Remove Haddock's dependency on `Cabal`

At this point, Haddock depended on Cabal-the-library solely for a
verbosity parser (which misleadingly accepts all sorts of verbosity
options that Haddock never uses). Now, the only dependency on Cabal
is for `haddock-test` (which uses Cabal to locate the Haddock interface
files of a couple boot libraries).

haddock-api/haddock-api.cabal

@@ -43,7 +43,6 @@ library
 
   -- this package typically supports only single major versions
   build-depends: base            ^>= 4.12.0
-               , Cabal           ^>= 2.4.0
                , ghc             ^>= 8.8
                , ghc-paths       ^>= 0.1.0.9
                , haddock-library ^>= 1.8.0
@@ -166,8 +165,7 @@ test-suite spec
     Haddock.Backends.Hyperlinker.Parser
     Haddock.Backends.Hyperlinker.Types
 
-  build-depends: Cabal           ^>= 2.4
-               , ghc             ^>= 8.8
+  build-depends: ghc             ^>= 8.8
                , ghc-paths       ^>= 0.1.0.9
                , haddock-library ^>= 1.8.0
                , xhtml           ^>= 3000.2.2

haddock-api/src/Haddock/Interface.hs

@@ -47,7 +47,6 @@ import Control.Exception (evaluate)
 import Data.List
 import qualified Data.Map as Map
 import qualified Data.Set as Set
-import Distribution.Verbosity
 import Text.Printf
 
 import Module (mkModuleSet, emptyModuleSet, unionModuleSet, ModuleSet)

haddock-api/src/Haddock/Options.hs

@@ -43,7 +43,6 @@ module Haddock.Options (
 import qualified Data.Char as Char
 import           Data.Version
 import           Control.Applicative
-import           Distribution.Verbosity
 import           FastString
 import           GHC ( DynFlags, Module, moduleUnitId )
 import           Haddock.Types
@@ -332,7 +331,7 @@ sinceQualification flags =
 verbosity :: [Flag] -> Verbosity
 verbosity flags =
   case [ str | Flag_Verbosity str <- flags ] of
-    []  -> normal
+    []  -> Normal
     x:_ -> case parseVerbosity x of
       Left e -> throwE e
       Right v -> v

haddock-api/src/Haddock/Utils.hs

@@ -49,7 +49,7 @@ module Haddock.Utils (
   MonadIO(..),
 
   -- * Logging
-  parseVerbosity,
+  parseVerbosity, Verbosity(..), silent, normal, verbose, deafening,
   out,
 
   -- * System tools
@@ -81,8 +81,6 @@ import System.Directory ( createDirectory, removeDirectoryRecursive )
 import System.IO ( hPutStr, hSetEncoding, IOMode(..), stderr, utf8, withFile )
 import System.IO.Unsafe ( unsafePerformIO )
 import qualified System.FilePath.Posix as HtmlPath
-import Distribution.Verbosity
-import Distribution.ReadE
 
 #ifndef mingw32_HOST_OS
 import qualified System.Posix.Internals
@@ -95,10 +93,27 @@ import MonadUtils ( MonadIO(..) )
 -- * Logging
 --------------------------------------------------------------------------------
 
+data Verbosity = Silent | Normal | Verbose | Deafening
+  deriving (Eq, Ord, Enum, Bounded, Show)
 
-parseVerbosity :: String -> Either String Verbosity
-parseVerbosity = runReadE flagToVerbosity
+silent, normal, verbose, deafening :: Verbosity
+silent    = Silent
+normal    = Normal
+verbose   = Verbose
+deafening = Deafening
 
+-- | Parse out a verbosity level. Inspired from Cabal's verbosity parsing.
+parseVerbosity :: String -> Either String Verbosity
+parseVerbosity "0" = Right Silent
+parseVerbosity "1" = Right Normal
+parseVerbosity "2" = Right Silent
+parseVerbosity "3" = Right Deafening
+parseVerbosity "silent"    = return Silent
+parseVerbosity "normal"    = return Normal
+parseVerbosity "verbose"   = return Verbose
+parseVerbosity "debug"     = return Deafening
+parseVerbosity "deafening" = return Deafening
+parseVerbosity other = Left ("Can't parse verbosity " ++ other)
 
 -- | Print a message to stdout, if it is not too verbose
 out :: MonadIO m

haddock.cabal

@@ -77,7 +77,6 @@ executable haddock
       deepseq,
       array,
       xhtml >= 3000.2 && < 3000.3,
-      Cabal >= 1.10,
       ghc-boot,
       ghc == 8.8.*,
       bytestring,