name = 'prog_listswap/3'
slug = 'list-manipulation language with semantics'

description = '''\
<p>Write a DCG for manipulating list elements. The first symbol in every word is <code>[begin]</code>, followed by any sequence of "instruction" symbols from the set {<code>left</code>, <code>right</code>, <code>swap</code>}, and finally <code>[end]</code>. The starting symbol should be named <code>prog_listswap</code>.</p>

<p>The meaning of a word (program) in this language has the form <code>In-->Out</code>, mapping from input to output lists. Besides the list contents, internal states also hold the current cursor position. The <code>left</code> and <code>right</code> instructions move the cursor one step in the given direction, while the <code>swap</code> instruction swaps the element under the cursor with its left neighbor (and fails if cursor is currently pointing to the first element of the list).</p>

<pre>
?- prog_listswap([1,2,3,4]-->Out, [begin,right,swap,end], []).
  Out = [2,1,3,4].
</pre>

<p>Helper predicate (already defined):<br />
&nbsp;&nbsp;<code>swap(List,I,NewList)</code> creates <code>NewList</code> by swapping the <code>I</code>th element with its left neighbor in <code>List</code></p>'''

hint = {}