Additional simplifier rules for Lets over Sequences

Pull any Literal out of the head of a Let Sequence and combine any 
following nested Let.

Author: jonathanhogg

src/flitter/language/tree.pyx

@@ -1497,10 +1497,18 @@ cdef class Let(Expression):
             sbody = (<Let>sbody).body
             resimplify = True
             touched = True
-        if not touched:
-            return self
         if remaining:
-            sbody = Let(tuple(remaining), sbody)
+            if type(sbody) is Sequence and type((<Sequence>sbody).expressions[0]) is Literal:
+                if len((<Sequence>sbody).expressions) > 2:
+                    sbody = Sequence(((<Sequence>sbody).expressions[0], Let(tuple(remaining), Sequence((<Sequence>sbody).expressions[1:]))))
+                else:
+                    if type((<Sequence>sbody).expressions[1]) is Let:
+                        resimplify = True
+                    sbody = Sequence(((<Sequence>sbody).expressions[0], Let(tuple(remaining), (<Sequence>sbody).expressions[1])))
+            elif touched:
+                sbody = Let(tuple(remaining), sbody)
+            else:
+                return self
         if resimplify:
             return sbody._simplify(context)
         return sbody

tests/test_language.py

@@ -111,8 +111,13 @@ def assertCodeOutput(self, code, output, with_errors=None, skip_simplifier=False
         while not isinstance(expr, Literal):
             if isinstance(expr, Let):
                 expr = expr.body
-            elif isinstance(expr, Sequence) and len(expr.expressions) == 2 and isinstance(expr.expressions[1], Export):
-                expr = expr.expressions[0]
+            elif isinstance(expr, Sequence) and len(expr.expressions) == 2:
+                if isinstance(expr.expressions[1], Export):
+                    expr = expr.expressions[0]
+                elif isinstance(expr.expressions[1], Let) and isinstance(expr.expressions[1].body, Export):
+                    expr = expr.expressions[0]
+                else:
+                    break
             else:
                 break
         if isinstance(expr, Export):

tests/test_simplifier.py

@@ -1038,6 +1038,14 @@ def test_unused_let_export(self):
                                 Let((PolyBinding(('x',), Add(Name('z'), Literal(1))),), Export()),
                                 dynamic={'z'}, unbound={'z', None})
 
+    def test_sequence_literal(self):
+        """Literals at head of a sequence body are pushed out of the let"""
+        self.assertSimplifiesTo(Let((PolyBinding(('x',), Add(Name('y'), Literal(1))),), Sequence((Literal(1), Name('x')))),
+                                Sequence((Literal(1), Let((PolyBinding(('x',), Add(Name('y'), Literal(1))),), Name('x')))), dynamic={'y'})
+        self.assertSimplifiesTo(Let((PolyBinding(('x',), Add(Name('y'), Literal(1))),), Sequence((Literal(1), Name('x'), Name('y')))),
+                                Sequence((Literal(1), Let((PolyBinding(('x',), Add(Name('y'), Literal(1))),), Sequence((Name('x'), Name('y')))))),
+                                dynamic={'y'})
+
 
 class TestCall(SimplifierTestCase):
     def test_dynamic(self):