GF.CompileOne: refactoring to reduce code duplication and improve readability

I prefer small functions with descriptive names over large monilithic chunks
of code, so I grouped the compiler passes called from compileSourceModule
into funcitons named frontend, middle and backend. This also makes decisions
about which passes to run clearly visible up front.

Also made some small changes in GF.Compile.

src/compiler/GF/Compile.hs

@@ -14,7 +14,7 @@ import GF.Infra.UseIO(IOE,FullPath,liftIO,getLibraryDirectory,putIfVerb,
                       justModuleName,extendPathEnv,putStrE,putPointE)
 import GF.Data.Operations(raise,(+++),err)
 
-import Control.Monad(foldM,when)
+import Control.Monad(foldM,when,(<=<))
 import GF.System.Directory(doesFileExist,getModificationTime)
 import System.FilePath((</>),isRelative,dropFileName)
 import qualified Data.Map as Map(empty,insert,elems) --lookup
@@ -69,7 +69,7 @@ compileModule opts1 env@(_,rfs) file =
   do file <- getRealFile file
      opts0 <- getOptionsFromFile file
      let curr_dir = dropFileName file
-     lib_dir  <- liftIO $ getLibraryDirectory (addOptions opts0 opts1)
+     lib_dir  <- getLibraryDirectory (addOptions opts0 opts1)
      let opts = addOptions (fixRelativeLibPaths curr_dir lib_dir opts0) opts1
      ps0 <- extendPathEnv opts
      let ps = nub (curr_dir : ps0)
@@ -85,7 +85,7 @@ compileModule opts1 env@(_,rfs) file =
       if exists
         then return file
         else if isRelative file
-               then do lib_dir <- liftIO $ getLibraryDirectory opts1
+               then do lib_dir <- getLibraryDirectory opts1
                        let file1 = lib_dir </> file
                        exists <- liftIO $ doesFileExist file1
                        if exists
@@ -94,24 +94,21 @@ compileModule opts1 env@(_,rfs) file =
                else raise (render ("File" <+> file <+> "does not exist."))
 
 compileOne' :: Options -> CompileEnv -> FullPath -> IOE CompileEnv
-compileOne' opts env@(srcgr,_) file =
-    extendCompileEnv env =<< compileOne opts srcgr file
+compileOne' opts env@(gr,_) = extendCompileEnv env <=< compileOne opts gr
 
 -- auxiliaries
 
---reverseModules (MGrammar ms) = MGrammar $ reverse ms
-
 -- | The environment
 type CompileEnv = (SourceGrammar,ModEnv)
 
 emptyCompileEnv :: CompileEnv
 emptyCompileEnv = (emptySourceGrammar,Map.empty)
 
-extendCompileEnv (gr,menv) (mfile,mo) = do
-  menv2 <- case mfile of
-    Just file -> do
-      let (mod,imps) = importsOfModule mo
-      t <- liftIO $ getModificationTime file
-      return $ Map.insert mod (t,imps) menv
-    _ -> return menv
-  return (prependModule gr mo,menv2) --- reverse later
+extendCompileEnv (gr,menv) (mfile,mo) =
+  do menv2 <- case mfile of
+                Just file ->
+                  do let (mod,imps) = importsOfModule mo
+                     t <- liftIO $ getModificationTime file
+                     return $ Map.insert mod (t,imps) menv
+                _ -> return menv
+     return (prependModule gr mo,menv2)