diff --git a/artiq/compiler/builtins.py b/artiq/compiler/builtins.py
index 47a89641c..c06f600d3 100644
--- a/artiq/compiler/builtins.py
+++ b/artiq/compiler/builtins.py
@@ -255,6 +255,6 @@ def is_allocated(typ):
     return not (is_none(typ) or is_bool(typ) or is_int(typ) or
                   is_float(typ) or is_range(typ) or
                   types._is_pointer(typ) or types.is_function(typ) or
-                  types.is_c_function(typ) or types.is_rpc_function(typ) or
+                  types.is_c_function(typ) or types.is_rpc(typ) or
                   types.is_method(typ) or types.is_tuple(typ) or
                   types.is_value(typ))
diff --git a/artiq/compiler/embedding.py b/artiq/compiler/embedding.py
index 006d274ad..426728fd5 100644
--- a/artiq/compiler/embedding.py
+++ b/artiq/compiler/embedding.py
@@ -428,9 +428,8 @@ class StitchingInferencer(Inferencer):
             if attr_name not in attributes:
                 # We just figured out what the type should be. Add it.
                 attributes[attr_name] = attr_value_type
-            elif not types.is_rpc_function(attr_value_type):
+            else:
                 # Does this conflict with an earlier guess?
-                # RPC function types are exempt because RPCs are dynamically typed.
                 try:
                     attributes[attr_name].unify(attr_value_type)
                 except types.UnificationError as e:
@@ -694,29 +693,22 @@ class Stitcher:
             # Let the rest of the program decide.
             return types.TVar()
 
-    def _quote_foreign_function(self, function, loc, syscall, flags):
+    def _quote_syscall(self, function, loc):
         signature = inspect.signature(function)
 
         arg_types = OrderedDict()
         optarg_types = OrderedDict()
         for param in signature.parameters.values():
-            if param.kind not in (inspect.Parameter.POSITIONAL_ONLY,
-                                  inspect.Parameter.POSITIONAL_OR_KEYWORD):
-                # We pretend we don't see *args, kwpostargs=..., **kwargs.
-                # Since every method can be still invoked without any arguments
-                # going into *args and the slots after it, this is always safe,
-                # if sometimes constraining.
-                #
-                # Accepting POSITIONAL_ONLY is OK, because the compiler
-                # desugars the keyword arguments into positional ones internally.
-                continue
+            if param.kind != inspect.Parameter.POSITIONAL_OR_KEYWORD:
+                diag = diagnostic.Diagnostic("error",
+                    "system calls must only use positional arguments; '{argument}' isn't",
+                    {"argument": param.name},
+                    self._function_loc(function),
+                    notes=self._call_site_note(loc, is_syscall=True))
+                self.engine.process(diag)
 
             if param.default is inspect.Parameter.empty:
-                arg_types[param.name] = self._type_of_param(function, loc, param,
-                                                            is_syscall=syscall is not None)
-            elif syscall is None:
-                optarg_types[param.name] = self._type_of_param(function, loc, param,
-                                                               is_syscall=False)
+                arg_types[param.name] = self._type_of_param(function, loc, param, is_syscall=True)
             else:
                 diag = diagnostic.Diagnostic("error",
                     "system call argument '{argument}' must not have a default value",
@@ -727,10 +719,8 @@ class Stitcher:
 
         if signature.return_annotation is not inspect.Signature.empty:
             ret_type = self._extract_annot(function, signature.return_annotation,
-                                           "return type", loc, is_syscall=syscall is not None)
-        elif syscall is None:
-            ret_type = builtins.TNone()
-        else: # syscall is not None
+                                           "return type", loc, is_syscall=True)
+        else:
             diag = diagnostic.Diagnostic("error",
                 "system call must have a return type annotation", {},
                 self._function_loc(function),
@@ -738,15 +728,23 @@ class Stitcher:
             self.engine.process(diag)
             ret_type = types.TVar()
 
-        if syscall is None:
-            function_type = types.TRPCFunction(arg_types, optarg_types, ret_type,
-                                               service=self.object_map.store(function))
-        else:
-            function_type = types.TCFunction(arg_types, ret_type,
-                                             name=syscall, flags=flags)
-
+        function_type = types.TCFunction(arg_types, ret_type,
+                                         name=function.artiq_embedded.syscall,
+                                         flags=function.artiq_embedded.flags)
         self.functions[function] = function_type
+        return function_type
 
+    def _quote_rpc(self, function, loc):
+        signature = inspect.signature(function)
+
+        if signature.return_annotation is not inspect.Signature.empty:
+            ret_type = self._extract_annot(function, signature.return_annotation,
+                                           "return type", loc, is_syscall=False)
+        else:
+            ret_type = builtins.TNone()
+
+        function_type = types.TRPC(ret_type, service=self.object_map.store(function))
+        self.functions[function] = function_type
         return function_type
 
     def _quote_function(self, function, loc):
@@ -780,9 +778,7 @@ class Stitcher:
                 elif function.artiq_embedded.syscall is not None:
                     # Insert a storage-less global whose type instructs the compiler
                     # to perform a system call instead of a regular call.
-                    self._quote_foreign_function(function, loc,
-                                                 syscall=function.artiq_embedded.syscall,
-                                                 flags=function.artiq_embedded.flags)
+                    self._quote_syscall(function, loc)
                 elif function.artiq_embedded.forbidden is not None:
                     diag = diagnostic.Diagnostic("fatal",
                         "this function cannot be called as an RPC", {},
@@ -792,14 +788,9 @@ class Stitcher:
                 else:
                     assert False
             else:
-                # Insert a storage-less global whose type instructs the compiler
-                # to perform an RPC instead of a regular call.
-                self._quote_foreign_function(function, loc, syscall=None, flags=None)
+                self._quote_rpc(function, loc)
 
-        function_type = self.functions[function]
-        if types.is_rpc_function(function_type):
-            function_type = types.instantiate(function_type)
-        return function_type
+        return self.functions[function]
 
     def _quote(self, value, loc):
         synthesizer = self._synthesizer(loc)
diff --git a/artiq/compiler/ir.py b/artiq/compiler/ir.py
index 8414c45c9..57936c48c 100644
--- a/artiq/compiler/ir.py
+++ b/artiq/compiler/ir.py
@@ -23,12 +23,19 @@ def is_basic_block(typ):
     return isinstance(typ, TBasicBlock)
 
 class TOption(types.TMono):
-    def __init__(self, inner):
-        super().__init__("option", {"inner": inner})
+    def __init__(self, value):
+        super().__init__("option", {"value": value})
 
 def is_option(typ):
     return isinstance(typ, TOption)
 
+class TKeyword(types.TMono):
+    def __init__(self, value):
+        super().__init__("keyword", {"value": value})
+
+def is_keyword(typ):
+    return isinstance(typ, TKeyword)
+
 class TExceptionTypeInfo(types.TMono):
     def __init__(self):
         super().__init__("exntypeinfo")
@@ -678,7 +685,7 @@ class GetAttr(Instruction):
             typ = obj.type.attributes[attr]
         else:
             typ = obj.type.constructor.attributes[attr]
-            if types.is_function(typ):
+            if types.is_function(typ) or types.is_rpc(typ):
                 typ = types.TMethod(obj.type, typ)
         super().__init__([obj], typ, name)
         self.attr = attr
diff --git a/artiq/compiler/transforms/artiq_ir_generator.py b/artiq/compiler/transforms/artiq_ir_generator.py
index a42cbe213..c5b25de7a 100644
--- a/artiq/compiler/transforms/artiq_ir_generator.py
+++ b/artiq/compiler/transforms/artiq_ir_generator.py
@@ -922,9 +922,6 @@ class ARTIQIRGenerator(algorithm.Visitor):
         if self.current_assign is None:
             return self.append(ir.GetAttr(obj, node.attr,
                                           name="{}.FLD.{}".format(_readable_name(obj), node.attr)))
-        elif types.is_rpc_function(self.current_assign.type):
-            # RPC functions are just type-level markers
-            return self.append(ir.Builtin("nop", [], builtins.TNone()))
         else:
             return self.append(ir.SetAttr(obj, node.attr, self.current_assign))
 
@@ -1719,7 +1716,7 @@ class ARTIQIRGenerator(algorithm.Visitor):
             self.engine.process(diag)
 
     def _user_call(self, callee, positional, keywords, arg_exprs={}):
-        if types.is_function(callee.type):
+        if types.is_function(callee.type) or types.is_rpc(callee.type):
             func     = callee
             self_arg = None
             fn_typ   = callee.type
@@ -1734,40 +1731,51 @@ class ARTIQIRGenerator(algorithm.Visitor):
         else:
             assert False
 
-        args = [None] * (len(fn_typ.args) + len(fn_typ.optargs))
-
-        for index, arg in enumerate(positional):
-            if index + offset < len(fn_typ.args):
-                args[index + offset] = arg
+        if types.is_rpc(fn_typ):
+            if self_arg is None:
+                args = positional
             else:
-                args[index + offset] = self.append(ir.Alloc([arg], ir.TOption(arg.type)))
+                args = [self_arg] + positional
 
-        for keyword in keywords:
-            arg = keywords[keyword]
-            if keyword in fn_typ.args:
-                for index, arg_name in enumerate(fn_typ.args):
-                    if keyword == arg_name:
-                        assert args[index] is None
-                        args[index] = arg
-                        break
-            elif keyword in fn_typ.optargs:
-                for index, optarg_name in enumerate(fn_typ.optargs):
-                    if keyword == optarg_name:
-                        assert args[len(fn_typ.args) + index] is None
-                        args[len(fn_typ.args) + index] = \
-                                self.append(ir.Alloc([arg], ir.TOption(arg.type)))
-                        break
+            for keyword in keywords:
+                arg = keywords[keyword]
+                args.append(self.append(ir.Alloc([ir.Constant(keyword, builtins.TStr()), arg],
+                                                 ir.TKeyword(arg.type))))
+        else:
+            args = [None] * (len(fn_typ.args) + len(fn_typ.optargs))
 
-        for index, optarg_name in enumerate(fn_typ.optargs):
-            if args[len(fn_typ.args) + index] is None:
-                args[len(fn_typ.args) + index] = \
-                        self.append(ir.Alloc([], ir.TOption(fn_typ.optargs[optarg_name])))
+            for index, arg in enumerate(positional):
+                if index + offset < len(fn_typ.args):
+                    args[index + offset] = arg
+                else:
+                    args[index + offset] = self.append(ir.Alloc([arg], ir.TOption(arg.type)))
 
-        if self_arg is not None:
-            assert args[0] is None
-            args[0] = self_arg
+            for keyword in keywords:
+                arg = keywords[keyword]
+                if keyword in fn_typ.args:
+                    for index, arg_name in enumerate(fn_typ.args):
+                        if keyword == arg_name:
+                            assert args[index] is None
+                            args[index] = arg
+                            break
+                elif keyword in fn_typ.optargs:
+                    for index, optarg_name in enumerate(fn_typ.optargs):
+                        if keyword == optarg_name:
+                            assert args[len(fn_typ.args) + index] is None
+                            args[len(fn_typ.args) + index] = \
+                                    self.append(ir.Alloc([arg], ir.TOption(arg.type)))
+                            break
 
-        assert None not in args
+            for index, optarg_name in enumerate(fn_typ.optargs):
+                if args[len(fn_typ.args) + index] is None:
+                    args[len(fn_typ.args) + index] = \
+                            self.append(ir.Alloc([], ir.TOption(fn_typ.optargs[optarg_name])))
+
+            if self_arg is not None:
+                assert args[0] is None
+                args[0] = self_arg
+
+            assert None not in args
 
         if self.unwind_target is None:
             insn = self.append(ir.Call(func, args, arg_exprs))
diff --git a/artiq/compiler/transforms/inferencer.py b/artiq/compiler/transforms/inferencer.py
index 460be6767..3d2bb0541 100644
--- a/artiq/compiler/transforms/inferencer.py
+++ b/artiq/compiler/transforms/inferencer.py
@@ -109,17 +109,11 @@ class Inferencer(algorithm.Visitor):
                     ]
 
                 attr_type = object_type.attributes[attr_name]
-                if types.is_rpc_function(attr_type):
-                    attr_type = types.instantiate(attr_type)
-
                 self._unify(result_type, attr_type, loc, None,
                             makenotes=makenotes, when=" for attribute '{}'".format(attr_name))
             elif types.is_instance(object_type) and \
                     attr_name in object_type.constructor.attributes:
                 attr_type = object_type.constructor.attributes[attr_name].find()
-                if types.is_rpc_function(attr_type):
-                    attr_type = types.instantiate(attr_type)
-
                 if types.is_function(attr_type):
                     # Convert to a method.
                     if len(attr_type.args) < 1:
@@ -155,6 +149,10 @@ class Inferencer(algorithm.Visitor):
                                     when=" while inferring the type for self argument")
 
                     attr_type = types.TMethod(object_type, attr_type)
+                elif types.is_rpc(attr_type):
+                    # Convert to a method. We don't have to bother typechecking
+                    # the self argument, since for RPCs anything goes.
+                    attr_type = types.TMethod(object_type, attr_type)
 
                 if not types.is_var(attr_type):
                     self._unify(result_type, attr_type,
@@ -871,6 +869,10 @@ class Inferencer(algorithm.Visitor):
             return # not enough info yet
         elif types.is_builtin(typ):
             return self.visit_builtin_call(node)
+        elif types.is_rpc(typ):
+            self._unify(node.type, typ.ret,
+                        node.loc, None)
+            return
         elif not (types.is_function(typ) or types.is_method(typ)):
             diag = diagnostic.Diagnostic("error",
                 "cannot call this expression of type {type}",
@@ -888,6 +890,10 @@ class Inferencer(algorithm.Visitor):
             typ = types.get_method_function(typ)
             if types.is_var(typ):
                 return # not enough info yet
+            elif types.is_rpc(typ):
+                self._unify(node.type, typ.ret,
+                            node.loc, None)
+                return
 
             typ_arity   = typ.arity() - 1
             typ_args    = OrderedDict(list(typ.args.items())[1:])
diff --git a/artiq/compiler/transforms/iodelay_estimator.py b/artiq/compiler/transforms/iodelay_estimator.py
index fff7470de..90bfefdb3 100644
--- a/artiq/compiler/transforms/iodelay_estimator.py
+++ b/artiq/compiler/transforms/iodelay_estimator.py
@@ -280,7 +280,7 @@ class IODelayEstimator(algorithm.Visitor):
                                   context="as an argument for delay_mu()")
             call_delay = value
         elif not types.is_builtin(typ):
-            if types.is_function(typ):
+            if types.is_function(typ) or types.is_rpc(typ):
                 offset = 0
             elif types.is_method(typ):
                 offset = 1
@@ -288,35 +288,38 @@ class IODelayEstimator(algorithm.Visitor):
             else:
                 assert False
 
-            delay = typ.find().delay.find()
-            if types.is_var(delay):
-                raise _UnknownDelay()
-            elif delay.is_indeterminate():
-                note = diagnostic.Diagnostic("note",
-                    "function called here", {},
-                    node.loc)
-                cause = delay.cause
-                cause = diagnostic.Diagnostic(cause.level, cause.reason, cause.arguments,
-                                              cause.location, cause.highlights,
-                                              cause.notes + [note])
-                raise _IndeterminateDelay(cause)
-            elif delay.is_fixed():
-                args = {}
-                for kw_node in node.keywords:
-                    args[kw_node.arg] = kw_node.value
-                for arg_name, arg_node in zip(list(typ.args)[offset:], node.args):
-                    args[arg_name] = arg_node
-
-                free_vars = delay.duration.free_vars()
-                node.arg_exprs = {
-                    arg: self.evaluate(args[arg], abort=abort,
-                                       context="in the expression for argument '{}' "
-                                               "that affects I/O delay".format(arg))
-                    for arg in free_vars
-                }
-                call_delay = delay.duration.fold(node.arg_exprs)
+            if types.is_rpc(typ):
+                call_delay = iodelay.Const(0)
             else:
-                assert False
+                delay = typ.find().delay.find()
+                if types.is_var(delay):
+                    raise _UnknownDelay()
+                elif delay.is_indeterminate():
+                    note = diagnostic.Diagnostic("note",
+                        "function called here", {},
+                        node.loc)
+                    cause = delay.cause
+                    cause = diagnostic.Diagnostic(cause.level, cause.reason, cause.arguments,
+                                                  cause.location, cause.highlights,
+                                                  cause.notes + [note])
+                    raise _IndeterminateDelay(cause)
+                elif delay.is_fixed():
+                    args = {}
+                    for kw_node in node.keywords:
+                        args[kw_node.arg] = kw_node.value
+                    for arg_name, arg_node in zip(list(typ.args)[offset:], node.args):
+                        args[arg_name] = arg_node
+
+                    free_vars = delay.duration.free_vars()
+                    node.arg_exprs = {
+                        arg: self.evaluate(args[arg], abort=abort,
+                                           context="in the expression for argument '{}' "
+                                                   "that affects I/O delay".format(arg))
+                        for arg in free_vars
+                    }
+                    call_delay = delay.duration.fold(node.arg_exprs)
+                else:
+                    assert False
         else:
             call_delay = iodelay.Const(0)
 
diff --git a/artiq/compiler/transforms/llvm_ir_generator.py b/artiq/compiler/transforms/llvm_ir_generator.py
index 816a7d6e8..acec6953a 100644
--- a/artiq/compiler/transforms/llvm_ir_generator.py
+++ b/artiq/compiler/transforms/llvm_ir_generator.py
@@ -177,7 +177,7 @@ class LLVMIRGenerator:
         typ = typ.find()
         if types.is_tuple(typ):
             return ll.LiteralStructType([self.llty_of_type(eltty) for eltty in typ.elts])
-        elif types.is_rpc_function(typ) or types.is_c_function(typ):
+        elif types.is_rpc(typ) or types.is_c_function(typ):
             if for_return:
                 return llvoid
             else:
@@ -229,7 +229,9 @@ class LLVMIRGenerator:
         elif ir.is_basic_block(typ):
             return llptr
         elif ir.is_option(typ):
-            return ll.LiteralStructType([lli1, self.llty_of_type(typ.params["inner"])])
+            return ll.LiteralStructType([lli1, self.llty_of_type(typ.params["value"])])
+        elif ir.is_keyword(typ):
+            return ll.LiteralStructType([llptr, self.llty_of_type(typ.params["value"])])
         elif ir.is_environment(typ):
             llty = self.llcontext.get_identified_type("env.{}".format(typ.env_name))
             if llty.elements is None:
@@ -618,7 +620,7 @@ class LLVMIRGenerator:
                                             size=llsize)
             llvalue = self.llbuilder.insert_value(llvalue, llalloc, 1, name=insn.name)
             return llvalue
-        elif not builtins.is_allocated(insn.type):
+        elif not builtins.is_allocated(insn.type) or ir.is_keyword(insn.type):
             llvalue = ll.Constant(self.llty_of_type(insn.type), ll.Undefined)
             for index, elt in enumerate(insn.operands):
                 llvalue = self.llbuilder.insert_value(llvalue, self.map(elt), index)
@@ -707,8 +709,8 @@ class LLVMIRGenerator:
     def get_global_closure(self, typ, attr):
         closure_type = typ.attributes[attr]
         assert types.is_constructor(typ)
-        assert types.is_function(closure_type)
-        if types.is_c_function(closure_type) or types.is_rpc_function(closure_type):
+        assert types.is_function(closure_type) or types.is_rpc(closure_type)
+        if types.is_c_function(closure_type) or types.is_rpc(closure_type):
             return None
 
         llty = self.llty_of_type(typ.attributes[attr])
@@ -1156,8 +1158,8 @@ class LLVMIRGenerator:
         elif builtins.is_range(typ):
             return b"r" + self._rpc_tag(builtins.get_iterable_elt(typ),
                                         error_handler)
-        elif ir.is_option(typ):
-            return b"o" + self._rpc_tag(typ.params["inner"],
+        elif ir.is_keyword(typ):
+            return b"k" + self._rpc_tag(typ.params["value"],
                                         error_handler)
         elif '__objectid__' in typ.attributes:
             return b"O"
@@ -1271,7 +1273,7 @@ class LLVMIRGenerator:
 
     def process_Call(self, insn):
         functiontyp = insn.target_function().type
-        if types.is_rpc_function(functiontyp):
+        if types.is_rpc(functiontyp):
             return self._build_rpc(insn.target_function().loc,
                                    functiontyp,
                                    insn.arguments(),
@@ -1303,7 +1305,7 @@ class LLVMIRGenerator:
         functiontyp = insn.target_function().type
         llnormalblock = self.map(insn.normal_target())
         llunwindblock = self.map(insn.exception_target())
-        if types.is_rpc_function(functiontyp):
+        if types.is_rpc(functiontyp):
             return self._build_rpc(insn.target_function().loc,
                                    functiontyp,
                                    insn.arguments(),
@@ -1392,7 +1394,7 @@ class LLVMIRGenerator:
             lleltsptr = llglobal.bitcast(lleltsary.type.element.as_pointer())
             llconst   = ll.Constant(llty, [ll.Constant(lli32, len(llelts)), lleltsptr])
             return llconst
-        elif types.is_function(typ):
+        elif types.is_rpc(typ) or types.is_function(typ):
             # RPC and C functions have no runtime representation.
             # We only get down this codepath for ARTIQ Python functions when they're
             # referenced from a constructor, and the value inside the constructor
diff --git a/artiq/compiler/types.py b/artiq/compiler/types.py
index 26fc24d95..cca1d15a9 100644
--- a/artiq/compiler/types.py
+++ b/artiq/compiler/types.py
@@ -94,9 +94,6 @@ class TVar(Type):
         else:
             return self.find().fold(accum, fn)
 
-    def map(self, fn):
-        return fn(self)
-
     def __repr__(self):
         if self.parent is self:
             return "<artiq.compiler.types.TVar %d>" % id(self)
@@ -141,21 +138,6 @@ class TMono(Type):
             accum = self.params[param].fold(accum, fn)
         return fn(accum, self)
 
-    def map(self, fn):
-        params = OrderedDict()
-        for param in self.params:
-            params[param] = self.params[param].map(fn)
-
-        attributes = OrderedDict()
-        for attr in self.attributes:
-            attributes[attr] = self.attributes[attr].map(fn)
-
-        self_copy = self.__class__.__new__(self.__class__)
-        self_copy.name = self.name
-        self_copy.params = params
-        self_copy.attributes = attributes
-        return fn(self_copy)
-
     def __repr__(self):
         return "artiq.compiler.types.TMono(%s, %s)" % (repr(self.name), repr(self.params))
 
@@ -202,9 +184,6 @@ class TTuple(Type):
             accum = elt.fold(accum, fn)
         return fn(accum, self)
 
-    def map(self, fn):
-        return fn(TTuple(list(map(lambda elt: elt.map(fn), self.elts))))
-
     def __repr__(self):
         return "artiq.compiler.types.TTuple(%s)" % repr(self.elts)
 
@@ -276,23 +255,6 @@ class TFunction(Type):
         accum = self.ret.fold(accum, fn)
         return fn(accum, self)
 
-    def _map_args(self, fn):
-        args = OrderedDict()
-        for arg in self.args:
-            args[arg] = self.args[arg].map(fn)
-
-        optargs = OrderedDict()
-        for optarg in self.optargs:
-            optargs[optarg] = self.optargs[optarg].map(fn)
-
-        return args, optargs, self.ret.map(fn)
-
-    def map(self, fn):
-        args, optargs, ret = self._map_args(fn)
-        self_copy = TFunction(args, optargs, ret)
-        self_copy.delay = self.delay.map(fn)
-        return fn(self_copy)
-
     def __repr__(self):
         return "artiq.compiler.types.TFunction({}, {}, {})".format(
             repr(self.args), repr(self.optargs), repr(self.ret))
@@ -308,35 +270,6 @@ class TFunction(Type):
     def __hash__(self):
         return hash((_freeze(self.args), _freeze(self.optargs), self.ret))
 
-class TRPCFunction(TFunction):
-    """
-    A function type of a remote function.
-
-    :ivar service: (int) RPC service number
-    """
-
-    attributes = OrderedDict()
-
-    def __init__(self, args, optargs, ret, service):
-        super().__init__(args, optargs, ret)
-        self.service = service
-        self.delay   = TFixedDelay(iodelay.Const(0))
-
-    def unify(self, other):
-        if isinstance(other, TRPCFunction) and \
-                self.service == other.service:
-            super().unify(other)
-        elif isinstance(other, TVar):
-            other.unify(self)
-        else:
-            raise UnificationError(self, other)
-
-    def map(self, fn):
-        args, optargs, ret = self._map_args(fn)
-        self_copy = TRPCFunction(args, optargs, ret, self.service)
-        self_copy.delay = self.delay.map(fn)
-        return fn(self_copy)
-
 class TCFunction(TFunction):
     """
     A function type of a runtime-provided C function.
@@ -368,11 +301,49 @@ class TCFunction(TFunction):
         else:
             raise UnificationError(self, other)
 
-    def map(self, fn):
-        args, _optargs, ret = self._map_args(fn)
-        self_copy = TCFunction(args, ret, self.name)
-        self_copy.delay = self.delay.map(fn)
-        return fn(self_copy)
+class TRPC(Type):
+    """
+    A type of a remote call.
+
+    :ivar ret: (:class:`Type`)
+        return type
+    :ivar service: (int) RPC service number
+    """
+
+    attributes = OrderedDict()
+
+    def __init__(self, ret, service):
+        assert isinstance(ret, Type)
+        self.ret, self.service = ret, service
+
+    def find(self):
+        return self
+
+    def unify(self, other):
+        if isinstance(other, TRPC) and \
+                self.service == other.service:
+            self.ret.unify(other.ret)
+        elif isinstance(other, TVar):
+            other.unify(self)
+        else:
+            raise UnificationError(self, other)
+
+    def fold(self, accum, fn):
+        accum = self.ret.fold(accum, fn)
+        return fn(accum, self)
+
+    def __repr__(self):
+        return "artiq.compiler.types.TRPC({})".format(repr(self.ret))
+
+    def __eq__(self, other):
+        return isinstance(other, TRPC) and \
+                self.service == other.service
+
+    def __ne__(self, other):
+        return not (self == other)
+
+    def __hash__(self):
+        return hash(self.service)
 
 class TBuiltin(Type):
     """
@@ -395,9 +366,6 @@ class TBuiltin(Type):
     def fold(self, accum, fn):
         return fn(accum, self)
 
-    def map(self, fn):
-        return fn(self)
-
     def __repr__(self):
         return "artiq.compiler.types.{}({})".format(type(self).__name__, repr(self.name))
 
@@ -490,9 +458,6 @@ class TValue(Type):
     def fold(self, accum, fn):
         return fn(accum, self)
 
-    def map(self, fn):
-        return fn(self)
-
     def __repr__(self):
         return "artiq.compiler.types.TValue(%s)" % repr(self.value)
 
@@ -543,10 +508,6 @@ class TDelay(Type):
         # delay types do not participate in folding
         pass
 
-    def map(self, fn):
-        # or mapping
-        return self
-
     def __eq__(self, other):
         return isinstance(other, TDelay) and \
                 (self.duration == other.duration and \
@@ -570,18 +531,6 @@ def TFixedDelay(duration):
     return TDelay(duration, None)
 
 
-def instantiate(typ):
-    tvar_map = dict()
-    def mapper(typ):
-        typ = typ.find()
-        if is_var(typ):
-            if typ not in tvar_map:
-                tvar_map[typ] = TVar()
-            return tvar_map[typ]
-        return typ
-
-    return typ.map(mapper)
-
 def is_var(typ):
     return isinstance(typ.find(), TVar)
 
@@ -616,8 +565,8 @@ def _is_pointer(typ):
 def is_function(typ):
     return isinstance(typ.find(), TFunction)
 
-def is_rpc_function(typ):
-    return isinstance(typ.find(), TRPCFunction)
+def is_rpc(typ):
+    return isinstance(typ.find(), TRPC)
 
 def is_c_function(typ, name=None):
     typ = typ.find()
@@ -732,7 +681,7 @@ class TypePrinter(object):
                 return "(%s,)" % self.name(typ.elts[0], depth + 1)
             else:
                 return "(%s)" % ", ".join([self.name(typ, depth + 1) for typ in typ.elts])
-        elif isinstance(typ, (TFunction, TRPCFunction, TCFunction)):
+        elif isinstance(typ, (TFunction, TCFunction)):
             args = []
             args += [ "%s:%s" % (arg, self.name(typ.args[arg], depth + 1))
                      for arg in typ.args]
@@ -746,12 +695,12 @@ class TypePrinter(object):
             elif not (delay.is_fixed() and iodelay.is_zero(delay.duration)):
                 signature += " " + self.name(delay, depth + 1)
 
-            if isinstance(typ, TRPCFunction):
-                return "[rpc #{}]{}".format(typ.service, signature)
             if isinstance(typ, TCFunction):
                 return "[ffi {}]{}".format(repr(typ.name), signature)
             elif isinstance(typ, TFunction):
                 return signature
+        elif isinstance(typ, TRPC):
+            return "[rpc #{}](...)->{}".format(typ.service, self.name(typ.ret, depth + 1))
         elif isinstance(typ, TBuiltinFunction):
             return "<function {}>".format(typ.name)
         elif isinstance(typ, (TConstructor, TExceptionConstructor)):
diff --git a/artiq/compiler/validators/monomorphism.py b/artiq/compiler/validators/monomorphism.py
index f30ac5288..0911deb40 100644
--- a/artiq/compiler/validators/monomorphism.py
+++ b/artiq/compiler/validators/monomorphism.py
@@ -30,7 +30,7 @@ class MonomorphismValidator(algorithm.Visitor):
         super().generic_visit(node)
 
         if isinstance(node, asttyped.commontyped):
-            if types.is_polymorphic(node.type) and not types.is_rpc_function(node.type):
+            if types.is_polymorphic(node.type):
                 note = diagnostic.Diagnostic("note",
                     "the expression has type {type}",
                     {"type": types.TypePrinter().name(node.type)},
diff --git a/artiq/coredevice/comm_generic.py b/artiq/coredevice/comm_generic.py
index b099acec0..35ef9b3ce 100644
--- a/artiq/coredevice/comm_generic.py
+++ b/artiq/coredevice/comm_generic.py
@@ -3,6 +3,7 @@ import logging
 import traceback
 from enum import Enum
 from fractions import Fraction
+from collections import namedtuple
 
 from artiq.coredevice import exceptions
 from artiq.language.core import int as wrapping_int
@@ -62,6 +63,9 @@ class RPCReturnValueError(ValueError):
     pass
 
 
+RPCKeyword = namedtuple('RPCKeyword', ['name', 'value'])
+
+
 class CommGeneric:
     def __init__(self):
         self._read_type = self._write_type = None
@@ -229,7 +233,8 @@ class CommGeneric:
             raise UnsupportedDevice("Unsupported runtime ID: {}"
                                     .format(runtime_id))
         gateware_version = self._read_chunk(self._read_length).decode("utf-8")
-        if gateware_version != software_version:
+        if gateware_version != software_version and \
+                gateware_version + ".dirty" != software_version:
             logger.warning("Mismatch between gateware (%s) "
                            "and software (%s) versions",
                            gateware_version, software_version)
@@ -298,7 +303,6 @@ class CommGeneric:
         logger.debug("running kernel")
 
     _rpc_sentinel = object()
-    _rpc_undefined = object()
 
     # See session.c:{send,receive}_rpc_value and llvm_ir_generator.py:_rpc_tag.
     def _receive_rpc_value(self, object_map):
@@ -332,27 +336,23 @@ class CommGeneric:
             stop  = self._receive_rpc_value(object_map)
             step  = self._receive_rpc_value(object_map)
             return range(start, stop, step)
-        elif tag == "o":
-            present = self._read_int8()
-            if present:
-                return self._receive_rpc_value(object_map)
-            else:
-                return self._rpc_undefined
+        elif tag == "k":
+            name  = self._read_string()
+            value = self._receive_rpc_value(object_map)
+            return RPCKeyword(name, value)
         elif tag == "O":
             return object_map.retrieve(self._read_int32())
         else:
             raise IOError("Unknown RPC value tag: {}".format(repr(tag)))
 
     def _receive_rpc_args(self, object_map, defaults):
-        args = []
-        default_arg_num = 0
+        args, kwargs = [], {}
         while True:
             value = self._receive_rpc_value(object_map)
             if value is self._rpc_sentinel:
-                return args
-            elif value is self._rpc_undefined:
-                args.append(defaults[default_arg_num])
-                default_arg_num += 1
+                return args, kwargs
+            elif isinstance(value, RPCKeyword):
+                kwargs[value.name] = value.value
             else:
                 args.append(value)
 
@@ -443,13 +443,13 @@ class CommGeneric:
         else:
             service = object_map.retrieve(service_id)
 
-        arguments   = self._receive_rpc_args(object_map, service.__defaults__)
-        return_tags = self._read_bytes()
-        logger.debug("rpc service: [%d]%r %r -> %s", service_id, service, arguments, return_tags)
+        args, kwargs = self._receive_rpc_args(object_map, service.__defaults__)
+        return_tags  = self._read_bytes()
+        logger.debug("rpc service: [%d]%r %r %r -> %s", service_id, service, args, kwargs, return_tags)
 
         try:
-            result = service(*arguments)
-            logger.debug("rpc service: %d %r == %r", service_id, arguments, result)
+            result = service(*args, **kwargs)
+            logger.debug("rpc service: %d %r %r == %r", service_id, args, kwargs, result)
 
             if service_id != 0:
                 self._write_header(_H2DMsgType.RPC_REPLY)
@@ -457,7 +457,7 @@ class CommGeneric:
                 self._send_rpc_value(bytearray(return_tags), result, result, service)
                 self._write_flush()
         except Exception as exn:
-            logger.debug("rpc service: %d %r ! %r", service_id, arguments, exn)
+            logger.debug("rpc service: %d %r %r ! %r", service_id, args, kwargs, exn)
 
             self._write_header(_H2DMsgType.RPC_EXCEPTION)
 
@@ -486,7 +486,13 @@ class CommGeneric:
                 for index in range(3):
                     self._write_int64(0)
 
-                (_, (filename, line, function, _), ) = traceback.extract_tb(exn.__traceback__, 2)
+                tb = traceback.extract_tb(exn.__traceback__, 2)
+                if len(tb) == 2:
+                    (_, (filename, line, function, _), ) = tb
+                elif len(tb) == 1:
+                    ((filename, line, function, _), ) = tb
+                else:
+                    assert False
                 self._write_string(filename)
                 self._write_int32(line)
                 self._write_int32(-1) # column not known
diff --git a/artiq/runtime/session.c b/artiq/runtime/session.c
index f2f8a8881..517614e90 100644
--- a/artiq/runtime/session.c
+++ b/artiq/runtime/session.c
@@ -862,21 +862,16 @@ static int send_rpc_value(const char **tag, void **value)
             break;
         }
 
-        case 'o': { // option(inner='a)
-            struct { int8_t present; struct {} contents; } *option = *value;
+        case 'k': { // keyword(value='a)
+            struct { const char *name; struct {} contents; } *option = *value;
             void *contents = &option->contents;
 
-            if(!out_packet_int8(option->present))
+            if(!out_packet_string(option->name))
                 return 0;
 
-            // option never appears in composite types, so we don't have
+            // keyword never appears in composite types, so we don't have
             // to accurately advance *value.
-            if(option->present) {
-                return send_rpc_value(tag, &contents);
-            } else {
-                skip_rpc_value(tag);
-                break;
-            }
+            return send_rpc_value(tag, &contents);
         }
 
         case 'O': { // host object
diff --git a/artiq/test/coredevice/test_embedding.py b/artiq/test/coredevice/test_embedding.py
index 5debdb2cf..867dd5fa3 100644
--- a/artiq/test/coredevice/test_embedding.py
+++ b/artiq/test/coredevice/test_embedding.py
@@ -59,6 +59,56 @@ class DefaultArgTest(ExperimentCase):
         self.assertEqual(exp.run(), 42)
 
 
+class _RPC(EnvExperiment):
+    def build(self):
+        self.setattr_device("core")
+
+    def args(self, *args) -> TInt32:
+        return len(args)
+
+    def kwargs(self, x="", **kwargs) -> TInt32:
+        return len(kwargs)
+
+    @kernel
+    def args0(self):
+        return self.args()
+
+    @kernel
+    def args1(self):
+        return self.args("A")
+
+    @kernel
+    def args2(self):
+        return self.args("A", 1)
+
+    @kernel
+    def kwargs0(self):
+        return self.kwargs()
+
+    @kernel
+    def kwargs1(self):
+        return self.kwargs(a="A")
+
+    @kernel
+    def kwargs2(self):
+        return self.kwargs(a="A", b=1)
+
+    @kernel
+    def args1kwargs2(self):
+        return self.kwargs("X", a="A", b=1)
+
+class RPCTest(ExperimentCase):
+    def test_args(self):
+        exp = self.create(_RPC)
+        self.assertEqual(exp.args0(), 0)
+        self.assertEqual(exp.args1(), 1)
+        self.assertEqual(exp.args2(), 2)
+        self.assertEqual(exp.kwargs0(), 0)
+        self.assertEqual(exp.kwargs1(), 1)
+        self.assertEqual(exp.kwargs2(), 2)
+        self.assertEqual(exp.args1kwargs2(), 2)
+
+
 class _Payload1MB(EnvExperiment):
     def build(self):
         self.setattr_device("core")
diff --git a/artiq/test/lit/embedding/error_rpc_default_unify.py b/artiq/test/lit/embedding/error_rpc_default_unify.py
deleted file mode 100644
index 92da137b7..000000000
--- a/artiq/test/lit/embedding/error_rpc_default_unify.py
+++ /dev/null
@@ -1,15 +0,0 @@
-# RUN: %python -m artiq.compiler.testbench.embedding +diag %s >%t
-# RUN: OutputCheck %s --file-to-check=%t
-
-from artiq.language.core import *
-from artiq.language.types import *
-
-# CHECK-L: <synthesized>:1: error: cannot unify int(width='a) with str
-# CHECK-L: ${LINE:+1}: note: expanded from here while trying to infer a type for an unannotated optional argument 'x' from its default value
-def foo(x=[1,"x"]):
-    pass
-
-@kernel
-def entrypoint():
-    # CHECK-L: ${LINE:+1}: note: in function called remotely here
-    foo()