add cons expression

ast.hs

@@ -16,6 +16,7 @@ data AST = Add AST AST
 		 | Lambda [(Pattern, [AST])]
 		 | Call String [AST]
 		 | UnitConst
+		 | Cons AST AST
 		 | ListConst [AST]
 		 | StrConst String
 		 | IntConst Integer

interp.hs

@@ -90,6 +90,13 @@ eval UnitConst = return UnitV
 
 eval (Block body) = foldr1 (>>) $ map eval body
 
+eval (Cons a b) = do
+	a' <- eval a
+	b' <- eval b
+	case b' of
+		ListV v' -> return $ ListV $ a':v'
+		_ -> error "cons: RHS must be a list"
+
 eval (ListConst v) =
 	mapM eval v >>= \xs ->
 		return $ ListV xs

parser.hs

@@ -110,7 +110,13 @@ call = do
 	symbol ")"
 	return $ Call name args'
 
-term = try block
+consExpr = do
+	x <- expr'
+	symbol "::"
+	y <- exprparser
+	return $ Cons x y
+
+expr' = try block
 	 <|> try funDef
 	 <|> try call
 	 <|> parens exprparser
@@ -119,6 +125,9 @@ term = try block
 	 <|> fmap StrConst stringLiteral
 	 <|> fmap IntConst integer
 
+term = try consExpr
+	<|> expr'
+
 seqStmt = sepBy1 statement semi
 
 program =