diff --git a/artiq/compiler/tools.py b/artiq/compiler/tools.py
index 7b1d0d7cf..d66e5abe9 100644
--- a/artiq/compiler/tools.py
+++ b/artiq/compiler/tools.py
@@ -1,4 +1,5 @@
 import ast
+from fractions import Fraction
 
 from artiq.language import core as core_language
 from artiq.language import units
@@ -13,6 +14,10 @@ def eval_ast(expr, symdict=dict()):
 def value_to_ast(value):
 	if isinstance(value, int):
 		return ast.Num(value)
+	elif isinstance(value, Fraction):
+		return ast.Call(func=ast.Name("Fraction", ast.Load()),
+			args=[ast.Num(value.numerator), ast.Num(value.denominator)],
+			keywords=[], starargs=None, kwargs=None)
 	elif isinstance(value, str):
 		return ast.Str(value)
 	else:
@@ -21,11 +26,35 @@ def value_to_ast(value):
 				return ast.Name(kg, ast.Load())
 		if isinstance(value, units.Quantity):
 			return ast.Call(
-					func=ast.Name("Quantity", ast.Load()),
-					args=[ast.Num(value.amount), ast.Name("base_"+value.unit.name+"_unit", ast.Load())],
-					keywords=[], starargs=None, kwargs=None)
+				func=ast.Name("Quantity", ast.Load()),
+				args=[value_to_ast(value.amount), ast.Name(value.unit.name+"_unit", ast.Load())],
+				keywords=[], starargs=None, kwargs=None)
 		return None
 
+class NotConstant(Exception):
+	pass
+
+def eval_constant(node):
+	if isinstance(node, ast.Num):
+		return node.n
+	elif isinstance(node, ast.Str):
+		return node.s
+	elif isinstance(node, ast.Call):
+		funcname = node.func.id
+		if funcname == "Fraction":
+			numerator, denominator = eval_constant(node.args[0]), eval_constant(node.args[1])
+			return Fraction(numerator, denominator)
+		elif funcname == "Quantity":
+			amount, unit = node.args
+			amount = eval_constant(amount)
+			try:
+				unit = getattr(units, unit.id)
+			except:
+				raise NotConstant
+			return units.Quantity(amount, unit)
+	else:
+		raise NotConstant
+
 def make_stmt_transformer(transformer_class):
 	def stmt_transformer(stmts, *args, **kwargs):
 		transformer = transformer_class(*args, **kwargs)