Add support for binding the result of an indented sequence to a name

Author: jonathanhogg

docs/language.md

@@ -559,6 +559,25 @@ ignored. If the vector is shorter, then the the additional names will be bound
 to items wrapped around from the start again. If the vector is `null` then all
 names will be bound to `null`.
 
+A let binding may also bind one or more names to the result of evaluating an
+indented "body" sequence of expressions. For example:
+
+```flitter
+let foo=
+    !foo bar=12
+        !baz
+```
+
+A let binding of this form may only have one name (or semicolon-separated list
+of names for an unpacked vector binding) followed by an `=`, a newline and then
+an indented sequence of expressions. This *sequence let* may contain any
+multi-line sequence expressions, the same as the body of a function or a loop.
+This is particularly useful for binding nested node structures to a name.
+
+If a semicolon-separated name list is provided in a sequence let, then the
+names will be bound to values following the unpacking logic described above,
+with the vector formed from the indented body being the source vector.
+
 Names introduced with a `let` can redefine engine-supplied values, like `beat`,
 and built-ins, like `sin`.
 

src/flitter/language/grammar.lark

@@ -27,6 +27,7 @@ sequence : expressions
 expressions : expression* let_expression? -> tuple
 
 let_expression : "let" multiline_bindings sequence -> let
+               | "let" name_list "=" _NL _INDENT sequence _DEDENT sequence -> sequence_let
                | function sequence -> let_function
                | "import" name_list "from" composition _NL sequence ->  let_import
 

src/flitter/language/parser.py

@@ -98,6 +98,9 @@ def template_call(self, function, bindings, sequence):
     def let_function(self, function, sequence):
         return tree.Let((tree.PolyBinding((function.name,), function),), sequence)
 
+    def sequence_let(self, names, value, sequence):
+        return tree.Let((tree.PolyBinding(names, value),), sequence)
+
     def anonymous_function(self, parameters, body):
         return tree.Function('<anon>', parameters, body)
 

tests/test_language.py

@@ -372,6 +372,27 @@ def test_contains(self):
 !foo x=1 y=1 z=0
             """, x=Vector((4, 5, 6)), y=Vector.range(10))
 
+    def test_sequence_let(self):
+        self.assertCodeOutput(
+            """
+let foo;bar =
+    !foo x=1
+        !bar
+        !baz
+    !bar b=2
+
+!frob
+    bar
+    foo
+            """,
+            """
+!frob
+ !bar b=2
+ !foo x=1
+  !bar
+  !baz
+            """)
+
 
 class ScriptTest(utils.TestCase):
     """