Allow Nix quasi-quoter to refer to and promote Haskell variables

src/Nix/TH.hs

@@ -1,32 +1,57 @@
+{-# LANGUAGE FlexibleInstances #-}
+{-# LANGUAGE LambdaCase #-}
 {-# LANGUAGE QuasiQuotes #-}
 {-# LANGUAGE TemplateHaskell #-}
+{-# LANGUAGE TypeSynonymInstances #-}
 
 {-# OPTIONS_GHC -Wno-missing-fields #-}
 
 module Nix.TH where
 
 import           Data.Fix
+import           Data.Foldable
 import           Data.Generics.Aliases
 import           Data.Set (Set)
 import qualified Data.Set as Set
 import qualified Data.Text as Text
 import           Language.Haskell.TH
 import           Language.Haskell.TH.Quote
+import           Nix.Atoms
 import           Nix.Expr
 import           Nix.Parser
 
 quoteExprExp :: String -> ExpQ
 quoteExprExp s = do
-    expr <- case parseNixText (Text.pack s) of
+    expr <- case parseNixTextLoc (Text.pack s) of
         Failure err -> fail $ show err
         Success e   -> return e
     dataToExpQ (const Nothing `extQ` metaExp (freeVars expr)) expr
 
-freeVars :: NExpr -> Set VarName
-freeVars = error "NYI: Implement an evaluator to find free variables"
+freeVars :: NExprLoc -> Set VarName
+freeVars = cata $ \case
+    NSym_ _ var -> Set.singleton var
+    Compose (Ann _ x) -> fold x
 
-metaExp :: Set VarName -> NExpr -> Maybe ExpQ
-metaExp fvs (Fix (NSym x)) | x `Set.member` fvs =
+class ToExpr a where
+  toExpr :: a -> NExprLoc
+
+instance ToExpr NExprLoc where
+  toExpr = id
+
+instance ToExpr VarName where
+  toExpr = Fix . NSym_ nullSpan
+
+instance ToExpr Int where
+  toExpr = Fix . NConstant_ nullSpan . NInt . fromIntegral
+
+instance ToExpr Integer where
+  toExpr = Fix . NConstant_ nullSpan . NInt
+
+instance ToExpr Float where
+  toExpr = Fix . NConstant_ nullSpan . NFloat
+
+metaExp :: Set VarName -> NExprLoc -> Maybe ExpQ
+metaExp fvs (Fix (NSym_ _ x)) | x `Set.member` fvs =
   Just [| toExpr $(varE (mkName (Text.unpack x))) |]
 metaExp _ _ = Nothing