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If this interests you, please do check out prolog. Declarative languages let you do something quite similar. In particular, they let you specify what you want t

If this interests you, please do check out prolog. Declarative languages let you do something quite similar. In particular, they let you specify what you want the computer to give you rather than what you want the computer to do. Believe it or not, a very good example of such a language which you might be familiar with is SQL.

PeterGriffin 12y ago
I don't believe it, because I use SQL every day, and I need to tell it which rows to lock when reading, whether to read them in share mode or fully, sometimes I need to help it figure out which indexes to use (of course I also have to manually set up the indexes too), when doing transactions I need to tell it how exactly to isolate the transaction, and I need to be very careful how I order my queries in order to avoid deadlocks. SQL is declarative only when you don't care about data integrity and performance. Maybe a bit better example for this kind of behavior are compilers. They're pretty "declarative" these days. You write what code you want to compile, and the compiler will pass it through a thousand transformation stages, deciding which trick to apply at every instruction, which code isn't really needed and so on. Of course, then you also need to deal with compilers getting it wrong too, so you start tweaking your code with special words like "volatile" and "restricted", mess with compiler options, and sometimes even disassemble code to see why turning the "fast" options makes your code slow. But in general, high quality compilers do this dance way better than SQL. So what's the moral here? Two things. First, calling something "declarative" doesn't really mean anything. It means the language is built on a very thick abstraction that lets the computer do more work than usual, so you can do less work. Second, thick abstractions are leaky. Works for basic cases, past that you'll still end up doing a lot of work, but now you have to fight your computer while doing it, as well. The Z3 Theorem Prover is an interesting toy, but I wouldn't trust it to do anything right in the real world. Much like most Microsoft Research projects, unfortunately.
- benjamincburns 12y ago
  So, when I say SQL, I meant ansi SQL, not PL/SQL, or any of the other various extensions of the language. Neither is purely declarative, but the prior is much closer. Also I wasn't suggesting that declarative languages are broadly useful (in fact I'd suggest quite the opposite), just that they are very interesting.
- Xixi 12y ago
  Prolog is a very different beast, in that it doesn't rely on any "thick" abstraction. It's based on resolution [1], thus the way code is executed is straightforward and unsurprising (providing you understand resolution first). If you have never tried it out you should, maybe you will like declarative languages a little bit more afterwards. [1] http://en.wikipedia.org/wiki/Resolution_%28logic%29 http://en.wikipedia.org/wiki/Resolution_%28logic%29
lelf 12y ago
Worth noting though that Prolog will do similar things only with finite domain solvers, which are all extensions to the language. Otherwise it's just a search with backtracking (read: brute-force) with well-defined semantics.
Dewie 12y ago
Prolog is the kind of language that is sold on being declarative, but ultimately you have to study how the unification algorithm works. And probably also sprinkle cuts around your code in order to not let the runtime do too much work, or maybe so that it will work correctly. Granted I've only used it in one university course (and only half of that course).

PeterGriffin 12y ago

I don't believe it, because I use SQL every day, and I need to tell it which rows to lock when reading, whether to read them in share mode or fully, sometimes I need to help it figure out which indexes to use (of course I also have to manually set up the indexes too), when doing transactions I need to tell it how exactly to isolate the transaction, and I need to be very careful how I order my queries in order to avoid deadlocks. SQL is declarative only when you don't care about data integrity and performance. Maybe a bit better example for this kind of behavior are compilers. They're pretty "declarative" these days. You write what code you want to compile, and the compiler will pass it through a thousand transformation stages, deciding which trick to apply at every instruction, which code isn't really needed and so on. Of course, then you also need to deal with compilers getting it wrong too, so you start tweaking your code with special words like "volatile" and "restricted", mess with compiler options, and sometimes even disassemble code to see why turning the "fast" options makes your code slow. But in general, high quality compilers do this dance way better than SQL. So what's the moral here? Two things. First, calling something "declarative" doesn't really mean anything. It means the language is built on a very thick abstraction that lets the computer do more work than usual, so you can do less work. Second, thick abstractions are leaky. Works for basic cases, past that you'll still end up doing a lot of work, but now you have to fight your computer while doing it, as well. The Z3 Theorem Prover is an interesting toy, but I wouldn't trust it to do anything right in the real world. Much like most Microsoft Research projects, unfortunately.

benjamincburns 12y ago
So, when I say SQL, I meant ansi SQL, not PL/SQL, or any of the other various extensions of the language. Neither is purely declarative, but the prior is much closer. Also I wasn't suggesting that declarative languages are broadly useful (in fact I'd suggest quite the opposite), just that they are very interesting.
Xixi 12y ago
Prolog is a very different beast, in that it doesn't rely on any "thick" abstraction. It's based on resolution [1], thus the way code is executed is straightforward and unsurprising (providing you understand resolution first). If you have never tried it out you should, maybe you will like declarative languages a little bit more afterwards. [1] http://en.wikipedia.org/wiki/Resolution_%28logic%29 http://en.wikipedia.org/wiki/Resolution_%28logic%29

lelf 12y ago

Worth noting though that Prolog will do similar things only with finite domain solvers, which are all extensions to the language. Otherwise it's just a search with backtracking (read: brute-force) with well-defined semantics.

Dewie 12y ago

Prolog is the kind of language that is sold on being declarative, but ultimately you have to study how the unification algorithm works. And probably also sprinkle cuts around your code in order to not let the runtime do too much work, or maybe so that it will work correctly. Granted I've only used it in one university course (and only half of that course).