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<div class="moz-cite-prefix">This piece of reasoning, if endorsed by
the libraries team, should be recorded somewhere else than just
email. Although I am not a fan of language-design-by-history, the
chain of events is at least extremely clear and rather defensible.<br>
<br>
Jacques<br>
<br>
On 2017-08-08 12:30 PM, Edward Kmett wrote:<br>
</div>
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<div dir="ltr">There was a great deal of monomorphization that
went into the Haskell 98 standard under the nominal goal of
trying to help newcomers to the language with simpler error
messages. For more you can read old mailing list posts from that
timeframe.
<div><br>
</div>
<div>As a technical aside, (<>) is used for Monoid (and
soon will upgrade to Semigroup) not MonadPlus, so the symbols
have diverged in sentiment. Not only that, but (++) and
(<>) get mixed in existing pretty printing code a good
deal, and have different fixities, and must, lest a bunch of
code silently change semantics. This was one reason why
(<>) was added (to match existing practice in the pretty
printing libraries) rather than generalizing (++).</div>
<div><br>
</div>
<div>
<div>Once we've added (<>), generalizing (++) becomes
less urgent and actually has some cons. Notably, there is a
subset of the community that finds the current form of map
and (++) potentially useful for teaching about lists. If
they generalized to become fmap and (<|>) or
(<>) then we create a redundant notation for an
existing thing, with no roadmap for replacing one with the
other, and lose the teaching tool.<br>
</div>
</div>
<div><br>
</div>
<div>
<div>As a general guideline, the core libraries committee has
been trying to avoid introducing redundant names that have
the exact same type signature, with possibly different
fixity, but where one is exported from the class and the
other isn't, because it makes it yet another detail you have
to memorize to know which one is the one in the class and
can be refined: the types simply don't tell you.</div>
<div><br>
</div>
<div>Having one version that is strictly more general enables
one subset of the community to simply forget about the other
one and move on, and another subset that aren't fans of
rampant abstraction to use the one with more specific type
when they want to clearly signal intent.</div>
<div><br>
</div>
<div>I'd be slightly more open to discussions about eventual
removal or exile of the redundant members to an appropriate
module than generalization under that guideline, but that
isn't a hill I'd want to die on. (++) is pretty well
embedded in Haskell's DNA.<br>
</div>
</div>
<div><br>
</div>
<div><b>tl;dr</b> It happened at first because of a great wave
of monomorphization, and there is at least a defensible reason
why it hasn't generalized back in the presence of other
changes that have happened in the meantime.</div>
<div><br>
</div>
<div>-Edward</div>
</div>
<div class="gmail_extra"><br>
<div class="gmail_quote">On Mon, Jul 3, 2017 at 1:29 PM, Doug
McIlroy <span dir="ltr"><<a
href="mailto:doug@cs.dartmouth.edu" target="_blank"
moz-do-not-send="true">doug@cs.dartmouth.edu</a>></span>
wrote:<br>
<blockquote class="gmail_quote" style="margin:0 0 0
.8ex;border-left:1px #ccc solid;padding-left:1ex"><span
class="">> What do you think of making (++) the same as
(<>)<br>
<br>
</span>This seems to be a call for returning to the old
situation in<br>
which (++) was an operator of class MonadPlus. Why was that<br>
abolished in Haskell 98?<br>
<div class="HOEnZb">
<div class="h5"><br>
Doug<br>
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