diff --git a/artiq/compiler/infer_types.py b/artiq/compiler/infer_types.py
new file mode 100644
index 000000000..7acebb65a
--- /dev/null
+++ b/artiq/compiler/infer_types.py
@@ -0,0 +1,134 @@
+from collections import namedtuple
+from fractions import gcd
+import ast
+
+from artiq.language import units
+
+def _lcm(a, b):
+	return a*b//gcd(a, b)
+
+TBool = namedtuple("TBool", "")
+TFloat = namedtuple("TFloat", "")
+TInt = namedtuple("TInt", "nbits")
+TFractionCD = namedtuple("TFractionCD", "denominator")
+TFraction = namedtuple("TFraction", "")
+
+class TypeAnnotation:
+	def __init__(self, t, unit=None):
+		self.t = t
+		self.unit = unit
+
+	def __repr__(self):
+		r = "TypeAnnotation("+str(self.t)
+		if self.unit is not None:
+			r += " <unit:"+str(self.unit.name)+">"
+		r += ")"
+		return r
+
+	def promote(self, ta):
+		if ta.unit != self.unit:
+			raise units.DimensionError
+		if isinstance(self.t, TBool):
+			if not isinstance(ta.t, TBool):
+				raise TypeError
+		elif isinstance(self.t, TFloat):
+			if not isinstance(ta.t, TFloat):
+				raise TypeError
+		elif isinstance(self.t, TInt):
+			if isinstance(ta.t, TInt):
+				self.t = TInt(max(self.t.nbits, ta.t.nbits))
+			elif isinstance(ta.t, (TFractionCD, TFraction)):
+				self.t = ta.t
+			else:
+				raise TypeError
+		elif isinstance(self.t, TFractionCD):
+			if isinstance(ta.t, TInt):
+				pass
+			elif isinstance(ta.t, TFractionCD):
+				self.t = TFractionCD(_lcm(self.t.denominator, ta.t.denominator))
+			elif isinstance(ta.t, TFraction):
+				self.t = TFraction()
+			else:
+				raise TypeError
+		elif isinstance(self.t, TFraction):
+			if not isinstance(ta.t, (TInt, TFractionCD, TFraction)):
+				raise TypeError
+		else:
+			raise TypeError
+
+def _get_expr_type(sym_to_type, node):
+	if isinstance(node, ast.NameConstant):
+		if isinstance(node.value, bool):
+			return TypeAnnotation(TBool())
+		else:
+			raise TypeError
+	elif isinstance(node, ast.Num):
+		if isinstance(node.n, int):
+			nbits = 32 if abs(node.n) < 2**31 else 64
+			return TypeAnnotation(TInt(nbits))
+		elif isinstance(node.n, float):
+			return TypeAnnotation(TFloat())
+		else:
+			raise TypeError
+	elif isinstance(node, ast.Name):
+		return sym_to_type[node.id]
+	elif isinstance(node, ast.UnaryOp):
+		return _get_expr_type(sym_to_type, node.operand)
+	elif isinstance(node, ast.Compare):
+		return TypeAnnotation(TBool())
+	elif isinstance(node, ast.BinOp):
+		l, r = _get_expr_type(sym_to_type, node.left), _get_expr_type(sym_to_type, node.right)
+		if isinstance(node.op, (ast.Add, ast.Sub)):
+			if l.unit != r.unit:
+				raise units.DimensionError
+			if isinstance(l.t, TFloat):
+				if isinstance(r.t, (TFloat, TInt, TFraction, TFractionCD)):
+					return l
+				else:
+					raise TypeError
+			if isinstance(l.t, TInt) and isinstance(r.t, TInt):
+				return TypeAnnotation(TInt(max(l.t.nbits, r.t.nbits)), l.unit)
+			if isinstance(l.t, TInt) and isinstance(r.t, (TFloat, TFraction, TFractionCD)):
+				return r
+			if isinstance(l.t, (TFractionCD, TFraction)) and isinstance(r.t, TFloat):
+				return r
+			if isinstance(l.t, TFractionCD) and isinstance(r.t, TInt):
+				return l
+			if isinstance(l.t, TFractionCD) and isinstance(r.t, TFractionCD):
+				return TypeAnnotation(TFractionCD(_lcm(l.t.denominator, r.t.denominator)), l.unit)
+			if isinstance(l.t, TFractionCD) and isinstance(r.t, TFraction):
+				return TypeAnnotation(TFraction())
+			if isinstance(l.t, TFraction) and isinstance(r.t, (TInt, TFractionCD, TFraction)):
+				return l
+			raise TypeError
+		else:
+			raise NotImplementedError
+	elif isinstance(node, ast.Call):
+		if node.func.id == "bool":
+			return TypeAnnotation(TBool())
+		elif node.func.id == "float":
+			return TypeAnnotation(TFloat())
+		elif node.func.id == "int":
+			return TypeAnnotation(TInt(32))
+		elif node.func.id == "int64":
+			return TypeAnnotation(TInt(64))
+		elif node.func.id == "Fraction":
+			if len(node.args) == 2 and isinstance(node.args[1], ast.Num):
+				if not isinstance(node.args[1].n, int):
+					raise TypeError
+				return TypeAnnotation(TFractionCD(node.args[1].n))
+			else:
+				return TypeAnnotation(TFraction())
+		elif node.func.id == "Quantity":
+			ta = _get_expr_type(sym_to_type, node.args[0])
+			ta.unit = getattr(units, node.args[1].id)
+			return ta
+		else:
+			raise NotImplementedError
+	else:
+		raise NotImplementedError
+
+if __name__ == "__main__":
+	import sys
+	testexpr = ast.parse(sys.argv[1], mode="eval")
+	print(_get_expr_type(dict(), testexpr.body))