HaLeX: HaLeX enables modelling, manipulation and visualization of regular languages

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This library was developed in the context of a programming methodology course for undergraduate students, and as a consequence, it was defined mainly for educational purposes. Indeed, it provides a clear, efficient and concise way to define, to understand and to manipulate regular languages in Haskell. Furthermore, the construction of the complete library has been proposed as assignment projects to the students following the course. HaLeX is now being used to support this course.

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Versions [RSS] 1.1, 1.1.1, 1.2, 1.2.1, 1.2.2, 1.2.4, 1.2.5, 1.2.6
Dependencies base (>4 && <5), HUnit, mtl, QuickCheck [details]
License LicenseRef-PublicDomain
Author João Saraiva
Maintainer João Saraiva <saraiva@di.uminho.pt>
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Home page http://www.di.uminho.pt/~jas/Research/HaLeX/HaLeX.html
Source repo head: git clone git://github.com/haslab/halex.git
Uploaded by joaoSaraiva at 2017-02-10T16:49:54Z
Reverse Dependencies 1 direct, 0 indirect [details]
Executables halex
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Readme for HaLeX-1.2.6

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HaLeX: A Haskell Library to Model, Manipulate and Visualize Regular Languages


Copyright João Saraiva Department of Computer Science, University of Minho, Braga, Portugal saraiva@di.uminho.pt

Version: 1.2.6 (January, 2017)

1- What is HaleX

HaLeX is a library of datatypes and functions implemented in Haskell that allows us to model, manipulate and animate regular languages.

This library was developed in the context of a programming methodology course for undergraduate students, and as a consequence, it was defined mainly for educational purposes.

2- Features of the Library

The library provides the following features:

  • The definition of deterministic finite automata, non-deterministic finite automata, and regular expressions directly and straightforwardly in Haskell.

  • The definition of the acceptance functions for all those models.

  • The transformation from regular expressions into non-deterministic finite automata (NDFA) and from NDFA into deterministic finite automata (DFA).

  • The transformation from NDFA and DFA into regular expressions

  • The minimization of the number of states of deterministic finite automata.

  • The equivalence of regular expressions and finite automata.

  • The graphical representation of finite automata.

  • The definition of reactive finite automata.

  • The automatic animation of the acceptance function of finite automata.

    The animations are produced in an external tool: the high-quality graph visualization system GraphViz. Thus, to be able to visualize and animate regular languages, you have to install GraphViz tool.

    The GraphViz system is public domain and it is available at:


3- The HaLeX Library

The library consists of the following modules:

  • RegExp.hs -> Regular Expressions

  • Dfa.hs -> Deterministic Finite Automata (DFA)

  • Ndfa.hs -> Non Deterministic Finite Automata (NDFA)

  • RegExp2Fa.hs -> Converts Regular expressions into Finite Automata

  • RegExpAsDiGraph.hs -> Graphic Representation of Regular Expressions

  • FaAsDiGraph.hs -> Graphic Representation of Finite Automata (using GraphViz language/tools)

  • FaOperations.hs -> Operations on Finite Automata (ndfa2dfa , dfa2ndfa, unions , concats, etc)

  • FaClasses.hs -> Type Classes to overload operations

  • Minimize.hs -> Minimization of the number of states

  • Equivalence.hs -> Equivalence of Regular Expressions/Automata

  • ReactiveDfa.hs -> Reactive Finite Automata

  • Dfa2MDfa.hs -> Produces a Reactive Dfa from a DFA

  • RegExpParser.hs -> Simple Parser for Concrete Regular Expressions (Unix like notation)

  • Parser.hs -> Basic Parser Combinators

  • Main.hs -> The Main Module of halex Tool (see next section)

  • MainAnim.hs -> The Main Fomule to run Animations

  • faAnin.lefty -> A script written in lefty (one of GraphViz tools) that animates the acceptance function

4- Using HaLeX: The halex Tool

The HaLeX library includes a useful tool to manipulate and vizualize regular languages: the halex tool. This is a batch tool that can be used in Unix pipes. It accepts as input a regular expression and it produces Haskell or graphic representations (graphviz) based on finite automata.

To install the halex tool, just compile the library modules using a Haskell compiler (see file INSTALL).

4.1 The synopsis of halex is:

Usage: halex options [file] ...

List of options: -N, -n --NDFA generate Non-Deterministic Finite Automaton -D, -d --DFA generate Deterministic Finite Automaton -M, -m --MinDfa generate Minimized Deterministic Finite Automaton -E, -e --Dfa with Effects generate Reactive Deterministic Finite Automaton -G, -g --graph generate GraphViz input file -S, -s --Sync State include a Synk State In the Graph Representation -R string, -r string --regular expression=string specify regular expression -o file --output=file specify output file -h, -? --help output a brief help message

4.2 Running halex: some Examples

  • Generating a Haskell-based NDFA

    halex -N -R"('+'|'-')?d*('.')?d+"

  • Producing the postscript of the graphic representation of a NDFA

    halex -N -G -R"('+'|'-')?d*('.')?d+" | dot -Tps

  • DFA with minimal number of states, visualized with dotty (one of GraphViz tools)

    halex -D -M -G -R"('+'|'-')?d*('.')?d+" | dotty -

  • Proving one law of the algebra of regular expressions

    halex -R"a*" -R"(a+)?"

4.2.1 Running one Animation

- First, we have to configure the path in the makefile
Make_Animation (subdirectory scripts) that produces the executable
for the animation. Update the variable HaLeX_DIR with the location
of the HaLeX library oin your machine.

- The above makefile, uses the Haskell main module MainAnim.hs
(subdirectory src) which calls the GraphViz tool lefty with the
script that animates the finite automata (file faAnim.lefty in
subdirectory scripts). Edit that module and update the path of
lefty_tool constant function.

- After that we are able to produce and run the animations. For
example, we can produce the reactive finite automata as

      halex -E -M -R"('+'|'-')?d*('.')?d+"

  which generates the automaton (in this case, the minimized
  automaton, due to the use of the -M option) in the file

- The Haskell module MainAnim is the main module to run the animations.
  It imports the previouly generated GenMDfa and it produces the

  Its main function accepts as argument the sentence to be
  accepted/animated by the acceptance function and calls the lefty

- The lefty tool interprets the lefty script (faAnim.lefty), which
  produces the animations. Lefty provides the text view of the
  script. To start running the animation we have to call the
  functions provided in the lefty script:

        - fa.init()
        - fa.main()

  which initialize the lefty tool and the animation. Write these
  functions in the top frame followed by return. At this moment, a
  new window will be displayed that contains the graphic
  representation of the input. The right button of the mouse
  provides a set of operations to run the animation
  (forwards/backwards, adjusting the speed, tracking the path,

6- Lecture/Exercise Notes

I have started developing the HaLeX library in 2000 in the context of a third year course on programming methodology. This course has a working load of 24 hours of theoretical classes and another 24 hours of laboratory classes, running for 12 weeks (\ie, a semester). The theoretical classes introduce the basic concepts of regular expressions, finite automata and context-free languages. HaLeX is used to support such classes. In the laboratory (a two hour class per week) the students have to solve exercises using a computer.

I have defined eleven exercise sets (one per week), using literate Haskell, that the students have to complete. Each set of exercises defines a module of the \HaLeX\ library. Thus, at the end of the course the students have a complete documentation of all the exercises and topics covered in the course, and, also, of the HaLeX library.

The Exercise Notes are (still in Portuguese...) avaliable at the HaLeX homepage.