replace Loop.{init,body}_arg with {init,body}_args

To reduce the unnecessary usage of `evaluable.Tuple`, this patch replaces the
loop init and body arg (singular) with init and body args (plural), similar to
the `args` parameter of `Evaluable`.

nutils/evaluable.py

@@ -4142,19 +4142,19 @@ class Loop(Evaluable):
     *   method ``evalf_loop_body(output, body_arg)``.
     '''
 
-    def __init__(self, loop_id: _LoopId, length: Array, init_arg: Evaluable, body_arg: Evaluable, *args, **kwargs):
+    def __init__(self, loop_id: _LoopId, length: Array, init_args: typing.Tuple[Evaluable, ...], body_args: typing.Tuple[Evaluable, ...], *args, **kwargs):
         assert isinstance(loop_id, _LoopId), f'loop_id={loop_id!r}'
         assert isinstance(length, Array), f'length={length!r}'
-        assert isinstance(init_arg, Evaluable), f'init_arg={init_arg!r}'
-        assert isinstance(body_arg, Evaluable), f'body_arg={init_arg!r}'
+        assert isinstance(init_args, tuple) and all(isinstance(arg, Evaluable) for arg in init_args), f'init_args={init_args!r}'
+        assert isinstance(body_args, tuple) and all(isinstance(arg, Evaluable) for arg in body_args), f'body_args={body_args!r}'
         self.loop_id = loop_id
         self.length = length
         self.index = _LoopIndex(loop_id, length)
-        self.init_arg = init_arg
-        self.body_arg = body_arg
-        if self.index in init_arg.arguments:
+        self.init_args = init_args
+        self.body_args = body_args
+        if any(self.index in arg.arguments for arg in init_args):
             raise ValueError('the loop initialization arguments must not depend on the index')
-        super().__init__(args=(length, init_arg, body_arg), *args, **kwargs)
+        super().__init__(args=(length, *init_args, *body_args), *args, **kwargs)
 
     @cached_property
     def _loop_block_id(self):
@@ -4172,7 +4172,7 @@ def _node(self, cache, subgraph, times, unique_loop_ids):
             return cached
 
         # Populate the `cache` with objects that do not depend on `self.index`.
-        stack = [self.init_arg, self.body_arg]
+        stack = [*self.init_args, *self.body_args]
         while stack:
             func = stack.pop()
             if self.index in func.arguments:
@@ -4199,8 +4199,8 @@ def arguments(self):
     @property
     def _loops(self):
         deps = util.IDSet([self])
-        deps |= self.init_arg._loops
-        deps |= self.body_arg._loops
+        for arg in itertools.chain(self.init_args, self.body_args):
+            deps |= arg._loops
         return deps.view()
 
 
@@ -4210,7 +4210,7 @@ def __init__(self, func: Array, shape: typing.Tuple[Array, ...], loop_id: _LoopI
         assert isinstance(func, Array) and func.dtype != bool, f'func={func!r}'
         assert func.ndim == len(shape)
         self.func = func
-        super().__init__(init_arg=Tuple(shape), body_arg=func, loop_id=loop_id, length=length, shape=shape, dtype=func.dtype)
+        super().__init__(init_args=shape, body_args=(func,), loop_id=loop_id, length=length, shape=shape, dtype=func.dtype)
 
     def _compile(self, builder):
         out, alloc_block_id = builder.new_empty_array_for_evaluable(self)
@@ -4335,7 +4335,7 @@ def __init__(self, func: Array, start: Array, stop: Array, concat_length: Array,
         if not self.func.ndim:
             raise ValueError('expected an array with at least one axis')
         shape = *func.shape[:-1], concat_length
-        super().__init__(init_arg=Tuple(shape), body_arg=Tuple((func, start, stop)), loop_id=loop_id, length=length, shape=shape, dtype=func.dtype)
+        super().__init__(init_args=shape, body_args=(func, start, stop), loop_id=loop_id, length=length, shape=shape, dtype=func.dtype)
 
     def _compile(self, builder):
         out, alloc_block_id = builder.new_empty_array_for_evaluable(self)