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<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>Nice idea, but I’m not yet convinced that it would work
well in practice.<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>First, there’s an exponential explosion in the number of
constructors required. Anything with an exponential is worrying.<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>Second, you presumably do not want massive code duplication. So
I assume that if you start with<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal style='text-indent:36.0pt'><span style='font-size:11.0pt;
font-family:"Calibri","sans-serif";color:#1F497D'>case x of { Node i n2 n2)
-> e<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>then you’ll desugar to something like this:<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'> let
fj i n1 n2 = e <o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'> in<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'> case
x of<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'> Node1
I -> fj i Nothing Nothing<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'> Node2
I n1 -> fj I (Just n1) Nothing<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'> …
etc for other cases…<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>This doesn’t look like a win, because you end up
allocating the (Just n1) things anyway! <o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>So maybe you want to *<b>duplicate</b>* e, so you generate<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'> case
x of<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'> Node1
I -> e[Nothing/n1, Nothing/n2]<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'> Node2
I n1 -> e[Just n1/n1, Nothing/n2]<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'> …
etc for other cases…<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>But now you not only have an exponential number of branches: in
each of those branches you duplicate an arbitrarily large expression e. That
doesn’t look attractive to me.<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>Simon<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'><o:p> </o:p></span></p>
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<p class=MsoNormal><b><span style='font-size:10.0pt;font-family:"Tahoma","sans-serif"'>From:</span></b><span
style='font-size:10.0pt;font-family:"Tahoma","sans-serif"'> glasgow-haskell-users-bounces@haskell.org
[mailto:glasgow-haskell-users-bounces@haskell.org] <b>On Behalf Of </b>Louis
Wasserman<br>
<b>Sent:</b> 21 July 2009 00:20<br>
<b>To:</b> glasgow-haskell-users@haskell.org<br>
<b>Subject:</b> Unpacking multi-type constructors<o:p></o:p></span></p>
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<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>I think this is a feasible idea. Whether this is in
fact a good idea is an entirely separate question, however, and I want
feedback.<br>
<br>
Currently, I believe that an UNPACK pragma used on a multi-constructor type has
no effect. For example,<br>
<br>
data Node = Node Int {-# UNPACK #-} !(Maybe Node) {-# UNPACK #-} !(Maybe Node)<br>
<br>
is the same as<br>
<br>
data Node = Node Int !(Maybe Node) !(Maybe Node)<br>
<br>
What I'd like is for this to translate into four constructors, one for each
combination of constructors for the UNPACK'd fields:<br>
<br>
data Node = Node0 Int -- Nothing, Nothing<o:p></o:p></p>
<div style='margin-left:60.0pt'>
<p class=MsoNormal>| Node1 Int Node -- Just, Nothing<br>
| Node2 Int Node -- Nothing, Just<br>
| Node3 Int Node Node -- Just, Just<o:p></o:p></p>
</div>
<p class=MsoNormal>The primary counterargument I can think of is that this can
result in a single-constructor type being turned into a multi-constructor type,
which runs the risk of interfering with GHC's sexcellent handling of
single-constructor types.<br>
<br>
The countercounterargument is, of course, that the {-# UNPACK #-} pragma
already behaves differently depending on the single-constructorness of the
underlying type, and that the obligation is already on the programmer to check
such things.<br>
<br>
For reference, could somebody point me to the place in GHC that currently takes
care of {-# UNPACK #-} pragmas, so I could -- for instance -- figure out
whether or not there's another reason that this idea isn't in place already?<br>
<br>
Louis Wasserman<br>
<a href="mailto:wasserman.louis@gmail.com">wasserman.louis@gmail.com</a><o:p></o:p></p>
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