Fix relative imports for dhall freeze --inplace

Fixes #617

This changes `dhall freeze` to use the directory of the supplied file path as
the root import instead of using ".".  This fixes the behavior of relative
imports when the root file is not located in the current working directory.

src/Dhall/Freeze.hs

@@ -13,7 +13,7 @@ import Data.Maybe (fromMaybe)
 import Data.Text
 import Dhall.Binary (ProtocolVersion(..))
 import Dhall.Core (Expr(..), Import(..), ImportHashed(..))
-import Dhall.Import (hashExpression, protocolVersion)
+import Dhall.Import (Status, hashExpression, protocolVersion)
 import Dhall.Parser (exprAndHeaderFromText, Src)
 import Dhall.Pretty (annToAnsiStyle, layoutOpts)
 import Lens.Family (set)
@@ -27,17 +27,12 @@ import qualified Data.Text.IO
 import qualified Dhall.Core
 import qualified Dhall.Import
 import qualified Dhall.TypeCheck
+import qualified System.FilePath
 import qualified System.IO
 
-readInput :: Maybe FilePath -> IO Text
-readInput = maybe Data.Text.IO.getContents Data.Text.IO.readFile
-
 -- | Retrieve an `Import` and update the hash to match the latest contents
-hashImport :: ProtocolVersion -> Import -> IO Import
-hashImport _protocolVersion import_ = do
-    let status =
-            set protocolVersion _protocolVersion (Dhall.Import.emptyStatus ".")
-
+hashImport :: Status IO -> ProtocolVersion -> Import -> IO Import
+hashImport status _protocolVersion import_ = do
     expression <- State.evalStateT (Dhall.Import.loadWith (Embed import_)) status
 
     case Dhall.TypeCheck.typeOf expression of
@@ -90,9 +85,24 @@ freeze
     -> ProtocolVersion
     -> IO ()
 freeze inplace _protocolVersion = do
-    text <- readInput inplace
+    (text, status) <- case inplace of
+        Nothing -> do
+            text <- Data.Text.IO.getContents
+
+            let status = set protocolVersion _protocolVersion (Dhall.Import.emptyStatus ".")
+            return (text, status)
+
+        Just file -> do
+            text <- Data.Text.IO.readFile file
+
+            let directory = System.FilePath.takeDirectory file
+
+            let status = set protocolVersion _protocolVersion (Dhall.Import.emptyStatus directory)
+
+            return (text, status)
+
     (header, parsedExpression) <- parseExpr srcInfo text
-    frozenExpression <- traverse (hashImport _protocolVersion) parsedExpression
+    frozenExpression <- traverse (hashImport status _protocolVersion) parsedExpression
     writeExpr inplace (header, frozenExpression)
         where
             srcInfo = fromMaybe "(stdin)" inplace