<div dir="ltr"><span style="font-family:arial,sans-serif;font-size:13px">Yep, search for the standard examples of Phantom Types.</span><br><div><span style="font-family:arial,sans-serif;font-size:13px"><br></span></div><div style>
<span style="font-family:arial,sans-serif;font-size:13px">Peter</span></div></div><div class="gmail_extra"><br><br><div class="gmail_quote">On 21 December 2012 20:49, Mike Meyer <span dir="ltr"><<a href="mailto:mwm@mired.org" target="_blank">mwm@mired.org</a>></span> wrote:<br>
<blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">While thinking about my next blog posting, I realized that it was a<br>
technique similar to what Perl does with the concept of "tainted"<br>
strings.<br>
<br>
If you're not familiar with it (and I'm probably getting the details<br>
wrong, not having written any perl in decades), a string that comes<br>
from an external source is considered "tainted". String constants in<br>
the program source are not. There's a builtin that can be used to say<br>
that declares a tainted string as not being tainted (presumably, the<br>
programmer has examined it somehow to determine this). Any string<br>
operations that return strings return tainted strings if any of the<br>
strings involved were tainted. Finally, functions that are subject to<br>
data injection attacks don't work on tainted strings.<br>
<br>
It seems like the Haskell type system ought to be able to support<br>
this. I can see a couple of approaches that might work (a "tainted"<br>
monad for use on the different string types or an Either-ish datatype)<br>
that might work, but don't have the chops to decide which approach<br>
might be better - or maybe that the answer is that it depends on the<br>
goal.<br>
<br>
I did some googling for this, but didn't turn up anything that seemed<br>
promising. Lots of stuff on type checking untrusted values, but no<br>
flagging strings as untrusted. Which leaves the questions...<br>
<br>
Is this actually a sane idea?<br>
<br>
Is there already a Haskell package that does this? Possibly part of a<br>
web framework?<br>
<br>
A package for another language, or a paper discussing doing this?<br>
<br>
Thoughts on other approaches than the two I mentioned?<br>
<br>
Thoughts on the best way do to do this?<br>
<br>
Thanks,<br>
<mike<br>
<br>
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</blockquote></div><br></div>