From 3c7025f9f3b72046cef1cb85abea7ebe99e29a0e Mon Sep 17 00:00:00 2001
From: John Wiegley <johnw@newartisans.com>
Date: Fri, 4 Jul 2014 02:06:56 -0500
Subject: [PATCH] Preliminary support for "let" syntax

---
 Nix/Parser.hs | 15 ++++++++++++---
 Nix/Types.hs  |  4 ++--
 2 files changed, 14 insertions(+), 5 deletions(-)

diff --git a/Nix/Parser.hs b/Nix/Parser.hs
index a578748..1055540 100644
--- a/Nix/Parser.hs
+++ b/Nix/Parser.hs
@@ -14,6 +14,7 @@ import           Nix.Internal
 import           Nix.Parser.Library
 import           Prelude hiding (elem)
 
+-- | The lexer for this parser is defined in 'Nix.Parser.Library'.
 nixApp :: Parser NExpr
 nixApp = go <$>
     someTill (whiteSpace *> nixExpr True)
@@ -55,6 +56,7 @@ nixTerm allowLambdas = choice
     , nixParens
     , nixList
     , nixPath
+    , nixLet
     , setLambdaStringOrSym allowLambdas
     ]
 
@@ -76,6 +78,11 @@ nixList = brackets (Fix . NList <$> many (nixTerm False)) <?> "list"
 nixPath :: Parser NExpr
 nixPath = try $ fmap mkPath $ mfilter ('/' `elem`) $ some (oneOf "A-Za-z_0-9.:/")
 
+nixLet :: Parser NExpr
+nixLet =  (Fix .) . NLet
+      <$> (symbol "let" *> nixBinders)
+      <*> (whiteSpace *> symbol "in" *> nixApp)
+
 -- | This is a bit tricky because we don't know whether we're looking at a set
 --   or a lambda until we've looked ahead a bit.  And then it may be neither,
 --   in which case we fall back to expected a plain string or identifier.
@@ -99,7 +106,7 @@ setLambdaStringOrSym allowLambdas = do
                 else keyName <?> "string"
 
 symName :: Parser Text
-symName = pack <$> ((:) <$> letter <*> many (alphaNum <|> char '.'))
+symName = pack <$> ((:) <$> letter <*> many (alphaNum <|> oneOf "._"))
 
 stringish :: Parser NExpr
 stringish =  (char '"' *> (merge <$> manyTill stringChar (char '"')))
@@ -124,6 +131,9 @@ argExpr =  (Fix . NArgSet . Map.fromList <$> argList)
 nvPair :: Parser (NExpr, NExpr)
 nvPair = (,) <$> keyName <*> (symbolic '=' *> nixApp)
 
+nixBinders :: Parser [(NExpr, NExpr)]
+nixBinders = nvPair `endBy` symbolic ';'
+
 keyName :: Parser NExpr
 keyName = (stringish <|> (mkSym <$> symName)) <* whiteSpace
 
@@ -138,8 +148,7 @@ setOrArgs = do
         else try (lookAhead lookaheadForSet)
     trace ("Do we have a set: " ++ show haveSet) $ return ()
     if haveSet
-        then braces (Fix . NSet sawRec <$> nvPair `endBy` symbolic ';')
-            <?> "set"
+        then braces (Fix . NSet sawRec <$> nixBinders) <?> "set"
         else do
             trace "parsing arguments" $ return ()
             args <- argExpr <?> "arguments"
diff --git a/Nix/Types.hs b/Nix/Types.hs
index c71cfbe..75d4bb1 100644
--- a/Nix/Types.hs
+++ b/Nix/Types.hs
@@ -100,7 +100,7 @@ data NExprF r
     | NArgSet (Map Text (Maybe r))
     | NSet  Bool [(r, r)]
 
-    | NLet r r
+    | NLet [(r, r)] r
     | NIf r r r
     | NWith r r
     | NAssert r r
@@ -157,7 +157,7 @@ dumpExpr = cata phi where
   phi (NList l)     = "NList [" ++ show l ++ "]"
   phi (NArgSet xs)  = "NArgSet " ++ show xs
   phi (NSet b xs)   = "NSet " ++ show b ++ " " ++ show xs
-  phi (NLet v e)    = "NLet " ++ v ++ " " ++ e
+  phi (NLet v e)    = "NLet " ++ show v ++ " " ++ e
   phi (NIf i t e)   = "NIf " ++ i ++ " " ++ t ++ " " ++ e
   phi (NWith c v)   = "NWith " ++ c ++ " " ++ v
   phi (NAssert e v) = "NAssert " ++ e ++ " " ++ v