[TYPES] Declarative vs imperative
adam at adamsmith.as
Tue Apr 23 04:57:03 EDT 2013
> It is not the job of our programs to instruct the computer.
> Rather, it is the job of the computer to execute our programs.
In the case of programming by example and related program synthesis
technologies, it really is our job to instruct the computer. We provide it
with an a possibly underdetermined set of functional constraints or
inconsistent set of training examples. The computer's job is to find any
one (of potentially many with equal scores) program, filling in the
details, that best matches the request.
I suppose one could argue that this isn't the activity of programming a
computer -- it's the activity of using some software that let's you make
queries against an implicitly defined database of possible structures that
could later be treated as executable programs (or hardware designs in other
applications). The physical machine is enacting some specific algorithm to
search the space of potential outputs and filter them by the constraints,
but details of this algorithm sink below the level of our perception in the
same way the microcode in modern CPUs is invisible even to the assembly
Whatever this activity, however, it is the hallmark of the
programmer/modeler/domain-engineer's job when using a (programming)
paradigm like answer set programming. We write documents in formal
languages that we call programs programs, and we "run" them to get output
with controllable properties that we expect. The implementers of the search
software have conspired with the hardware to offer us the illusion of
programming against an inherently nondeterministic machine in which nature
finds a way such that we only observe it guessing valid solutions.
Certainly, this is an illusion offered by abstraction levels, squinting,
and convention, but so are all of the potential levels one might program at
below it all the way down to the quantum level. In any nondeterminstic
programming setting, it's the programmer's intent to enlist support of
something outside of their own code (somewhere between a hardware random
number generator and a SAT solver) to carry out some form of "filling in
Meanwhile, I'll agree that this phenomenon isn't too relevant when
describing the nature of a C compiler (superoptimizing compilers written as
nondeterministic programs aside).
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