View source for Lucid

__TOC__

== Introduction ==

Weblogs Forum: [http://www.artima.com/forums/flat.jsp?forum=106&thread=102839 Fluid Programming in Lucid] -- good examples, and very good links.

[http://i.csc.uvic.ca/home/hei/ihei.cgi?context=%3Ctop:%3C%22lucid%22%3E%3E Lucid page] written by [http://i.csc.uvic.ca/home/hei/ihei.cgi?context=%3C%3E Bill Wadge], including an  [http://i.csc.uvic.ca/home/LucidPrimer/LPS.ise excerpt from the book "Lucid, the Dataflow Programming Language"].

[http://en.wikipedia.org/wiki/Lucid Wikipedia article] (yet a stub, but useful links).

== Details ==

The most detailed online material on the details of this language I could find: Raganswamy Jagannathan, Chris Dodd. [http://newton.cs.concordia.ca/%7egipsy/publications/jagannathan96gluguide.ps GLU programmer's guide] (downloadable as the 4th paper of [http://newton.cs.concordia.ca/%7egipsy/publications.html GIPSY Publications]). The first pages seem to discuss other problems, but it is worth of reading further, because a detailed description of Lucid's syntax and semantics is hiding inside this paper (section 3.1.2 on pages 22--38). This paper is part of the [http://newton.cs.concordia.ca/%7egipsy/ Gipsy Project Home Page] (GIPSY: A General Intensional Programming System).

== Implementation ==

=== Fast-food ===

A fast-food implementation:
* [[Media:Lucid.lhs|Lucid.lhs]]
* [[Media:Test.lhs|Test.lhs]]
written to mimick and test the following Lucid example (I have made it a little Haskell-like -- it is not original Lucid syntax, but it is not an embedding into Haskell language, either -- in fact, it is a didactic intermediate syntax):
<code>
 average n = sum n / card
 sum n = '''first''' n '''`fby`''' (sum n + '''next''' n)
 card = 1 '''`fby`''' succ card
</code>
This program computes the ''moving average'' of the input stream:
<center>
{| border=1 cellspacing=0 cellpadding=5
| style="background:#efefef;" | '''Snapshot'''
| style="background:#efefef;" | '''Input'''
| style="background:#efefef;" | '''Output'''
|- 
| 0
| <code>&nbsp;</code>
| <code> </code>
|-
| 1
| <code>10</code>
| <code>10</code>
|- 
| 2
| <code>10</code>, <code>20</code>
| <code>10</code>, <code>15</code> 
|- 
| 3
| <code>10</code>, <code>20</code>, <code>30</code>
| <code>10</code>, <code>15</code>, <code>20</code>
|- 
| 4
| <code>10</code>, <code>20</code>, <code>30</code>, <code>40</code>
| <code>10</code>, <code>15</code>, <code>20</code>, <code>25</code>
|- 
| 5
| <code>10</code>, <code>20</code>, <code>30</code>, <code>40</code>, <code>50</code>
| <code>10</code>, <code>15</code>, <code>20</code>, <code>25</code>, <code>30</code>
|}
</center>
In fact, <code>first</code>, <code>next</code>, <code>fby</code> are not necessarily primitives in Lucid, but this fast-food implementation treats them as primitives.

The above example may suggest that Lucid be necessarily synchronous -- it reacts on inputs in a one-output-for-each-input way. This is not the case: Lucid has general, unrestricted stream processing abilities (thus able to implement also asynchronous processes).

=== Arrow ===

A better implementation could be written using the [[arrow]] library available at the [http://www.haskell.org/arrows/download.html Downloads page of Arrows: A General Interface to Computation] written by [http://www.galois.com/~antony/ Antony Courtney],
[http://www.cs.nott.ac.uk/~nhn/ Henrik Nilsson] and
[http://www.soi.city.ac.uk/~ross/ Ross Paterson].

=== Comonad ===

A professional approach based on [[comonad]]s:
see [http://www.cs.ioc.ee/~tarmo/papers/aplas05.pdf The Essence of Dataflow Programming] paper written by [http://www.cs.ioc.ee/~tarmo/ Tarmo Uustalu].
Comments on this paper can be found on [http://lambda-the-ultimate.org/node/988 Lambda the Ultimate] (but the  link to the mentioned given there seems to be broken, I have given an updated one here). The links to pages introducing the concept of comonad are [http://www.cs.helsinki.fi/u/ekarttun/comonad/ Comonads and Haskell] written by [http://www.haskell.org/tmrwiki/EinarKarttunen Einar Karttunen].