Fifth-generation programming language
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A fifth-generation programming language (abbreviated 5GL) is a programming language based on solving problems using constraints given to the program, rather than using an algorithm written by a programmer. Most constraint-based and logic programming languages and some declarative languages are fifth-generation languages.
While fourth-generation programming languages are designed to build specific programs, fifth-generation languages are designed to make the computer solve a given problem without the programmer. This way, the programmer only needs to worry about what problems need to be solved and what conditions need to be met, without worrying about how to implement a routine or algorithm to solve them. Fifth-generation languages are used mainly in artificial intelligence research. Prolog, OPS5, and Mercury are examples of fifth-generation languages.
In the 1980s, fifth-generation languages were considered to be the wave of the future, and some predicted that they would replace all other languages for system development, with the exception of low-level languages. Most notably, from 1982 to 1993 Japan put much research and money into their fifth generation computer systems project, hoping to design a massive computer network of machines using these tools.
However, as larger programs were built, the flaws of the approach became more apparent. It turns out that starting from a set of constraints defining a particular problem, deriving an efficient algorithm to solve it is a very difficult problem in itself. This crucial step cannot yet be automated and still requires the insight of a human programmer.
Today,[when?] fifth-generation languages are back as a possible level of computer language. A number[who?] of software vendors currently claim that their software meets the visual "programming" requirements of the 5GL concept.
Common misconception 
From time to time vendors in the application development space have claimed 5GL (5th generation language) capabilities. Most of the time they actually sell 4GLs with a higher level of automation and knowledge base. Since the 5GL awareness has dropped because the hype of the 1980s quickly faded away and the projects were eventually all dropped; this has opened doors to the vendors to re-use the term in marketing their new tools, without causing much controversy among the current generations of programmers.