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    <p>If you are writing a Prolog system as an exercise, don't expect that you get too much done. After all, it is quite an effort.</p> <p>To start with, aim for the <a href="http://www.complang.tuwien.ac.at/ulrich/iso-prolog/prologue#status_quo" rel="nofollow">core of the ISO standard</a>, that is 13211-1:1995 including Cor.1:2007 and Cor.2:2012. That core is pretty much supported by many systems like: IF, SWI, YAP, B, GNU, SICStus, Jekejeke, Minerva. So while this core just covers the very basics, it will be still a lot of work to you.</p> <p>Then, you can consider what further direction you want to go. From a standard's viewpoint these are <strong>implementation specific</strong> extensions. Systems pretty much differ in the way they offer extensions, so there is no clear way to choose. The most popular systems are <a href="http://sicstus.sics.se/" rel="nofollow">SICStus</a> (commercial) and <a href="http://www.swi-prolog.org/" rel="nofollow">SWI</a> (open source). An open source system with better conformance than SWI is <a href="http://www.gprolog.org/" rel="nofollow">GNU</a>.</p> <p>You are putting a lot of quite debatable implications into your question, so let me try to sort some out:</p> <p><strong>Price of standards.</strong> ISO standards do cost something - these documents have a certain legal status - depending on your country and legislation. Freely available web documents can serve as evidence only. See for example the <a href="http://webstore.ansi.org/FindStandards.aspx?SearchString=9899:2011" rel="nofollow">C standard</a> which you get for the same prices: One official high price (USD 285) and a reduced one by INCITS (USD 30). The difference is only the cover sheet. At least, you can get the Prolog standard for a significantly reduced price.</p> <p><strong>Relevance.</strong> There is just one standard. And systems conform quite closely. Where they differ, they differ rather randomly. As an example, look at this <a href="http://www.complang.tuwien.ac.at/ulrich/iso-prolog/conformity_assessment" rel="nofollow">detailed comparison of syntax</a> which covers both reading and writing terms. Typically, such differences are reported by users who get hit by one or another difference. These differences are nowhere formally defined.</p>
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