Add syntactic sugar for nested for loops

Author: jonathanhogg

docs/language.md

@@ -703,6 +703,23 @@ There is actually a more convenient `polar(theta)` function that does the same
 thing as `zip(cos(theta), sin(theta))`. Arguably, it would be even neater to
 implement a clock face using `!transform rotate=` instead.
 
+Some syntactic sugar is provided to make directly nested loops more readable.
+For example:
+
+```flitter
+for x in ..N
+    for y in ..N
+        for z in ..N
+            !sphere position=x;y;z size=0.5
+```
+
+may be written as the more concise:
+
+```flitter
+for x in ..N, y in ..N, z in ..N
+    !sphere position=x;y;z size=0.5
+```
+
 Again, loops may also be used in-line in non-sequence expressions with syntax
 borrowed from Python:
 
@@ -716,6 +733,23 @@ This will evaluate to:
 !line points=0;0;1;5;2;10;3;15;4;20
 ```
 
+There is no special syntax for nested in-line loops as these can already be
+simply written as:
+
+```flitter
+let grid = (x;y) for x in ..N for y in ..N
+```
+
+It is worth noting that – following the reversed notation – these loops operate
+in the *reverse* order to nested non-inline loops, in that this example is
+equivalent to:
+
+```flitter
+let grid = ((x;y) for x in ..N) for y in ..N
+```
+
+and therefore the *last* loop is outermost.
+
 ## Function calling
 
 Functions are called in the normal way, with the name of the function followed

examples/bounce.fl

@@ -65,6 +65,5 @@ let SIZE=1080;1080
                             !light position=0 falloff=1;0;1;0 color=color radius=RADII[i]
                     -- These additional directional lights are just here to create some
                     -- reflections on the glass box that lend it more presence
-                    for x in (-1;1)
-                        for y in (-1;1)
-                            !light color=0.01 direction=x;y;0.5+0.1*y-0.05*x
+                    for x in (-1;1), y in (-1;1)
+                        !light color=0.01 direction=x;y;0.5+0.1*y-0.05*x

src/flitter/language/grammar.lark

@@ -33,10 +33,12 @@ let_expression : "let" multiline_bindings sequence -> let
 ?expression : node _NL
             | node _NL _INDENT sequence _DEDENT -> append
             | "@" atom [attribute_bindings] _NL [_INDENT sequence _DEDENT] -> template_call
-            | "for" name_list "in" conditional _NL _INDENT sequence _DEDENT -> loop
+            | "for" iterators _NL _INDENT sequence _DEDENT -> loop
             | "if" conditions ["else" _NL _INDENT sequence _DEDENT] -> if_else
             | _EOF -> export
 
+iterators : name_list "in" conditional ("," name_list "in" conditional)* -> tuple
+
 function : "func" NAME _LPAREN parameters _RPAREN _NL _INDENT sequence _DEDENT
 
 parameters : (parameter ("," parameter)*)? -> tuple

src/flitter/language/parser.py

@@ -77,6 +77,12 @@ def inline_if_else(self, then, condition, else_):
     def inline_loop(self, body, names, source):
         return tree.For(names, source, body)
 
+    def loop(self, iterators, expr):
+        while iterators:
+            expr = tree.For(iterators[-2], iterators[-1], expr)
+            iterators = iterators[:-2]
+        return expr
+
     def call(self, function, args):
         args = list(args)
         bindings = []
@@ -120,7 +126,6 @@ def anonymous_function(self, parameters, body):
     logical_or = tree.Or
     logical_xor = tree.Xor
     lookup = tree.Lookup
-    loop = tree.For
     lt = tree.LessThan
     modulo = tree.Modulo
     multiply = tree.Multiply