Abstract
We introduce here a domain-specific language, PLEB. The Piecewise-Local Expression Builder interpreter (plebby) is an interactive system for defining, manipulating, and classifying regular formal languages. The interactive theorem-proving environment provides a generalization of regular expressions with which one can intuitively construct languages via constraints. These constraints retain their semantics upon extension to larger alphabets. The system allows one to decide implications and equalities, either at the language level (with a specified alphabet) or at the logical level (across all possible alphabets). Additionally, one can decide membership in a number of predefined classes, or arbitrary algebraic varieties. With several views of a language, including multiple algebraic structures, the system provides ample opportunity to explore and understand properties of languages.
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Notes
- 1.
At https://github.com/vvulpes0/Language-Toolkit-2/tree/develop one finds the latest unstable version of the software, and full stable releases can be found at https://hackage.haskell.org/package/language-toolkit.
- 2.
In ascii, the word boundaries are %| (left) and |% (right), while angle-brackets are represented by less-than and greater-than signs. Other equivalences are given in Table 2 on page 302.
- 3.
Available at https://hackage.haskell.org/package/finite-semigroups.
- 4.
Available at https://github.com/vvulpes0/amalgam.
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Acknowledgments
The system described in this work owes its creation to the wonderful Theory of Computation course taught by Jim Rogers at Earlham College. Further enhancements arose from work with Jeffrey Heinz at Stony Brook University. And much gratitude is extended to the anonymous reviewers for their helpful suggestions.
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Appendix
Appendix
This appendix contains selected worked exercises from various textbooks.
1.1 Exercise 2.1 from McNaughton and Papert [27]
“Decide whether each of the Figures 2.2–2.8 represents a locally testable event. Decide further whether it is locally testable in the strict sense.” We cover only figures 2.4, 2.7 and 2.8. These figures are represented by the following at &t files, named mp-2-1-4.att, mp-2-1-7.att and mp-2-1-8.att, respectively.
Here, the tool directly answers the exercises, even providing additional information regarding the factor size \(k\) for the language locally testable in the strict sense.
1.2 5.1 Exercises from Sipser [41]
In the third edition of “Introduction to the Theory of Computation”, Sipser [41] asks students to construct state diagrams for various regular languages. Exercise 1.4 focuses on intersections, 1.5 on complements, and 1.6 has assorted other languages. We select a small sample to cover here, all over the alphabet \(\varSigma =\{a,b\}\):
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1.4e \(\{w|w\text { starts with an a and has at most one b}\}\)
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1.5c \(\{w|w\text { contains neither the substrings ab nor ba}\}\)
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1.6n All strings except the empty string
As an aside, exercise 1.6 uses \(\varSigma =\{0,1\}\) in the original.
Figure 2 depicts the results. Rejecting sink states are omitted from the display and must be filled in by hand.
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Lambert, D. (2024). System Description: A Theorem-Prover for Subregular Systems: The Language Toolkit and Its Interpreter, Plebby. In: Gibbons, J., Miller, D. (eds) Functional and Logic Programming. FLOPS 2024. Lecture Notes in Computer Science, vol 14659. Springer, Singapore. https://doi.org/10.1007/978-981-97-2300-3_16
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