🚀 go-pugleaf

In article <517@quintus.UUCP>, ok@quintus.uucp (Richard A. O'Keefe) writes:
> I wonder whether there is any virtue in declarativeness as such, or whether
> the real issue may not be hidden dependencies within a program?

If a language is  declarative,  then it is  possible to apply  program
transformations in order to gain  efficiencies which are not otherwise
possible, or to compile for  a parallel processor  without needing  to
change  (or recompile for the  same machine, but   different number of
processors) the code.  Even if the dependencies are up front, and your
program is essentially declarative you lose  a  lot of this potential,
and it's  hard  work for the  compiler writer (as I've found   from my
PhD).  For example one  single assignment, intended  to improve  space
and time  efficiency working under   normal assumptions,  can actually
force   a   parallel  program   to run    sequentially.   Things  like
higher-order functions or dynamic variables are also very hard to sort
out  when you allow   assignment.   I/O has much  the  same  effect as
assignment.

Note that this isn't  to say that an  expert armed with an  imperative
language and the   appropriate scheduling constructs  (PAR, SEQ  etc.)
can't  write efficient  parallel  programs, but surely    the   aim of
language  designers  is  to   make   programming  easier  and  machine
independent (and that includes independent of the number of processors
available).  Declarative languages make this possible, at some cost in
compiler-writer time.

Incidentally, at Imperial College, London, they have a pure functional
language with lazy data  structures  (infinite lists),  called  HOPE+,
which  runs faster   than compiled C on a   Sun-3, at  least for  some
examples (benchmarking is very emotive so I won't go into details!).
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