Change dhall-repl to dhall repl (#435)

Fixes #411

This moves `dhall-repl` to become a `repl` subcommand of the `dhall`
executable for command-line consistency

This also extends the REPL logic to respect the command-line `--explain` flag

dhall.cabal

@@ -169,16 +169,19 @@ Library
         directory                   >= 1.3      && < 1.4 ,
         filepath                    >= 1.4      && < 1.5 ,
         formatting                  >= 6.3      && < 6.4 ,
+        haskeline                   >= 0.7.3.0  && < 0.8 ,
         http-client                 >= 0.4.30   && < 0.6 ,
         http-client-tls             >= 0.2.0    && < 0.4 ,
         insert-ordered-containers   >= 0.2.1.0  && < 0.3 ,
         lens-family-core            >= 1.0.0    && < 1.3 ,
         megaparsec                  >= 6.1.1    && < 6.6 ,
         memory                      >= 0.14     && < 0.15,
+        mtl                         >= 2.2.1    && < 2.3 ,
         optparse-applicative                       < 0.15,
         parsers                     >= 0.12.4   && < 0.13,
         prettyprinter               >= 1.2.0.1  && < 1.3 ,
         prettyprinter-ansi-terminal >= 1.1.1    && < 1.2 ,
+        repline                     >= 0.1.6.0  && < 0.2 ,
         scientific                  >= 0.3.0.0  && < 0.4 ,
         template-haskell                           < 2.14,
         text                        >= 0.11.1.0 && < 1.3 ,
@@ -187,6 +190,7 @@ Library
         vector                      >= 0.11.0.0 && < 0.13
     if !impl(ghc >= 8.0)
       Build-Depends: semigroups == 0.18.*
+      Build-Depends: transformers == 0.4.2.*
 
     Exposed-Modules:
         Dhall,
@@ -197,6 +201,7 @@ Library
         Dhall.Main
         Dhall.Parser,
         Dhall.Pretty,
+        Dhall.Repl
         Dhall.TH,
         Dhall.Tutorial,
         Dhall.TypeCheck
@@ -211,23 +216,6 @@ Executable dhall
     Build-Depends: base, dhall
     GHC-Options: -Wall
 
-Executable dhall-repl
-    Hs-Source-Dirs: dhall-repl
-    Main-Is: Main.hs
-    Build-Depends:
-        base             >= 4        && < 5   ,
-        ansi-terminal                         ,
-        dhall                                 ,
-        haskeline        >= 0.7.3.0  && < 0.8 ,
-        mtl              >= 2.2.1    && < 2.3 ,
-        repline          >= 0.1.6.0  && < 0.2 ,
-        prettyprinter                         ,
-        prettyprinter-ansi-terminal           ,
-        text
-    if !impl(ghc >= 8.0)
-      Build-Depends: transformers == 0.4.2.*
-    GHC-Options: -Wall
-
 Executable dhall-format
     Hs-Source-Dirs: dhall-format
     Main-Is: Main.hs

src/Dhall/Main.hs

@@ -32,6 +32,7 @@ import qualified Data.Text.Prettyprint.Doc                 as Pretty
 import qualified Data.Text.Prettyprint.Doc.Render.Terminal as Pretty
 import qualified Dhall.Core
 import qualified Dhall.Parser
+import qualified Dhall.Repl
 import qualified Dhall.TypeCheck
 import qualified Options.Applicative
 import qualified System.Console.ANSI
@@ -43,7 +44,7 @@ data Options = Options
     , plain   :: Bool
     }
 
-data Mode = Default | Version | Resolve | Type | Normalize
+data Mode = Default | Version | Resolve | Type | Normalize | Repl
 
 parseOptions :: Parser Options
 parseOptions = Options <$> parseMode <*> parseExplain <*> parsePlain
@@ -66,6 +67,7 @@ parseMode =
     <|> subcommand "resolve"   "Resolve an expression's imports" Resolve
     <|> subcommand "type"      "Infer an expression's type"      Type
     <|> subcommand "normalize" "Normalize an expression"         Normalize
+    <|> subcommand "repl"      "Interpret expressions in a REPL" Repl
     <|> pure Default
   where
     subcommand name description mode =
@@ -207,6 +209,9 @@ command (Options {..}) = do
 
             render System.IO.stdout (Dhall.Core.normalize inferredType)
 
+        Repl -> do
+            Dhall.Repl.repl explain
+
 main :: IO ()
 main = do
     options <- Options.Applicative.execParser parserInfoOptions

src/Dhall/Repl.hs

@@ -2,7 +2,7 @@
 {-# language NamedFieldPuns #-}
 {-# language OverloadedStrings #-}
 
-module Main ( main ) where
+module Dhall.Repl ( repl ) where
 
 import Control.Exception ( SomeException(SomeException), displayException, throwIO )
 import Control.Monad.IO.Class ( MonadIO, liftIO )
@@ -13,6 +13,7 @@ import Data.List ( foldl' )
 import qualified Data.Text as Text
 import qualified Data.Text.Prettyprint.Doc as Pretty
 import qualified Data.Text.Prettyprint.Doc.Render.Terminal as Pretty ( renderIO )
+import qualified Dhall
 import qualified Dhall.Context
 import qualified Dhall.Core as Dhall ( Var(V), Expr, normalize )
 import qualified Dhall.Pretty
@@ -26,22 +27,25 @@ import qualified System.Console.Repline as Repline
 import qualified System.IO
 
 
-main :: IO ()
-main =
-  evalStateT
-    ( Repline.evalRepl
-        "⊢ "
-        ( dontCrash . eval )
-        options
+repl :: Bool -> IO ()
+repl explain = if explain then Dhall.detailed io else io
+  where
+    io =
+      evalStateT
+        ( Repline.evalRepl
+            "⊢ "
+            ( dontCrash . eval )
+            options
         ( Repline.Word completer )
-        greeter
-    )
-    emptyEnv
+            greeter
+        )
+        (emptyEnv { explain })
 
 
 data Env = Env
   { envBindings :: Dhall.Context.Context Binding
   , envIt :: Maybe Binding
+  , explain :: Bool
   }
 
 
@@ -50,6 +54,7 @@ emptyEnv =
   Env
     { envBindings = Dhall.Context.empty
     , envIt = Nothing
+    , explain = False
     }
 
 
@@ -146,9 +151,11 @@ typeCheck expr = do
   env <-
     get
 
+  let wrap = if explain env then Dhall.detailed else id
+
   case Dhall.typeWith ( bindingType <$> envToContext env ) expr of
     Left e ->
-      liftIO ( throwIO e )
+      liftIO ( wrap (throwIO e) )
 
     Right a ->
       return a