This PR adds language entries for the two types of programs that use the .lp extension; answer set programs and linear programs. They're both common, and they shouldn't be added independently to avoid widespread misclassification.

Description

Adding Answer Set Programming was first suggested in #3867, but it was difficult to assess popularity at the time because GitHub code search didn't support special characters. #3867 (comment) points out that .lp isn't necessarily exclusive to ASP, but fortunately Prolog (the most similar commonly used logic programming language) has coalesced on .pl.

#4860 discussed adding a language group for ASP (the language varies slightly across different solvers). The included grammar is quite generic, so I don't believe there's a need for specific sub-grammars/languages.

#4860 also points at the .lp is also used for "linear programs" (common human-readable input format for LP solvers). These can be told apart by looking for the required objective statement. Gurobi docs:

The first section in an LP file is the objective section. This section begins with one of the following six keywords: minimize, maximize, minimum, maximum, min, or max. Capitalization is ignored. This keyword may appear alone, or it may be immediately followed by multi-objectives, which indicates that the model contains multiple objective functions.

Answer Set Programming

Answer Set Programming is a logic programming paradigm which dates back to the 1990s. The .lp (logic program) file extension has been used for ASP programs since at least 1998, when it was used by lparse and the smodels solver.

Today, there are many ASP-focused repos on GitHub. Here are more than a hundred that opted into the #answer-set-programming topic.

Checklist:

• I am adding a new language.
  • The extension of the new language is used in hundreds of repositories on GitHub.com.
    • Search results for each extension:
      • .lp with :-: https://github.com/search?type=code&q=NOT+is%3Afork+path%3A*.lp+%3A-
        • Returns 36k files as of December 2024. I would conservatively estimate >1000 repos, assuming 36 .lp files on average.
  • I have included a real-world usage sample for all extensions added in this PR:
    • Sample source(s):
      • From Clingo:
        • 15puzzle-encoding.lp
        • onmodel-py.lp
      • From Draco:
        • apt.lp
    • Sample license(s):
      • Clingo is MIT
      • Draco is BSD 3 Clause
  • I have included a syntax highlighting grammar: https://github.com/nickswalker/asp-syntax-highlight
  • I have added a color
    • Hex value: #A9CC29
    • Rationale: A green that contrasts with the blue of Python, which is commonly used in ASP projects.
  • I have updated the heuristics to distinguish my language from others using the same extension.

Linear Programming

Linear programs are mathematical models of linearly related variables. The lp file format goes back at least 20 years as part of CPLEX.

Checklist

• I am adding a new language.
  • The extension of the new language is used in hundreds of repositories on GitHub.com.
    • Search results for each extension:
      • .lp with Subject To https://github.com/search?type=code&q=NOT+is%3Afork+path%3A*.lp+%22Subject+To%22
      • Returns 69k files as of December 2024.
  • I have included a real-world usage sample for all extensions added in this PR:
    • Sample source(s):
      • Multiobj.lp
      • diet.lp
    • Sample license(s):
      • Multiobj MIT
      • diet.lp Apache 2.0
  • I have included a syntax highlighting grammar
  • I have added a color
    • Hex value: #DD2113
    • Rationale: Used by the popular Gurobi solver package
  • I have updated the heuristics to distinguish my language from others using the same extension.