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+
+{%- extends "base.xhtml" -%}
+
+
+
+{%- block title -%}Thue Version 2a{%- endblock -%}
+
+
+
+{%- block content %}
+<h4>Thue Version 2a</h4>
+
+<p>Git repository: <a href="/cgi-bin/cgit.cgi/esoteric">Link</a></p>
+
+<h5>1/1/2017</h5>
+
+<p><a href="http://esolangs.org/wiki/Thue" class="external">Thue</a> is an esoteric programming
+language based on unrestricted grammars. A Thue program consists of a number of rules detailing a
+sequence of symbols to replace and a sequence of symbols to replace with, and an initial state of
+the program. Applicable rules are then applied to the inital state in a random order until no more
+are applicable, at which point the program terminates.</p>
+
+<p>An example Thue program that increments a binary number surrounded by '_' characters:</p>
+<div class="precontain">
+<code>
+1_::=1++
+0_::=1
+01++::=10
+11++::=1++0
+_0::=_
+_1++::=10
+::=
+_1111111_
+</code>
+</div>
+
+<p>The before and after symbols in each rule are separated by '::=' and the list of rules is ended
+by a blank rule. Note that whitespace in rules and the inital state is NOT ignored.</p>
+
+<p>Output is handled by prefixing the right hand side with '~', which causes those symbols to go to
+stdout and the replacement in the program to be the empty string.</p>
+
+<p>The traditional Hello World program:</p>
+<div class="precontain">
+<code>
+a::=~Hello World!
+::=
+a
+</code>
+</div>
+
+<p>Input is handled by having the right hand side of a rule be ':::', which causes the left hand
+side symbols to be replaced with a line from the standard output. Unfortunately, this immediately
+causes problems.</p>
+
+<p>The following is an innocent piece of code that accepts a single line of input and does nothing
+more. Maybe.</p>
+<div class="precontain">
+<code>
+a::=:::
+::=
+a
+</code>
+</div>
+
+<p>If a string involving the letter 'a' is entered into the above program, the single input rule
+will again become applicable and another line of input will be obtained. In other words, the input
+in Thue is unescaped and allows direct code injection into a program.</p>
+
+<p>To solve this problem, I've constructed a slightly modified version of Thue that I'm calling
+version 2a. In this version, all symbols obtained through stdin are treated as different from
+ordinary symbols. Rules can refer to and manipulate symbols obtained through stdin by surrounding
+them in double quotes.</p>
+
+<p>A rule that replaces an ordinary symbol 'a' with 'abc':</p>
+<div class="precontain">
+<code>
+a::=abc
+</code>
+</div>
+
+<p>A rule that replaces the letter 'a' that was obtained from stdin with 'abc':</p>
+<div class="precontain">
+<code>
+"a"::=abc
+</code>
+</div>
+
+<p>For convenience, a number of escaped characters are also available:</p>
+<div class="precontain">
+<pre>
+\\ -> backslash
+\r -> return
+\n -> newline
+\: -> colon
+\" -> double quote
+\EOT -> end of file
+</pre>
+</div>
+
+<p>While this doesn't solve all the problems Thue has (try writing a Thue program that asks for and
+greets the user by name!) it should solve this one particular issue.</p>
+{% endblock -%}
+
+