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manual: add a section about aliasing restrictions for parameter passing; Spark inspired

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Araq
2019-05-06 14:46:48 +02:00
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doc/manual_experimental.rst
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@@ -1606,3 +1606,28 @@ validation errors:
If the taint mode is turned off, ``TaintedString`` is simply an alias for
``string``.
Aliasing restrictions in parameter passing
==========================================
**Note**: The aliasing restrictions are currently not enforced by the
implementation and need to be fleshed out futher.
"Aliasing" here means that the underlying storage locations overlap in memory
at runtime. An "output parameter" is a parameter of type ``var T``, an input
parameter is any parameter that is not of type ``var``.
1. Two output parameters should never be aliased.
2. An input and an output parameter should not be aliased.
3. An output parameter should never be aliased with a global or thread local
variable referenced by the called proc.
4. An input parameter should not be aliased with a global or thread local
variable updated by the called proc.
One problem with rules 3 and 4 is that they affect specific global or thread
local variables, but Nim's effect tracking only tracks "uses no global variable"
via ``.noSideEffect``. The rules 3 and 4 can also be approximated by a different rule:
5. A global or thread local variable (or a location derived from such a location)
can only passed to a parameter of a ``.noSideEffect`` proc.