Add an internal "dhall haskell-syntax-tree" command (#1553)

… for debugging.

dhall/dhall.cabal

@@ -431,6 +431,7 @@ Library
         parsers                     >= 0.12.4   && < 0.13,
         prettyprinter               >= 1.5.1    && < 1.6 ,
         prettyprinter-ansi-terminal >= 1.1.1    && < 1.2 ,
+        pretty-simple                              < 4   ,
         profunctors                 >= 3.1.2    && < 5.6 ,
         repline                     >= 0.2.1.0  && < 0.3 ,
         serialise                   >= 0.2.0.0  && < 0.3 ,

dhall/src/Dhall/Main.hs

@@ -50,6 +50,7 @@ import qualified Data.Aeson
 import qualified Data.Aeson.Encode.Pretty
 import qualified Data.ByteString.Lazy
 import qualified Data.ByteString.Lazy.Char8
+import qualified Data.Map
 import qualified Data.Text
 import qualified Data.Text.IO
 import qualified Data.Text.Prettyprint.Doc                 as Pretty
@@ -76,7 +77,7 @@ import qualified System.Exit                               as Exit
 import qualified System.IO
 import qualified System.FilePath
 import qualified Text.Dot
-import qualified Data.Map
+import qualified Text.Pretty.Simple
 
 -- | Top-level program options
 data Options = Options
@@ -130,6 +131,7 @@ data Mode
     | Encode { file :: Input, json :: Bool }
     | Decode { file :: Input, json :: Bool }
     | Text { file :: Input }
+    | SyntaxTree { file :: Input }
 
 data ResolveMode
     = Dot
@@ -158,11 +160,12 @@ parseOptions =
         f True  = Censor
         f False = NoCensor
 
-subcommand :: String -> String -> Parser a -> Parser a
+subcommand' :: Bool -> String -> String -> Parser a -> Parser a
-subcommand name description parser =
+subcommand' internal name description parser =
     Options.Applicative.hsubparser
         (   Options.Applicative.command name parserInfo
         <>  Options.Applicative.metavar name
+        <>  if internal then Options.Applicative.internal else mempty
         )
   where
     parserInfo =
@@ -171,6 +174,12 @@ subcommand name description parser =
             <>  Options.Applicative.progDesc description
             )
 
+subcommand :: String -> String -> Parser a -> Parser a
+subcommand = subcommand' False
+
+internalSubcommand :: String -> String -> Parser a -> Parser a
+internalSubcommand = subcommand' True
+
 parseMode :: Parser Mode
 parseMode =
         subcommand
@@ -229,6 +238,10 @@ parseMode =
             "text"
             "Render a Dhall expression that evaluates to a Text literal"
             (Text <$> parseFile)
+    <|> internalSubcommand
+            "haskell-syntax-tree"
+            "Output the parsed syntax tree (for debugging)"
+            (SyntaxTree <$> parseFile)
     <|> (   Default
         <$> parseFile
         <*> parseOutput
@@ -745,7 +758,14 @@ command (Options {..}) = do
                     System.IO.withFile file System.IO.WriteMode (`Data.Text.IO.hPutStr` tags)
 
                 StandardOutput -> Data.Text.IO.putStrLn tags
-            
+
+        SyntaxTree {..} -> do
+            expression <- getExpression file
+
+            let denoted :: Expr Void Import
+                denoted = Dhall.Core.denote expression
+
+            Text.Pretty.Simple.pPrintNoColor denoted
 
 -- | Entry point for the @dhall@ executable
 main :: IO ()

nix/pretty-simple.nix

@@ -0,0 +1,27 @@
+{ mkDerivation, aeson, ansi-terminal, base, bytestring, Cabal
+, cabal-doctest, containers, criterion, doctest, Glob, mtl
+, optparse-applicative, QuickCheck, stdenv, template-haskell, text
+, transformers
+}:
+mkDerivation {
+  pname = "pretty-simple";
+  version = "3.1.0.0";
+  sha256 = "ea8a062fe7a535a0778ea0f85442551b9a67fad95979fc3e4c617399452c775c";
+  configureFlags = [ "-fbuildexample" "-fbuildexe" ];
+  isLibrary = true;
+  isExecutable = true;
+  setupHaskellDepends = [ base Cabal cabal-doctest ];
+  libraryHaskellDepends = [
+    ansi-terminal base containers mtl text transformers
+  ];
+  executableHaskellDepends = [
+    aeson base bytestring optparse-applicative text
+  ];
+  testHaskellDepends = [
+    base doctest Glob QuickCheck template-haskell
+  ];
+  benchmarkHaskellDepends = [ base criterion text ];
+  homepage = "https://github.com/cdepillabout/pretty-simple";
+  description = "pretty printer for data types with a 'Show' instance";
+  license = stdenv.lib.licenses.bsd3;
+}

stack-lts-6.yaml

@@ -48,6 +48,7 @@ extra-deps:
 - parser-combinators-1.0.0
 - prettyprinter-1.5.1@sha256:fca87c3e2611d3499a0341a59857e9b424a23f31646e4737d535a18582284f96,5375
 - prettyprinter-ansi-terminal-1.1.1.2
+- pretty-simple-3.1.0.0@sha256:d9d045de35be6a2d78a9b3b38d39b13caba96c0ef5ba9d4b44b9b7c2c50e7fa1
 - primitive-0.6.3.0
 - process-1.6.2.0
 - profunctors-5.4