Add the ExtraChecks module to do occurs check and check the type of main

Elm.cabal

@@ -68,6 +68,7 @@ Library
                        Type.Constrain.Literal,
                        Type.Constrain.Pattern,
                        Type.Environment,
+                       Type.ExtraChecks,
                        Type.Fragment,
                        Type.Inference,
                        Type.PrettyPrint,
@@ -136,6 +137,7 @@ Executable elm
                        Type.Constrain.Literal,
                        Type.Constrain.Pattern,
                        Type.Environment,
+                       Type.ExtraChecks,
                        Type.Fragment,
                        Type.Inference,
                        Type.PrettyPrint,

compiler/Type/ExtraChecks.hs

@@ -0,0 +1,15 @@
+-- This module contains checks to be run *after* type inference has
+-- completed successfully. At that point we still need to do occurs
+-- checks and ensure that `main` has an acceptable type.
+module Type.ExtraChecks where
+
+import qualified Data.Map as Map
+import Type.State (Env)
+
+mainCheck :: Env -> IO (Either String Env)
+mainCheck env =
+    case Map.lookup "main" env of
+      Nothing -> return (Right env)
+      Just var -> return (Right env)
+
+occursCheck = undefined

compiler/Type/State.hs

@@ -24,10 +24,12 @@ data Pool = Pool {
 
 emptyPool = Pool { maxRank = outermostRank, inhabitants = [] }
 
+type Env = Map.Map String Variable
+
 -- Keeps track of the environment, type variable pool, and a list of errors
 data SolverState = SS {
-    sEnv :: Map.Map String Variable,
-    sSavedEnv :: Map.Map String Variable,
+    sEnv :: Env,
+    sSavedEnv :: Env,
     sPool :: Pool,
     sMark :: Int,
     sErrors :: [IO P.Doc]
@@ -77,7 +79,7 @@ switchToPool pool = modifyPool (\_ -> pool)
 getPool :: StateT SolverState IO Pool
 getPool = sPool <$> get
 
-getEnv :: StateT SolverState IO (Map.Map String Variable)
+getEnv :: StateT SolverState IO Env
 getEnv = sEnv <$> get
 
 saveLocalEnv :: StateT SolverState IO ()