🚀 go-pugleaf

I have been reading "Elements of Functional Programming", by Chris Reade,
and it has caused me to reconsider the relative merits of logic versus
functional programming languages.  Specifically, I am considering the merits
of Prolog versus ML or Miranda.

Most Prolog predicates implement functions, so why not be honest and just
use a functional language?  It is very difficult, if not impossible, to
write Prolog relations which work robustly in "all directions", i.e., solve
for their inputs given their outputs.  O.K., it's possible to write a parser
which can be run backward, converting a syntax tree to a token stream.  But
once you add a symbol table and a code generator, you might as well be
honest:  you have implemented a function mapping a token stream into object
code, it probably isn't reversible, and you probably don't even care.  The
nice thing about a functional language is that the functional dependencies
are documented within the language, rather than as extraneous "modes" or in
comments.

In Prolog, there is a wide divergence between theory and practice, but in a
functional language, the theory itself is practical.  Most practical Prolog
programs use constructs which are impossible to define within the pure Horn
clause model -- they are riddled with constructs such as cut, not, and
findall, and sometimes even assert and retract.  These detract from the
value of a Prolog program as a mathematical specification.  The only impure
constructs I've seen in functional programming are exceptions and
destructive assignment, and these are very rarely used.  In the "Elements of
Functional Programming", each of these is used exactly once in order to
implement some low-level stuff.  Every other program is defined purely
functionally, which means that it has well-defined semantics within lambda
calculus.

Furthermore, functional programming languages are strongly typed and
higher-order, which cannot be said for Prolog.  That allows me to do things
like manipulate graphs as full-fledged objects, rather than as data
scattered around in extensional relations.  I can do that in pure Prolog,
too, but it ain't easy.

Are my points mistaken, or are there some features of Prolog which are just
so wonderful that they eclipse all the bad things?  Maybe logical variables
or something?  I have quite a bit of experience with Prolog, but none
whatsoever with ML or Miranda.


Patrick Chkoreff   719-590-5983           ---  R.S.V.P. ---
{hpfcla,hplabs}!hpldola!patch