Add a toXML serializer for values

Relates to issue #109

Nix/Builtins.hs

@@ -56,6 +56,7 @@ import           Nix.Scope
 import           Nix.Stack
 import           Nix.Thunk
 import           Nix.Utils
+import           Nix.XML
 import           System.FilePath
 import           System.Posix.Files
 
@@ -132,8 +133,11 @@ builtinsList = sequence [
     , add' Normal   "hashString"                 hashString
     , add  Normal   "readFile"                   readFile_
     , add  Normal   "readDir"                    readDir_
-    , add' Normal   "toJSON"                     (arity1 $ decodeUtf8 . LBS.toStrict . A.encodingToLazyByteString . toEncodingSorted)
+    , add' Normal   "toJSON"
+      (arity1 $ decodeUtf8 . LBS.toStrict . A.encodingToLazyByteString
+                           . toEncodingSorted)
     , add  Normal   "fromJSON"                   fromJSON
+    , add  Normal   "toXML"                      toXML_
     , add  Normal   "typeOf"                     typeOf
     , add2 Normal   "partition"                  partition_
     , add0 Normal   "currentSystem"              currentSystem
@@ -593,6 +597,10 @@ fromJSON t = do
         Left jsonError -> throwError $ "builtins.fromJSON: " ++ jsonError
         Right v -> toValue v
 
+toXML_ :: MonadBuiltins e m => NThunk m -> m (NValue m)
+toXML_ = force >=> normalForm >=> \x ->
+    pure $ NVStr (Text.pack (toXML x)) mempty
+
 typeOf :: MonadBuiltins e m => NThunk m -> m (NValue m)
 typeOf t = do
     v <- force t

Nix/XML.hs

@@ -0,0 +1,42 @@
+{-# LANGUAGE LambdaCase #-}
+
+module Nix.XML where
+
+import           Data.Fix
+import qualified Data.HashMap.Lazy as M
+import qualified Data.Text as Text
+import           Nix.Atoms
+import           Nix.Monad
+import           Text.XML.Light
+
+toXML :: Functor m => NValueNF m -> String
+toXML = (.) ((++ "\n") .
+             ("<?xml version='1.0' encoding='utf-8'?>\n" ++) .
+             ppElement .
+             (\e -> Element (unqual "expr") [] [Elem e] Nothing))
+        $ cata
+        $ \case
+    NVConstant a -> case a of
+        NInt n   -> elem "int" "value" (show n)
+        NFloat f -> elem "float" "value" (show f)
+        NBool b  -> elem "bool" "value" (if b then "true" else "false")
+        NNull    -> Element (unqual "null") [] [] Nothing
+        NUri u   -> elem "uri" "value" (Text.unpack u)
+
+    NVStr t _ -> elem "string" "value" (Text.unpack t)
+    NVList l  -> Element (unqual "list") [] (Elem <$> l) Nothing
+
+    NVSet s   -> Element (unqual "attrs") []
+        (map (\(k, v) -> Elem (Element (unqual "attr")
+                                      [Attr (unqual "name") (Text.unpack k)]
+                                      [Elem v] Nothing))
+             (M.toList s)) Nothing
+
+    NVClosure _ _p _  ->
+        Element (unqual "function") []
+                (error "NYI: XML function param attrset") Nothing
+    NVLiteralPath fp -> elem "path" "value" fp
+    NVEnvPath p      -> elem "path" "value" p
+    NVBuiltin name _ -> elem "function" "name" name
+  where
+    elem n a v = Element (unqual n) [Attr (unqual a) v] [] Nothing

default.nix

@@ -18,7 +18,7 @@ let
       , tasty, tasty-hunit, tasty-th, text, transformers, trifecta
       , unordered-containers, these, optparse-applicative, interpolate
       , process, exceptions, bytestring, mtl, monadlist, base16-bytestring
-      , cryptohash, template-haskell, syb
+      , cryptohash, template-haskell, syb, xml
       }:
       mkDerivation {
         pname = "hnix";
@@ -31,7 +31,7 @@ let
           parsers regex-tdfa regex-tdfa-text semigroups text transformers
           trifecta unordered-containers these process directory filepath
           exceptions bytestring mtl monadlist base16-bytestring cryptohash
-          template-haskell syb
+          template-haskell syb xml
         ];
         executableHaskellDepends = [
           ansi-wl-pprint base containers data-fix deepseq optparse-applicative

hnix.cabal

@@ -27,6 +27,7 @@ Library
     Nix.Scope
     Nix.Stack
     Nix.Eval
+    Nix.XML
     Nix.Thunk
     Nix.Lint
     Nix.Monad
@@ -75,6 +76,7 @@ Library
     , unix
     , syb
     , vector
+    , xml
   if flag(parsec)
     Cpp-options: -DUSE_PARSEC
     Build-depends: parsec

shell.nix

@@ -18,7 +18,7 @@ let
       , tasty, tasty-hunit, tasty-th, text, transformers, trifecta
       , unordered-containers, these, optparse-applicative, free
       , interpolate, process, exceptions, bytestring, mtl, monadlist
-      , base16-bytestring, cryptohash, template-haskell, syb
+      , base16-bytestring, cryptohash, template-haskell, syb, xml
       }:
       mkDerivation {
         pname = "hnix";
@@ -31,7 +31,7 @@ let
           parsers regex-tdfa regex-tdfa-text semigroups text transformers
           trifecta unordered-containers these free process directory
           filepath exceptions bytestring mtl monadlist base16-bytestring
-          cryptohash template-haskell syb
+          cryptohash template-haskell syb xml
         ];
         executableHaskellDepends = [
           ansi-wl-pprint base containers data-fix deepseq optparse-applicative