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    copied!<p>Maybe my partial solution will help to clarify what I meant in the question.</p> <p>Let's say you have a somewhat non-trivial parser:</p> <pre><code>class MyParser &lt; Parslet::Parser rule(:dollars) { match('[0-9]').repeat(1).as(:dollars) } rule(:comma_separated_dollars) { match('[0-9]').repeat(1, 3).as(:dollars) &gt;&gt; ( match(',') &gt;&gt; match('[0-9]').repeat(3, 3).as(:dollars) ).repeat(1) } rule(:cents) { match('[0-9]').repeat(2, 2).as(:cents) } rule(:currency) { (str('$') &gt;&gt; (comma_separated_dollars | dollars) &gt;&gt; str('.') &gt;&gt; cents).as(:currency) # order is important in (comma_separated_dollars | dollars) } end </code></pre> <p>Now if we want to parse a fixed-width Currency string; this isn't the easiest thing to do. Of course, you could figure out exactly how to express the repeat expressions in terms of the final width, but it gets really unnecessarily tricky, especially in the comma separated case. Also, in my use case, currency is really just one example. I want to be able to have an easy way to come up with fixed-width definitions for adresses, zip codes, etc....</p> <p>This seems like something that should be handle-able by a PEG. I managed to write a prototype version, using <a href="https://github.com/kschiess/parslet/blob/master/lib/parslet/atoms/lookahead.rb" rel="nofollow">Lookahead</a> as a template:</p> <pre><code>class FixedWidth &lt; Parslet::Atoms::Base attr_reader :bound_parslet attr_reader :width def initialize(width, bound_parslet) # :nodoc: super() @width = width @bound_parslet = bound_parslet @error_msgs = { :premature =&gt; "Premature end of input (expected #{width} characters)", :failed =&gt; "Failed fixed width", } end def try(source, context) # :nodoc: pos = source.pos teststring = source.read(width).to_s if (not teststring) || teststring.size != width return error(source, @error_msgs[:premature]) #if not teststring &amp;&amp; teststring.size == width end fakesource = Parslet::Source.new(teststring) value = bound_parslet.apply(fakesource, context) return value if not value.error? source.pos = pos return error(source, @error_msgs[:failed]) end def to_s_inner(prec) # :nodoc: "FIXED-WIDTH(#{width}, #{bound_parslet.to_s(prec)})" end def error_tree # :nodoc: Parslet::ErrorTree.new(self, bound_parslet.error_tree) end end # now we can easily define a fixed-width currency rule: class SHPParser rule(:currency15) { FixedWidth.new(15, currency &gt;&gt; str(' ').repeat) } end </code></pre> <p>Of course, this is a pretty hacked solution. Among other things, line numbers and error messages are not good inside of a fixed width constraint. I would love to see this idea implemented in a better fashion.</p>
 

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