On Mon, Feb 9, 2009 at 11:06 AM, Tillmann Rendel <span dir="ltr"><<a href="mailto:rendel@cs.au.dk">rendel@cs.au.dk</a>></span> wrote:<br><div class="gmail_quote"><blockquote class="gmail_quote" style="border-left: 1px solid rgb(204, 204, 204); margin: 0pt 0pt 0pt 0.8ex; padding-left: 1ex;">
Gregg Reynolds wrote::<div class="Ih2E3d"><br>
<blockquote class="gmail_quote" style="border-left: 1px solid rgb(204, 204, 204); margin: 0pt 0pt 0pt 0.8ex; padding-left: 1ex;">
My original question was motivated by the observation that a human reader of<br>
an expression of the form "e >>= f" , on seeing that f is constant, may pull<br>
the constant value out of f, disregard e and dispense with the application f<br>
e. <br>
</blockquote>
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While a human reader may well do that, but it would be correct or wrong depending on the definition of >>=. The same is of course true for compilers. By the way, there is no "application f e".<br>
</blockquote><div><br>I guess it would help if I got the notation right. My intended meaning was f* e, where * is the Kleisli star. Sorry about that. <br></div><blockquote class="gmail_quote" style="border-left: 1px solid rgb(204, 204, 204); margin: 0pt 0pt 0pt 0.8ex; padding-left: 1ex;">
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An example where it would be wrong to ignore e:<br>
<br>
sum ([1, 2] >>= const [21])<br>
<br>
This expression should evaluate to sum [21, 21] = 42, not sum [21] = 21.<br>
</blockquote><div><br>Sigh. I hate it when this happens. Just when I thought I had it figured out, it turns out I'm clueless. This is very enlightening and should definitely be included in any monad tutorial. Actually you don't even need "sum" and "const" to demo the point, "[1,2] >>= \x -> [21]" evals to "[21, 21]" in ghci. And I have absolutely no idea why. Very mysterious, the Kleisli star. :(<br>
<br>Back to the drawing board!<br><br>-gregg</div></div><br>