[Swift-devel] more data-ordered execution

[Ben Clifford]

I've been working on moving all assignments and mapper parameter 
evaluation into a data-ordered mode, rather than the present situation 
where some expressions are evaluated as they are first seen and therefore 
must be fully evaluatable at that point (without any yet-to-be-initialized 
values).

This is a problem which has been around since the dawn of Swift; and seems 
to trip people up with some regularity (knowledge of what causes this 
problem and what doesn't is somewhat contingent on knowing 'too much' 
about the innards of the Swift implementation).

I'm making steady progress, but encountering plenty of stuff on the way 
that is stopping this from being a quick fix.

Array assignments: at present, a = [1,2,3] doesn't work or mean anything 
outside of a variable declaration. so you can say:

int a[] = [1,2,3];

but not:

int a[];
a=[1,2,3];

I done some work to fix that, though I'm still a little hazy on what the 
correct semantics should be for complex data structures involving files.

Having got assignment mostly working, I then change assignment behaviour 
so that initialisations in variable declarations are compiled ot the same 
code as if their initialisation assignment was separate (so int a = 7; now 
evalautes to the code that would have been previously produced by int a; a 
= 7; )

For mapper parameters, I make a variable for each mapper parameter, which 
is then initialized (as with any variable now) in proper data-dependant 
order; and modify data node handling so that mappers are not initialized 
until all their parameter variables are closed. This is perhaps not the 
most efficient way to do things - at the moment I'm interested in getting 
correct behaviour rather than efficiency.

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