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Declarative languages are defined by the fact that in them, you don't tell the compiler how to do things. You just describe the result you want, and it's the compilers job to get there. Yes, this makes the compiler rather hard to write.

SQL is probably the primary example. Prolog is also well-known.

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For anyone who may not know, Prolog does this in a pretty interesting way. You have some declarations that make up your program, Prolog takes these and then uses them to conduct a logical proof (like in discrete math). You negate the premise (proof by contradiction), and then try to show that this will cause a contradiction (line 5 says Bob is Human, but line 42 says Bob is not Human). If you can find such a contradiction, your premise was true.

It does this by using backtracking, which is really interesting in itself if you're not familiar with it.

Also fascinating is the amazing and innovative virtual machine that drives Prolog: the Warren Abstract Machine.

See: http://wambook.sourceforge.net/

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