execution ideas

Author: linas

Design-G.md

@@ -21,5 +21,45 @@ need to run locally before getting pushed off to the GPU, and the rules
 for these could be very arcane and detailed and prone to bugs.
 
 Another possibility is to have an `(ExecOnGPULink AST)`, similar to
-`(PureExec AST)`
+`(PureExec AST)`. In fact, it could be PureExec, given extensions.
+Lets review.
 
+`PureExec` was originally invented to allow "pure execution" i.e.
+side-effect free execution of some AST by grabbing a temp AtomSpace,
+child of the current AtomSpace, and running the AST there. Any changes
+to the AtomSpace are thus made only to this temp space, and are thus
+discarded when execution completes. Thus, the execution is "pure".
+
+Next `PureExec` got used to define tail-recursive loops. The structure
+here is
+```
+ (DefineLink (DefinedProcedutre "foo")
+ (PureExec AST (DefinedProcedutre "foo")))
+```
+so the AST performs a single-step of the loop, and the
+`DefinedProcedure` recurses. The `CondLink` can be used to limit
+recursion. This is more or less just like tail recursion on scheme.
+
+Note that an earlier realization of this idea was with the behavior
+trees infrastructure, with `SequentialAndLink`. This used evaluation,
+not execution, and the `SequentialAnd` had a built-in "keep going or
+stop" character in it, that avoided the need for an explicit `CondLink`.
+It's inappropriate for execution, because the returned Value is
+generally not interpretable as a bool flag "keep going".
+
+Until `PureExec` was introduced, there wasn't anything analogous for
+execution. The `ParallelLink` and cousins run threads, but there was no
+`SerialLink` mostly because step-wise execution was not needed.
+
+The whole Threaded/Serial Atomese toolset needs to be revised to be
+cleaner. ThreadJoinLink should be marked obsolete. PureExec needs to
+be redesigned as described below.
+
+Since the initial idea of `PureExec` was purity, it could not be used
+for tail-recursive, serialized execution in the current AtomSpace: so an
+argument is added: which AtomSpace the execution
+
+TODO List
+---------
+Writiing the above, the TODO list expands:
+*