[WIP] Start decoupling macro expansion from Haskell (and co.)

CHANGELOG

@@ -5,3 +5,6 @@
 11/17/2019
   - [WIP] Continued work from last week. While wiring everything up, Polysemy became very, very unhappy. I spent some time trying to figure out what I was doing wrong, but with no luck. Eventually, I ended up removing `Sem.Reader (Backend backendEffs)` altogether in favor of just passing `Backend backendEffs` around as a parameter. I've since been running into what I think is a variation of https://github.com/polysemy-research/polysemy/issues/280 (which I opened yesterday after finding an MVCE).
     I'm not too great with typeclass-fu, so this is taking a bit longer than I had anticipated (unfortunately).
+
+11/10/2019
+  - [WIP] Break out Haskell-specific code in `Axel.Macros` into a `Backend` interface. Adapt `Axel.File` to use this new system.

src/Axel/Haskell/File.hs

@@ -22,7 +22,12 @@ import qualified Axel.Eff.Process as Effs (Process)
 import qualified Axel.Eff.Resource as Effs (Resource)
 import qualified Axel.Eff.Restartable as Effs (Restartable)
 import Axel.Haskell.Convert (convertFile)
-import Axel.Macros (handleFunctionApplication, processProgram)
+import Axel.Macros
+  ( HaskellBackendEffs
+  , handleFunctionApplication
+  , haskellBackend
+  , processProgram
+  )
 import Axel.Normalize (normalizeStatement, withExprCtxt)
 import Axel.Parse (parseMultiple', parseSource)
 import Axel.Parse.AST (Expression(Symbol))
@@ -103,13 +108,16 @@ transpileSource ::
   -> Sem.Sem effs SM.Output
 transpileSource filePath source =
   toHaskell . statementsToProgram <$>
-  (parseSource (Just filePath) source >>=
-   processProgram
-     @fileExpanderEffs
-     @funAppExpanderEffs
-     handleFunctionApplication
-     (void . transpileFileInPlace)
-     filePath)
+  Sem.runReader
+    haskellBackend
+    (parseSource (Just filePath) source >>=
+     processProgram
+       @fileExpanderEffs
+       @funAppExpanderEffs
+       @HaskellBackendEffs
+       handleFunctionApplication
+       (void . transpileFileInPlace)
+       filePath)
 
 convertFileInPlace ::
      (Sem.Members '[ Effs.Console, Effs.FileSystem, Sem.Error Error, Effs.FileSystem] effs)