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From "Steve Loughran" <>
Subject Re: What is a 'declarative' language [ was Re: [VOTE] vote on general direction ...]
Date Mon, 26 Mar 2001 20:38:01 GMT

> The stuff I did find was about Prolog, so I'm in the process of reading it
> all. I'll report back when I have something concrete and useful to add to
> the discussion.

okay, I have to own up as to having done a fair bit of Prolog in the mid/lat
eighties, first as an undergrad at Edinburgh Uni, later on in cube land when
we still all thought that natural language and AI would solve everyone's
problems. I've been avoiding joining in the discussion so far, as it was as
amusing as watching VB programmers trying to explain the concept of
'inheritance' to each other, having nothing but VB as a common language.

Prolog is declarative. And it's a representation of logic (First order
predicate calculus, horn clause subset). And it's a resolution process
(depth first searching).

The resolution process is the key -in prolog, if a declaration proves false,
the system backtracks to its last choice point and tries the next path

So you could have some declaration like (I think I've forgotten some of the
syntax, but things starting with capital letters are variables which are
bound to when needed.)

delivers :- builds, tests, deploys.

builds :- compiles([source,tests])

%% compile a file passing in a list of options
compiles([H]) :- javac(H, flags([optimise(true)|debug(false)])).
compiles([H|T]) :- compiles([H]),compiles(T).

tests :- junit(options).

deploys :- deploys-internal, deploys-staging.
deploys:- failure-handler(deployment).

deploys-staging:- is-reachable("http:/staging/"),
deploys-staging:- echo("staging not reachable), fail.

deploys-internal:- is-reachable("http:/internal/"),
deploys-internal:- is-reachable("http:/internal:8080/"),

failure-handler(Stage) :- page("my-pager",["failure in",Stage]);

So, this looks very like an ant build file, with build test and deploy
targets. And the execution process is similar -we wont deploy unless the
tests suceed. But look at the deployment process, where I have two targets
(clauses) of the same name for deploys-staging. If anything goes wrong with
the first one, the one where we upload stuff, the system backtracks to the
last choice point -in this case the second clause. so it runs down there
instead. And if deployment completely goes pear shaped I would get paged

So, here is a declarative language with decision making 'implicit in the

Then look at the compile task(s). There I am using list recursion as an
alternative to direct iteration. So you can pass a list of sub-actions to
'compile' and it will do them one by one.

So even though it is declarative it still supports iteration -in a
predictable order, and branching. And it also has a very wierd way of going
wrong -where a single typo is enough to send the engine into reverse,
backtracking and then heading off down some obscure path. That and the
maintenance grief it could cause is why I am not proposing a Prolog like
language as the build tool language. Where prolog would be good is in
letting you specify the 'when things go wrong' process, but, outside of
deployment, where can things go wrong that you can handle gracefully?

Actually the list based invocation is nice. We need more list aware
languages :-)


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