Merge pull request #202 from NixOS/print-ast

Add an AST pretty-printing mode

main/Main.hs

@@ -41,7 +41,8 @@ data Nixfmt = Nixfmt
     width :: Width,
     check :: Bool,
     quiet :: Bool,
-    verify :: Bool
+    verify :: Bool,
+    ast :: Bool
   }
   deriving (Show, Data, Typeable)
 
@@ -60,7 +61,11 @@ options =
         verify =
           False
             &= help
-              "Apply sanity checks on the output after formatting"
+              "Apply sanity checks on the output after formatting",
+        ast =
+          False
+            &= help
+              "Pretty print the internal AST, only for debugging"
       }
       &= summary ("nixfmt v" ++ showVersion version)
       &= help "Format Nix source code"
@@ -128,6 +133,7 @@ toTargets Nixfmt{check = True, files = paths} = map checkFileTarget <$> collectA
 type Formatter = FilePath -> Text -> Either String Text
 
 toFormatter :: Nixfmt -> Formatter
+toFormatter Nixfmt{ast = True} = Nixfmt.printAst
 toFormatter Nixfmt{width, verify = True} = Nixfmt.formatVerify width
 toFormatter Nixfmt{width, verify = False} = Nixfmt.format width
 

nixfmt.cabal

@@ -82,6 +82,7 @@ library
     , scientific       >= 0.3.0 && < 0.4.0
     , text             >= 1.2.3 && < 2.2
     , transformers
+    , pretty-simple
   default-language:    Haskell2010
   ghc-options:
     -Wall

src/Nixfmt.hs

@@ -4,18 +4,21 @@ module Nixfmt (
   Width,
   format,
   formatVerify,
+  printAst,
 )
 where
 
 import Data.Bifunctor (bimap, first)
 import Data.Either (fromRight)
 import Data.Text (Text, unpack)
-import Nixfmt.Parser (file)
+import Data.Text.Lazy (toStrict)
+import qualified Nixfmt.Parser as Parser
 import Nixfmt.Predoc (layout)
 import Nixfmt.Pretty ()
 import Nixfmt.Types (Expression, ParseErrorBundle, Whole (..), walkSubprograms)
 import qualified Text.Megaparsec as Megaparsec (parse)
 import Text.Megaparsec.Error (errorBundlePretty)
+import Text.Pretty.Simple (pShow)
 
 -- import Debug.Trace (traceShow, traceShowId)
 
@@ -27,7 +30,13 @@ type Width = Int
 format :: Width -> FilePath -> Text -> Either String Text
 format width filename =
   bimap errorBundlePretty (layout width)
-    . Megaparsec.parse file filename
+    . Megaparsec.parse Parser.file filename
+
+-- | Pretty print the internal AST for debugging
+printAst :: FilePath -> Text -> Either String Text
+printAst path unformatted = do
+  Whole unformattedParsed' _ <- first errorBundlePretty . Megaparsec.parse Parser.file path $ unformatted
+  Left (unpack $ toStrict $ pShow unformattedParsed')
 
 -- Same functionality as `format`, but add sanity checks to guarantee the following properties of the formatter:
 -- - Correctness: The formatted output parses, and the parse tree is identical to the input's
@@ -65,7 +74,7 @@ formatVerify width path unformatted = do
                 <> unpack (layout width (fromRight (error "TODO") $ parse $ layout width minimized))
         else Right formattedOnce
   where
-    parse = first errorBundlePretty . Megaparsec.parse file path
+    parse = first errorBundlePretty . Megaparsec.parse Parser.file path
     pleaseReport x = path <> ": " <> x <> " This is a bug in nixfmt. Please report it at https://github.com/NixOS/nixfmt"
 
 minimize :: Expression -> (Expression -> Bool) -> Expression