<div dir="ltr">Were any of these objections put forward in 1993 when Mark P. Jones published on constructor classes [1] ? Everything about this follows directly and uniquely from that design.<div><br></div><div>[1]: <a href="http://web.cecs.pdx.edu/~mpj/pubs/fpca93.pdf">http://web.cecs.pdx.edu/~mpj/pubs/fpca93.pdf</a></div></div><div class="gmail_extra"><br><div class="gmail_quote">On Wed, Feb 24, 2016 at 7:38 AM, Augustsson, Lennart <span dir="ltr"><<a href="mailto:Lennart.Augustsson@sc.com" target="_blank">Lennart.Augustsson@sc.com</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d">Of course a pair is a container of two things (which can have different types).<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d">You can come up with some different definition of what it means to be a container, so that a pair is no longer a container of two things, but this is just obfuscation.<u></u><u></u></span></p>
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<p class="MsoNormal"><b><span lang="EN-US" style="font-size:10.0pt;font-family:"Tahoma","sans-serif"">From:</span></b><span lang="EN-US" style="font-size:10.0pt;font-family:"Tahoma","sans-serif""> Libraries [mailto:<a href="mailto:libraries-bounces@haskell.org" target="_blank">libraries-bounces@haskell.org</a>]
<b>On Behalf Of </b>Nathan Bouscal<br>
<b>Sent:</b> 24 February 2016 13:29<br>
<b>To:</b> Haskell Libraries<span class=""><br>
<b>Subject:</b> Re: Haskell Foldable Wats (Was: Add conspicuously missing Functor instances for tuples)<u></u><u></u></span></span></p>
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<p class="MsoNormal">On Wed, Feb 24, 2016 at 1:04 PM, Henrik Nilsson <<a href="mailto:Henrik.Nilsson@nottingham.ac.uk" target="_blank">Henrik.Nilsson@nottingham.ac.uk</a>> wrote:<u></u><u></u></p><div><div class="h5">
<p class="MsoNormal">Hi,<br>
<br>
On 02/24/2016 11:08 AM, Fumiaki Kinoshita wrote:<u></u><u></u></p>
<p class="MsoNormal">Thinking tuples of as multi-element containers is not recommended. A<br>
tuple (a, b) is, a pair of one 'a' and one 'b';<u></u><u></u></p>
<p class="MsoNormal"><br>
Which, to me, at least, very much sounds like a container of two<br>
elements?<u></u><u></u></p>
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<p class="MsoNormal">You can use essentially the same argument to say that [a] sounds like a container of any number of elements, therefore there shouldn't be anything wrong with [1, 'foo']. It's not uncommon in programming for "what a thing naively sounds
like" to be quite different from "what a thing actually is". <i>Tuples are not lists</i>.<u></u><u></u></p>
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<p class="MsoNormal">I agree that there's room for confusion, but there is room for confusion in
<i>a lot</i> of parts of Haskell, especially for people who bring a lot of preconceived notions with them. We should try to make the transition easier for them, but to me that looks a lot more like "really good error messages" and less like pointedly ignoring
the structure of types that might be confusing.<u></u><u></u></p>
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<p class="MsoNormal"> <u></u><u></u></p>
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<br>
Seriosuly, if, as a result of tuples being instances of Functor and<br>
Foldable etc., the end result is confusion to the point that<br>
many no longer understand a tuple simply as a container of a certain<br>
number of elements, then that's another case in point against<br>
this whole design. (In particular the Foldable part: while I personally<br>
don't find the functor instances particularly compelling or useful,<br>
they seem less likely to seriously bite.)<u></u><u></u></p>
<p class="MsoNormal" style="margin-left:4.8pt">as Foldable works on<br>
values pointed by the rightmost type argument, 1 should be the only<br>
reasonable result of 'length'.<br>
<br>
data TwoThree a b = TwoThree a a b b b<br>
<br>
What should 'length (TwoThree "Foo" "Bar" 0 1 2)' be?<u></u><u></u></p>
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<br>
A static type error, perhaps?<br>
<br>
(As indeed it will be unless the appropriate instances are made<br>
for TwoThree. But I am guessing we should understand TwoThree<br>
as a tuple here.)<u></u><u></u></p>
<p class="MsoNormal" style="margin-right:0cm;margin-bottom:12.0pt;margin-left:4.8pt">
Looking at only<br>
the expression, 5 might seem to make sense, but is not meaningful<br>
considering the type.<u></u><u></u></p>
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Best,<br>
<br>
/Henrik<br>
-- <br>
Henrik Nilsson<br>
School of Computer Science<br>
The University of Nottingham<br>
<a href="mailto:nhn@cs.nott.ac.uk" target="_blank">nhn@cs.nott.ac.uk</a><br>
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<br></blockquote></div><br><br clear="all"><div><br></div>-- <br><div class="gmail_signature"><div dir="ltr"><div><div dir="ltr"><div dir="ltr">Chris Allen<br><div><span style="font-size:12.8000001907349px">Currently working on </span><a href="http://haskellbook.com" target="_blank">http://haskellbook.com</a></div></div></div></div></div></div>
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