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fixes #1286; object case transitions are now sound

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Andreas Rumpf
2019-05-26 23:10:34 +02:00
parent 247fa431de
commit 49e686ab4e
14 changed files with 123 additions and 166 deletions
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doc/manual.rst
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@@ -1538,10 +1538,23 @@ the ``case`` statement: The branches in a ``case`` section may be indented too.
In the example the ``kind`` field is called the `discriminator`:idx:\: For
safety its address cannot be taken and assignments to it are restricted: The
new value must not lead to a change of the active object branch. For an object
branch switch ``system.reset`` has to be used. Also, when the fields of a
particular branch are specified during object construction, the corresponding
discriminator value must be specified as a constant expression.
new value must not lead to a change of the active object branch. Also, when the
fields of a particular branch are specified during object construction, the
corresponding discriminator value must be specified as a constant expression.
Instead of changing the active object branch, replace the old object in memory
with a new one completely:
.. code-block:: nim
var x = Node(kind: nkAdd, leftOp: Node(kind: nkInt, intVal: 4),
rightOp: Node(kind: nkInt, intVal: 2))
# change the node's contents:
x[] = NodeObj(kind: nkString, strVal: "abc")
Starting with version 0.20 ``system.reset`` cannot be used anymore to support
object branch changes as this never was completely memory safe.
As a special rule, the discriminator kind can also be bounded using a ``case``
statement. If possible values of the discriminator variable in a