diff --git a/src/Convert/EmptyArgs.hs b/src/Convert/EmptyArgs.hs
index 1b727eb..662b609 100644
--- a/src/Convert/EmptyArgs.hs
+++ b/src/Convert/EmptyArgs.hs
@@ -46,8 +46,8 @@ traverseFunctionsM (MIPackageItem (Function ml t f decls stmts)) = do
 traverseFunctionsM other = return other
 
 convertExpr :: Idents -> Expr -> Expr
-convertExpr functions (Call Nothing func (Args [] [])) =
-    Call Nothing func (Args args [])
+convertExpr functions (Call (Ident func) (Args [] [])) =
+    Call (Ident func) (Args args [])
     where args = if Set.member func functions
             then [Just $ Number "0"]
             else []
diff --git a/src/Convert/Interface.hs b/src/Convert/Interface.hs
index eef569f..ba9ca66 100644
--- a/src/Convert/Interface.hs
+++ b/src/Convert/Interface.hs
@@ -186,6 +186,7 @@ convertDescription _ _ other = other
 -- add a prefix to all standard identifiers in a module item
 prefixModuleItems :: Identifier -> ModuleItem -> ModuleItem
 prefixModuleItems prefix =
+    prefixMIPackageItem .
     traverseDecls prefixDecl .
     traverseExprs (traverseNestedExprs prefixExpr) .
     traverseLHSs  (traverseNestedLHSs  prefixLHS )
@@ -200,6 +201,19 @@ prefixModuleItems prefix =
         prefixLHS :: LHS -> LHS
         prefixLHS (LHSIdent x) = LHSIdent (prefix ++ x)
         prefixLHS other = other
+        prefixMIPackageItem (MIPackageItem item) =
+            MIPackageItem $ prefixPackageItem prefix item
+        prefixMIPackageItem other = other
+
+-- add a prefix to all standard identifiers in a package item
+prefixPackageItem :: Identifier -> PackageItem -> PackageItem
+prefixPackageItem prefix (Function lifetime t x decls stmts) =
+    Function lifetime t x' decls stmts
+    where x' = prefix ++ x
+prefixPackageItem prefix (Task     lifetime   x decls stmts) =
+    Task     lifetime   x' decls stmts
+    where x' = prefix ++ x
+prefixPackageItem _ other = other
 
 lookupType :: [ModuleItem] -> Expr -> (Type, [Range])
 lookupType items (Ident ident) =
diff --git a/src/Convert/KWArgs.hs b/src/Convert/KWArgs.hs
index d6821d7..dfe5173 100644
--- a/src/Convert/KWArgs.hs
+++ b/src/Convert/KWArgs.hs
@@ -44,11 +44,11 @@ collectTFDecls name decls =
         getInput _ = Nothing
 
 convertExpr :: TFs -> Expr -> Expr
-convertExpr _ (orig @ (Call Nothing _ (Args _ []))) = orig
-convertExpr tfs (Call Nothing func (Args pnArgs kwArgs)) =
+convertExpr _ (orig @ (Call _ (Args _ []))) = orig
+convertExpr tfs (Call (Ident func) (Args pnArgs kwArgs)) =
     case tfs Map.!? func of
-        Nothing -> Call Nothing func (Args pnArgs kwArgs)
-        Just ordered -> Call Nothing func (Args args [])
+        Nothing -> Call (Ident func) (Args pnArgs kwArgs)
+        Just ordered -> Call (Ident func) (Args args [])
             where
                 args = pnArgs ++ (map snd $ sortOn position kwArgs)
                 position (x, _) = elemIndex x ordered
diff --git a/src/Convert/NestPI.hs b/src/Convert/NestPI.hs
index 253db10..80365be 100644
--- a/src/Convert/NestPI.hs
+++ b/src/Convert/NestPI.hs
@@ -82,12 +82,12 @@ collectPIsM _ = return ()
 
 -- writes down the names of subroutine invocations
 collectSubroutinesM :: Stmt -> Writer Idents ()
-collectSubroutinesM (Subroutine Nothing f _) = tell $ Set.singleton f
+collectSubroutinesM (Subroutine (Ident f) _) = tell $ Set.singleton f
 collectSubroutinesM _ = return ()
 
 -- writes down the names of function calls and identifiers
 collectIdentsM :: Expr -> Writer Idents ()
-collectIdentsM (Call Nothing x _) = tell $ Set.singleton x
+collectIdentsM (Call (Ident x) _) = tell $ Set.singleton x
 collectIdentsM (Ident x)          = tell $ Set.singleton x
 collectIdentsM _ = return ()
 
diff --git a/src/Convert/Package.hs b/src/Convert/Package.hs
index a7fa1c1..5998c4e 100644
--- a/src/Convert/Package.hs
+++ b/src/Convert/Package.hs
@@ -154,22 +154,14 @@ traverseModuleItem existingItemNames packages (MIPackageItem (Import x y)) =
         items = map snd $ filter (filterer . fst) $ packageItems
 traverseModuleItem _ _ item =
     (traverseExprs $ traverseNestedExprs traverseExpr) $
-    (traverseStmts traverseStmt) $
     (traverseTypes $ traverseNestedTypes traverseType) $
     item
     where
 
         traverseExpr :: Expr -> Expr
         traverseExpr (PSIdent x y) = Ident $ x ++ "_" ++ y
-        traverseExpr (Call (Just ps) f args) =
-            Call Nothing (ps ++ "_" ++ f) args
         traverseExpr other = other
 
-        traverseStmt :: Stmt -> Stmt
-        traverseStmt (Subroutine (Just ps) f args) =
-            Subroutine Nothing (ps ++ "_" ++ f) args
-        traverseStmt other = other
-
         traverseType :: Type -> Type
         traverseType (Alias (Just ps) xx rs) =
             Alias Nothing (ps ++ "_" ++ xx) rs
diff --git a/src/Convert/Simplify.hs b/src/Convert/Simplify.hs
index e340841..3ee62b4 100644
--- a/src/Convert/Simplify.hs
+++ b/src/Convert/Simplify.hs
@@ -56,13 +56,13 @@ convertExpr info (DimFn f v e) =
     DimFn f v e'
     where
         e' = simplify $ substitute info e
-convertExpr info (Call Nothing "$clog2" (Args [Just e] [])) =
+convertExpr info (Call (Ident "$clog2") (Args [Just e] [])) =
     if clog2' == clog2
         then clog2
         else clog2'
     where
         e' = simplify $ substitute info e
-        clog2 = Call Nothing "$clog2" (Args [Just e'] [])
+        clog2 = Call (Ident "$clog2") (Args [Just e'] [])
         clog2' = simplify clog2
 convertExpr info (Mux cc aa bb) =
     if before == after
diff --git a/src/Convert/SizeCast.hs b/src/Convert/SizeCast.hs
index eefca79..b5566de 100644
--- a/src/Convert/SizeCast.hs
+++ b/src/Convert/SizeCast.hs
@@ -65,7 +65,7 @@ traverseExprM =
                     lift $ tell $ Set.singleton (s, sg)
                     let f = castFnName s sg
                     let args = Args [Just e] []
-                    return $ Call Nothing f args
+                    return $ Call (Ident f) args
                 _ -> return $ Cast (Right s) e
         convertExprM other = return other
 
diff --git a/src/Convert/Struct.hs b/src/Convert/Struct.hs
index 2f74cab..622d054 100644
--- a/src/Convert/Struct.hs
+++ b/src/Convert/Struct.hs
@@ -51,10 +51,10 @@ convertDescription (description @ Part{}) =
             traverseExprsM traverseExprM      >>=
             traverseAsgnsM traverseAsgnM
         traverseStmtM :: Stmt -> State Types Stmt
-        traverseStmtM (Subroutine Nothing f args) = do
+        traverseStmtM (Subroutine expr args) = do
             stateTypes <- get
-            return $ uncurry (Subroutine Nothing) $
-                convertCall structs stateTypes f args
+            return $ Subroutine expr $
+                convertCall structs stateTypes expr args
         traverseStmtM stmt =
             traverseStmtLHSsM  traverseLHSM  stmt >>=
             traverseStmtExprsM traverseExprM      >>=
@@ -152,7 +152,7 @@ convertType structs t1 =
 
 -- writes down the names of called functions
 collectCallsM :: Expr -> Writer Idents ()
-collectCallsM (Call Nothing f _) = tell $ Set.singleton f
+collectCallsM (Call (Ident f) _) = tell $ Set.singleton f
 collectCallsM _ = return ()
 
 collectTFArgsM :: ModuleItem -> Writer Types ()
@@ -328,8 +328,8 @@ convertAsgn structs types (lhs, expr) =
                     show (Set.toList extraNames) ++ " that are not in " ++
                     show structTf
             else if Map.member structTf structs then
-                Call Nothing
-                    (packerFnName structTf)
+                Call
+                    (Ident $ packerFnName structTf)
                     (Args (map (Just . snd) items) [])
             else
                 Pattern items
@@ -464,14 +464,15 @@ convertAsgn structs types (lhs, expr) =
                     (_, []) -> Implicit Unspecified []
                     (tf, rs) -> tf $ tail rs
                 (_, i') = convertSubExpr i
-        convertSubExpr (Call Nothing f args) =
-            (retType, uncurry (Call Nothing) $ convertCall structs types f args)
+        convertSubExpr (Call e args) =
+            (retType, Call e $ convertCall structs types e' args)
             where
-                retType = case Map.lookup f types of
-                    Nothing -> Implicit Unspecified []
-                    Just t -> t
-        convertSubExpr (Call (Just x) f args) =
-            (Implicit Unspecified [], Call (Just x) f args)
+                (_, e') = convertSubExpr e
+                retType = case e' of
+                    Ident f -> case Map.lookup f types of
+                        Nothing -> Implicit Unspecified []
+                        Just t -> t
+                    _ -> Implicit Unspecified []
         convertSubExpr (String s) = (Implicit Unspecified [], String s)
         convertSubExpr (Number n) = (Implicit Unspecified [], Number n)
         convertSubExpr (Time   n) = (Implicit Unspecified [], Time   n)
@@ -536,10 +537,13 @@ convertAsgn structs types (lhs, expr) =
             where fieldMap = Map.fromList $ map swap fields
 
 -- attempts to convert based on the assignment-like contexts of TF arguments
-convertCall :: Structs -> Types -> Identifier -> Args -> (Identifier, Args)
-convertCall structs types f (Args pnArgs kwArgs) =
-    (f, args)
+convertCall :: Structs -> Types -> Expr -> Args -> Args
+convertCall structs types fn (Args pnArgs kwArgs) =
+    case fn of
+        Ident _ -> args
+        _ -> Args pnArgs kwArgs
     where
+        Ident f = fn
         idxs = map show ([0..] :: [Int])
         args = Args
             (map snd $ map convertArg $ zip idxs pnArgs)
diff --git a/src/Convert/Traverse.hs b/src/Convert/Traverse.hs
index 3fe851f..284d97e 100644
--- a/src/Convert/Traverse.hs
+++ b/src/Convert/Traverse.hs
@@ -257,7 +257,7 @@ traverseSinglyNestedStmtsM fullMapper = cs
             return $ If u e s1' s2'
         cs (Timing event stmt) = fullMapper stmt >>= return . Timing event
         cs (Return expr) = return $ Return expr
-        cs (Subroutine ps f exprs) = return $ Subroutine ps f exprs
+        cs (Subroutine expr exprs) = return $ Subroutine expr exprs
         cs (Trigger blocks x) = return $ Trigger blocks x
         cs (Assertion a) =
             traverseAssertionStmtsM fullMapper a >>= return . Assertion
@@ -455,11 +455,12 @@ traverseNestedExprsM mapper = exprMapper
             e' <- exprMapper e
             l' <- mapM exprMapper l
             return $ Stream o e' l'
-        em (Call    ps f (Args l p)) = do
+        em (Call  e (Args l p)) = do
+            e' <- exprMapper e
             l' <- mapM maybeExprMapper l
             pes <- mapM maybeExprMapper $ map snd p
             let p' = zip (map fst p) pes
-            return $ Call ps f (Args l' p')
+            return $ Call e' (Args l' p')
         em (UniOp      o e) =
             exprMapper e >>= return . UniOp o
         em (BinOp      o e1 e2) = do
@@ -711,11 +712,12 @@ traverseStmtExprsM exprMapper = flatStmtMapper
     flatStmtMapper (If u cc s1 s2) =
         exprMapper cc >>= \cc' -> return $ If u cc' s1 s2
     flatStmtMapper (Timing event stmt) = return $ Timing event stmt
-    flatStmtMapper (Subroutine ps f (Args l p)) = do
+    flatStmtMapper (Subroutine e (Args l p)) = do
+        e' <- exprMapper e
         l' <- mapM maybeExprMapper l
         pes <- mapM maybeExprMapper $ map snd p
         let p' = zip (map fst p) pes
-        return $ Subroutine ps f (Args l' p')
+        return $ Subroutine e' (Args l' p')
     flatStmtMapper (Return expr) =
         exprMapper expr >>= return . Return
     flatStmtMapper (Trigger blocks x) = return $ Trigger blocks x
diff --git a/src/Language/SystemVerilog/AST/Expr.hs b/src/Language/SystemVerilog/AST/Expr.hs
index 4f54a1f..d9e5de0 100644
--- a/src/Language/SystemVerilog/AST/Expr.hs
+++ b/src/Language/SystemVerilog/AST/Expr.hs
@@ -46,7 +46,7 @@ data Expr
     | Repeat  Expr [Expr]
     | Concat  [Expr]
     | Stream  StreamOp Expr [Expr]
-    | Call    (Maybe Identifier) Identifier Args
+    | Call    Expr Args
     | UniOp   UniOp Expr
     | BinOp   BinOp Expr Expr
     | Mux     Expr Expr Expr
@@ -75,7 +75,7 @@ instance Show Expr where
     show (BinOp   o a b) = printf "(%s %s %s)" (show a) (show o) (show b)
     show (Dot     e n  ) = printf "%s.%s"      (show e) n
     show (Mux     c a b) = printf "(%s ? %s : %s)" (show c) (show a) (show b)
-    show (Call   ps f l) = printf "%s%s%s"     (maybe "" (++ "::") ps) f (show l)
+    show (Call    e l  ) = printf "%s%s"       (show e) (show l)
     show (Cast tore e  ) = printf "%s'(%s)"    (showEither tore) (show e)
     show (DimsFn  f v  ) = printf "%s(%s)"     (show f) (showEither v)
     show (DimFn   f v e) = printf "%s(%s, %s)" (show f) (showEither v) (show e)
@@ -184,7 +184,7 @@ simplify (orig @ (Repeat (Number n) exprs)) =
 simplify (Concat [expr]) = expr
 simplify (Concat exprs) =
     Concat $ filter (/= Concat []) exprs
-simplify (orig @ (Call Nothing "$clog2" (Args [Just (Number n)] []))) =
+simplify (orig @ (Call (Ident "$clog2") (Args [Just (Number n)] []))) =
     case readNumber n of
         Nothing -> orig
         Just x -> Number $ show $ clog2 x
diff --git a/src/Language/SystemVerilog/AST/Stmt.hs b/src/Language/SystemVerilog/AST/Stmt.hs
index 6a2dd87..4815f2a 100644
--- a/src/Language/SystemVerilog/AST/Stmt.hs
+++ b/src/Language/SystemVerilog/AST/Stmt.hs
@@ -48,7 +48,7 @@ data Stmt
     | If      (Maybe UniquePriority) Expr Stmt Stmt
     | Timing  Timing Stmt
     | Return  Expr
-    | Subroutine (Maybe Identifier) Identifier Args
+    | Subroutine Expr Args
     | Trigger Bool Identifier
     | Assertion Assertion
     | Continue
@@ -84,7 +84,7 @@ instance Show Stmt where
                 where showInit (l, e) = showAssign (l, AsgnOpEq, e)
             showAssign :: (LHS, AsgnOp, Expr) -> String
             showAssign (l, op, e) = printf "%s %s %s" (show l) (show op) (show e)
-    show (Subroutine ps x a) = printf "%s%s%s;" (maybe "" (++ "::") ps) x aStr
+    show (Subroutine e a) = printf "%s%s;" (show e) aStr
         where aStr = if a == Args [] [] then "" else show a
     show (AsgnBlk o v e) = printf "%s %s %s;" (show v) (show o) (show e)
     show (Asgn    t v e) = printf "%s <= %s%s;" (show v) (maybe "" showPad t) (show e)
diff --git a/src/Language/SystemVerilog/Parser/Parse.y b/src/Language/SystemVerilog/Parser/Parse.y
index 357b214..7796e2d 100644
--- a/src/Language/SystemVerilog/Parser/Parse.y
+++ b/src/Language/SystemVerilog/Parser/Parse.y
@@ -592,7 +592,6 @@ DeclTokens(delim) :: { [DeclToken] }
 DeclToken :: { DeclToken }
   : DeclOrStmtToken   { $1 }
   | ParameterBindings { DTParams   $1 }
-  | PortBindings      { DTInstance $1 }
 
 DeclOrStmtTokens(delim) :: { [DeclToken] }
   : DeclOrStmtToken                  delim  { [$1] }
@@ -612,6 +611,7 @@ DeclOrStmtToken :: { DeclToken }
   | LHSConcat      { DTConcat   $1 }
   | PartialType    { DTType     $1 }
   | "." Identifier { DTDot      $2 }
+  | PortBindings   { DTInstance $1 }
   | Signing        { DTSigning  $1 }
   | Lifetime       { DTLifetime $1 }
   | Identifier "::" Identifier { DTPSIdent $1 $3 }
@@ -904,8 +904,8 @@ StmtAsgn :: { Stmt }
   : LHS AsgnOp Expr ";"                        { AsgnBlk $2 $1 $3 }
   | LHS IncOrDecOperator ";"                   { AsgnBlk (AsgnOp $2) $1 (Number "1") }
   | LHS "<=" opt(DelayOrEventControl) Expr ";" { Asgn $3 $1 $4 }
-  |                 Identifier ";" { Subroutine (Nothing) $1 (Args [] []) }
-  | Identifier "::" Identifier ";" { Subroutine (Just $1) $3 (Args [] []) }
+  | LHS          ";" { Subroutine (lhsToExpr $1) (Args [] []) }
+  | LHS CallArgs ";" { Subroutine (lhsToExpr $1) $2 }
 StmtNonAsgn :: { Stmt }
   : StmtBlock(BlockKWSeq, "end" ) { $1 }
   | StmtBlock(BlockKWPar, "join") { $1 }
@@ -920,8 +920,6 @@ StmtNonBlock :: { Stmt }
   | Unique "if" "(" Expr ")" Stmt %prec NoElse { If $1 $4 $6 Null }
   | "for" "(" ForInit ForCond ForStep ")" Stmt { For $3 $4 $5 $7 }
   | Unique CaseKW "(" Expr ")" Cases "endcase" { Case $1 $2 $4 (fst $6) (snd $6) }
-  |                 Identifier CallArgs ";"    { Subroutine (Nothing) $1 $2 }
-  | Identifier "::" Identifier CallArgs ";"    { Subroutine (Just $1) $3 $4 }
   | TimingControl Stmt                         { Timing $1 $2 }
   | "return" Expr ";"                          { Return $2 }
   | "return"      ";"                          { Return Nil }
@@ -1102,8 +1100,7 @@ Expr :: { Expr }
   | String                      { String $1 }
   | Number                      { Number $1 }
   | Time                        { Time   $1 }
-  |                 Identifier CallArgs { Call (Nothing) $1 $2 }
-  | Identifier "::" Identifier CallArgs { Call (Just $1) $3 $4 }
+  | Expr CallArgs               { Call $1 $2 }
   | DimsFn "(" TypeOrExpr ")"   { DimsFn $1 $3 }
   | DimFn  "(" TypeOrExpr ")"   { DimFn  $1 $3 (Number "1") }
   | DimFn  "(" TypeOrExpr "," Expr ")" { DimFn $1 $3 $5 }
diff --git a/src/Language/SystemVerilog/Parser/ParseDecl.hs b/src/Language/SystemVerilog/Parser/ParseDecl.hs
index d2cc208..9f01e7c 100644
--- a/src/Language/SystemVerilog/Parser/ParseDecl.hs
+++ b/src/Language/SystemVerilog/Parser/ParseDecl.hs
@@ -38,8 +38,8 @@ module Language.SystemVerilog.Parser.ParseDecl
 , parseDTsAsDeclsOrAsgns
 ) where
 
-import Data.List (elemIndex, findIndex, findIndices)
-import Data.Maybe (fromJust, mapMaybe)
+import Data.List (elemIndex, findIndex, findIndices, partition)
+import Data.Maybe (mapMaybe)
 
 import Language.SystemVerilog.AST
 
@@ -202,22 +202,34 @@ parseDTsAsDecl tokens =
 -- [PUBLIC]: parser for single block item declarations or assign or arg-less
 -- subroutine call statetments
 parseDTsAsDeclOrAsgn :: [DeclToken] -> ([Decl], [Stmt])
-parseDTsAsDeclOrAsgn [DTIdent     f] = ([], [Subroutine (Nothing) f (Args [] [])])
-parseDTsAsDeclOrAsgn [DTPSIdent p f] = ([], [Subroutine (Just  p) f (Args [] [])])
+parseDTsAsDeclOrAsgn [DTIdent     f] = ([], [Subroutine (Ident     f) (Args [] [])])
+parseDTsAsDeclOrAsgn [DTPSIdent p f] = ([], [Subroutine (PSIdent p f) (Args [] [])])
 parseDTsAsDeclOrAsgn tokens =
-    if (isAsgn (last tokens) || tripLookahead tokens) && lhs /= Nothing
-        then ([], [constructor (fromJust lhs) expr])
+    if (isStmt (last tokens) || tripLookahead tokens) && maybeLhs /= Nothing
+        then ([], [stmt])
         else (parseDTsAsDecl tokens, [])
     where
-        (constructor, expr) = case last tokens of
-            DTAsgn     op e -> (AsgnBlk op, e)
-            DTAsgnNBlk mt e -> (Asgn    mt, e)
+        stmt = case last tokens of
+            DTAsgn     op e -> AsgnBlk op lhs e
+            DTAsgnNBlk mt e -> Asgn    mt lhs e
+            DTInstance args -> Subroutine (lhsToExpr lhs) (instanceToArgs args)
             _ -> error $ "invalid block item decl or stmt: " ++ (show tokens)
-        lhs = takeLHS $ init tokens
-        isAsgn :: DeclToken -> Bool
-        isAsgn (DTAsgnNBlk      _ _) = True
-        isAsgn (DTAsgn _ _) = True
-        isAsgn _ = False
+        maybeLhs = takeLHS $ init tokens
+        Just lhs = maybeLhs
+        isStmt :: DeclToken -> Bool
+        isStmt (DTAsgnNBlk{}) = True
+        isStmt (DTAsgn{}) = True
+        isStmt (DTInstance{}) = True
+        isStmt _ = False
+
+-- converts port bindings to call args
+instanceToArgs :: [PortBinding] -> Args
+instanceToArgs bindings =
+    Args pnArgs kwArgs
+    where
+        (pnBindings, kwBindings) = partition (null . fst) bindings
+        pnArgs = map snd pnBindings
+        kwArgs = kwBindings
 
 -- [PUBLIC]: parser for comma-separated declarations or assignment lists; this
 -- is only used for `for` loop initialization lists
diff --git a/test/basic/interface_func.sv b/test/basic/interface_func.sv
new file mode 100644
index 0000000..c8dca05
--- /dev/null
+++ b/test/basic/interface_func.sv
@@ -0,0 +1,11 @@
+interface Foo;
+    function bar;
+        input integer x;
+        return x * x;
+    endfunction
+endinterface
+
+module top;
+    Foo foo();
+    initial $display(foo.bar(3));
+endmodule
diff --git a/test/basic/interface_func.v b/test/basic/interface_func.v
new file mode 100644
index 0000000..2004673
--- /dev/null
+++ b/test/basic/interface_func.v
@@ -0,0 +1,7 @@
+module top;
+    function bar;
+        input integer x;
+        bar = x * x;
+    endfunction
+    initial $display(bar(3));
+endmodule