Pretty print nix expressions

Nix.hs

@@ -3,7 +3,9 @@ module Main where
 import Data.Map as Map
 import Nix.Eval
 import Nix.Parser
+import Nix.Pretty
 import Nix.Types
+import Text.PrettyPrint.HughesPJ 
 import System.Environment
 
 nix :: FilePath -> IO ()
@@ -12,6 +14,7 @@ nix path = do
     case res of
         Failure e -> error $ "Parse failed: " ++ show e
         Success n -> do
+            putStrLn $ render (prettyNix n)
             top <- evalExpr n (Fix (NVSet Map.empty)) -- evaluate top level
             print top
 

Nix/Pretty.hs

@@ -0,0 +1,42 @@
+module Nix.Pretty where
+
+import Text.PrettyPrint.HughesPJ
+import Nix.Types
+import Data.Map (toList)
+import Data.Text (Text)
+
+prettyBind :: (NExpr, NExpr) -> Doc
+prettyBind (n, v) = prettyNix n <+> equals <+> prettyNix v <> semi
+
+prettySetArg :: (Text, Maybe NExpr) -> Doc
+prettySetArg (n, Nothing) = text (show n)
+prettySetArg (n, Just v) = text (show n) <+> text "?" <+> prettyNix v
+
+prettyFold = foldr (<+>) empty
+
+prettyNix :: NExpr -> Doc
+prettyNix (Fix expr) = go expr where
+  go (NConstant atom) = text $ show atom
+  go (NOper oper) = text $ show oper
+  go (NList list) = lbrack <+> (prettyFold $ map prettyNix list) <+> rbrack
+
+  go (NArgSet args) = lbrace <+> (prettyFold $ map prettySetArg $ toList args) <+> rbrace
+
+  go (NSet rec list) = 
+    (if rec then text "rec" else empty)
+    <+> lbrace <+> (foldr (<+>) empty $ map prettyBind list) <+> rbrace
+
+  go (NLet binds body) = text "let"
+  go (NIf cond trueBody falseBody) =
+    text "if" <+> prettyNix cond
+    <+> text "then" <+> prettyNix trueBody
+    <+> text "else" <+> prettyNix falseBody
+
+  go (NWith scope body) = text "with" <+> prettyNix scope <> semi <+> prettyNix body
+  go (NAssert cond body) = text "assert" <+> prettyNix cond <> semi <+> prettyNix body
+  go (NInherit attrs) = text "inherit"
+
+  go (NVar e) = prettyNix e
+  go (NApp fun arg) = prettyNix fun <+> prettyNix arg
+  go (NAbs args body) = prettyNix args <> colon <+> prettyNix body
+

default.nix

@@ -1,5 +1,5 @@
 { cabal, parsers, trifecta, text, ansiWlPprint, parsec, transformers
-, useParsec ? true
+, pretty, useParsec ? true
 }:
 
 cabal.mkDerivation (self: rec {
@@ -11,6 +11,7 @@ cabal.mkDerivation (self: rec {
   noHaddock = true;
   buildDepends = [
     ansiWlPprint
+    pretty
     text
     transformers
   ] ++ (if useParsec then [ parsec ] else [ parsers trifecta ]);

hnix.cabal

@@ -45,6 +45,7 @@ Library
       base                     >= 4.3          && < 5
     , ansi-wl-pprint
     , containers
+    , pretty
     , text
     , transformers
   if flag(parsec)
@@ -80,6 +81,7 @@ executable hnix
     , hnix
     , ansi-wl-pprint
     , containers
+    , pretty
     , text
     , transformers
   if flag(parsec)