### More on I/O, effects and logic programming

I've been having some talks on #prolog about the best way to implement effects in logic programming languages and have come across some really good information, due in large part to "ski" who hangs out on #prolog.

There seem to be quite a few ways to implement effects for I/O. I'm not sure exactly at this point how many of them will be appropriate to implementing non-monotonic logic, by which I mean allowing the data store to grow/shrink.

Apparently the Monad strategy can move into the logic programming scene without too much difficulty. There are even a few papers about using monads in lambda-prolog that I've come across lately. This method has a lot going for it, as it seems to have worked fairly well in haskell and clean.

Another way is using linearity. This has a lot of appeal since linear logic has a well understood declarative semantics and it doesn't force us to use higher order constructs in order to achieve I/O. I'm not sure how this would interact with changing the data store though.

Mercury uses the linearity approach. First we declare the main predicate to be of a determinate mode, meaning that there is no way that we can have other choices. now the I/O resource is exhausted when used and we don't have to worry about backtracking over the use of the resource.

All of this new information makes the end goal of a fully declarative system where effects can take place seem more feasible.

### Managing state in Prolog monadically, using DCGs.

Prolog is a beautiful language which makes a lot of irritating rudimentary rule application and search easy. I have found it is particularly nice when trying to deal with compilers which involve rule based transformation from a source language L to a target language L'.

However, the management of these rules generally requires keeping track of a context, and this context has to be explicitly threaded through the entire application, which involves a lot of irritating and error prone sequence variables. This often leads to your code looking something a bit like this:

compile(seq(a,b),(ResultA,ResultB),S0,S2) :- compile(a,ResultA,S0,S1), compile(b,ResultB,S1,S2).
While not the worst thing, I've found it irritating and ugly, and I've made a lot of mistakes with incorrectly sequenced variables. It's much easier to see sequence made explicitly textually in the code.

While they were not designed for this task, but rather for parsing, DCGs turn out to be a convenient …

### Generating etags automatically when needed

Have you ever wanted M-. (the emacs command which finds the definition of the term under the cursor) to just "do the right thing" and go to the most current definition site, but were in a language that didn't have an inferior process set-up to query about source locations correctly (as is done in lisp, ocaml and some other languages with sophisticated emacs interfaces)?

Well, fret no more. Here is an approach that will let you save the appropriate files and regenerate your TAGS file automatically when things change assuring that M-. takes you to the appropriate place.

You will have to reset the tags-table-list or set it when you first use M-. and you'll want to change the language given to find and etags in the 'create-prolog-tags function (as you're probably not using prolog), but otherwise it shouldn't require much customisation.

And finally, you will need to run etags once manually, or run 'M-x create-prolog-tags' in order to get the initia…

### Formalisation of Tables in a Dependent Language

I've had an idea kicking about in my head for a while of making query plans explicit in SQL in such a way that one can be assured that the query plan corresponds to the SQL statement desired. The idea is something like a Curry-Howard in a relational setting. One could infer the plan from the SQL, the SQL from the plan, or do a sort of "type-checking" to make sure that the plan corresponds to the SQL.

The devil is always in the details however. When I started looking at the primitives that I would need, it turns out that the low level table joining operations are actually not that far from primitive SQL statement themselves. I decided to go ahead and formalise some of what would be necessary in Agda in order get a better feel for the types of objects I would need and the laws which would be required to demonstrate that a plan corresponded with a statement.

Dependent types are very powerful and give you plenty of rope to hang yourself. It's always something of…