import: opts
   main-is: Main.hs
-  hs-source-dirs: ./
+  hs-source-dirs: app
   ghc-options: -threaded
   build-depends:
     , base

hmp3-ng.cabal

@@ -11,85 +11,49 @@ import Syntax
 tests :: TestTree
 tests = testGroup "Lexer"
     [ testGroup "doP (status messages)"
-        [ testCase "0 is Stopped"        $ status (doP "0")   @?= Stopped
-        , testCase "1 is Paused"         $ status (doP "1")   @?= Paused
-        , testCase "2 is Playing"        $ status (doP "2")   @?= Playing
-        , testCase "3 is Playing"        $ status (doP "3")   @?= Playing
-        , testCase "empty is Playing"    $ status (doP "")    @?= Playing
-        , testCase "garbage is Playing"  $ status (doP "xyz") @?= Playing
+        [ testCase "0 is Stopped"        $ doP "0"   @?= S Stopped
+        , testCase "1 is Paused"         $ doP "1"   @?= S Paused
+        , testCase "2 is Playing"        $ doP "2"   @?= S Playing
+        , testCase "3 is Playing"        $ doP "3"   @?= S Playing
+        , testCase "empty is Playing"    $ doP ""    @?= S Playing
+        , testCase "garbage is Playing"  $ doP "xyz" @?= S Playing
         ]
     , testGroup "doF (frame messages)"
-        [ testCase "all four fields parse" $ do
-            let Frame{..} = frame (doF "123 456 12.34 56.78")
-            currentFrame @?= 123
-            framesLeft   @?= 456
-            currentTime  @?= 12.34
-            timeLeft     @?= 56.78
-        , testCase "negative timeLeft is clamped to zero" $ do
-            timeLeft (frame (doF "0 0 0.00 -1.00")) @?= 0
+        [ testCase "all four fields parse" $
+            doF "123 456 12.34 56.78" @?= R (Frame 123 456 12.34 56.78)
+        , testCase "negative timeLeft is clamped to zero" $
+            doF "0 0 0.00 -1.00"      @?= R (Frame 0 0 0.00 0)
         ]
     , testGroup "doS (info messages)"
         [ testCase "userinfo combines version, bitrate, kHz" $
-            userinfo (info (doS "1.0 1 44100 stereo 0 0 2 0 0 0 128 0"))
-                @?= "mpeg 1.0 128kbit/s 44kHz"
+            doS "1.0 1 44100 stereo 0 0 2 0 0 0 128 0"
+                @?= I (Info "mpeg 1.0 128kbit/s 44kHz")
         ]
     , testGroup "doI (track info / id3)"
         [ testCase "non-id3 input becomes Left filename" $
-            fileLeft (doI "song.mp3") @?= "song.mp3"
+            doI "song.mp3"
+                @?= F (File (Left "song.mp3"))
         , testCase "non-id3 input is trimmed" $
-            fileLeft (doI "  song.mp3  ") @?= "song.mp3"
-        , testCase "id3 with title only" $ do
-            let i = fileRight (doI ("ID3:" <> field30 "Title"))
-            id3title  i @?= "Title"
-            id3artist i @?= ""
-            id3album  i @?= ""
-            id3str    i @?= "Title"
-        , testCase "id3 with title and artist" $ do
-            let i = fileRight (doI ("ID3:" <> field30 "Title" <> field30 "Artist"))
-            id3title  i @?= "Title"
-            id3artist i @?= "Artist"
-            id3album  i @?= ""
-            id3str    i @?= "Artist : Title"
-        , testCase "id3 with title, artist, and album" $ do
-            let i = fileRight (doI ("ID3:" <> field30 "Title"
-                                          <> field30 "Artist"
-                                          <> field30 "Album"))
-            id3title  i @?= "Title"
-            id3artist i @?= "Artist"
-            id3album  i @?= "Album"
-            id3str    i @?= "Artist : Album : Title"
+            doI "  song.mp3  "
+                @?= F (File (Left "song.mp3"))
+        , testCase "id3 with title only" $
+            doI ("ID3:" <> field30 "Title")
+                @?= F (File (Right (Id3 "Title" "" "" "Title")))
+        , testCase "id3 with title and artist" $
+            doI ("ID3:" <> field30 "Title" <> field30 "Artist")
+                @?= F (File (Right (Id3 "Title" "Artist" "" "Artist : Title")))
+        , testCase "id3 with title, artist, and album" $
+            doI ("ID3:" <> field30 "Title" <> field30 "Artist" <> field30 "Album")
+                @?= F (File (Right (Id3 "Title" "Artist" "Album" "Artist : Album : Title")))
         , testCase "id3 with empty title falls back to Left of trimmed input" $
             -- mpg123 sometimes returns ID3 records with a blank title; rather than
             -- present an empty track name, the parser exposes the raw line.
-            fileLeft (doI ("ID3:" <> field30 "" <> field30 "Artist"))
-                @?= "ID3:" <> P.replicate 30 ' ' <> "Artist"
+            doI ("ID3:" <> field30 "" <> field30 "Artist")
+                @?= F (File (Left ("ID3:" <> P.replicate 30 ' ' <> "Artist")))
         ]
     ]
 
-------------------------------------------------------------------------
--- Helpers
-
 -- Pad/truncate a ByteString to exactly 30 characters with trailing spaces,
 -- matching the fixed-width field convention used by mpg123 ID3 output.
 field30 :: P.ByteString -> P.ByteString
 field30 b = P.take 30 (b <> P.replicate 30 ' ')
-
-status :: Msg -> Status
-status (S s) = s
-status _     = error "expected S"
-
-frame :: Msg -> Frame
-frame (R f) = f
-frame _     = error "expected R"
-
-info :: Msg -> Info
-info (I i) = i
-info _     = error "expected I"
-
-fileLeft :: Msg -> P.ByteString
-fileLeft (F (File (Left  s))) = s
-fileLeft _                    = error "expected F (File (Left _))"
-
-fileRight :: Msg -> Id3
-fileRight (F (File (Right i))) = i
-fileRight _                    = error "expected F (File (Right _))"