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Marshall

Marshall is a programming language for exact real arithmetic based on ideas from Abstract Stone Duality (ASD) and the construction of the Dedekind reals in ASD. See also Andrej Bauer's talk on Efficient computations with Dedekind reals.

The main idea of Marshall is that a real number x is given as a Dedekind cut, i.e., as a pair of predicates describing which numbers are smaller than x and which ones larger. For example, the sqrt function is defined in Marshall as

let sqrt =
  fun a : real =>
    cut x
      left  (x < 0 \/ x * x < a)
      right (x > 0 /\ x * x > a)

See example.asd for more examples.

Prerequisites

To compile Marshall you will need Ocaml, version 4.12 or later, the OCaml dune build system, the menhir parser generator, and the large number library num. The best way to obtain these is to use the OCaml package manager opam. Once you have got OPAM going, run

opam install dune menhir num

Optionally, you may install the rlwrap or ledit command-line editing wrappers. Marshall will use them automatically in the interactive mode.

Compilation

Compilation should run smoothly or not at all. Type

dune build

If all goes well, an executable marshall.exe will appear in the main directory.

Examples

There are examples in the example subdirectory. It may be instructive to look at prelude.asd.

A small Haskell implementation

In etc/haskell you can find a small Haskell implementation of real numbers that somewhat follows the ideas of Marshall. It was implemented as part of a graduate course on computable topology in 2009 at the Faculty of Computer Science and Informatics, University of Ljubljana. Of particular help might be the notes etc/haskell/notes.tex.

Compiling with MPFR

In theory Marshall can be compiled with the MPFR library, but the current configuration is not set up to use it. If you manage to do it in a nice way, please contact us. Here are some obsolete instruction that may or may not work:

  1. Install mpfr through OPAM, as well as its dependencies.
  2. Add mpfr as a dependency to dune-project.
  3. Change src/main.ml so that it uses the MPFR interval arithmetic instead of Big_num.

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