🚀 go-pugleaf

jdmorris@ix.netcom.com (Jason D. Morris) wrote:
>Assuming you allow 'pure' and 'clean', the compiler would then have
>to check to make sure your functions were in fact 'pure' and 'clean'.
>If the compiler could do such checking as part of its array of
>optimizations, why not just do it for every function as
>part of a general optimization step?

The problem is the compiler needs to know the "purity" of the
functions a given function calls during its compilation, or it can't
optimize the code.  You would thus have to compile all the called
functions first, and have that information available while  you
compile that given function. For reasons I hope are reasonably
obvious, this is impossible.  Conceivably you could have a compilation
system that does a first pass compilation over all of the code,
looking for purity of functions, and then a second pass to do the
actual compilation, but that's a bit beyond the capabilities of most
modern compilers...

With the proposed idea, a function that claims to pure and isn't would
be tagged with a compiler error.  This might be problematic with
templates, as instantiation for a particular type may lead to non-pure
functions being called.  Also, changing a function so that it is no
longer pure may lead to a cascade of changes to other functions, as
they would lose their purity tag.  There are probably some other
problems I may not have noticed also (libraries that depend on other,
canging, libraries?)

Before making this part of the standard, it would be nice to have some
sample evidence of how much of a performance difference it makes in
meaningful programs.  In general, you can simulate this by saving in a
variable what you would use multiple calls to get (this is not as nice
for normal programming as having tags, however, as it would require
substantially more work to change if the called function is no longer
pure, or (even worse) might not get changed when it needs to be. 

Andrew Bell
abell@mindspring.com
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