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<div class="moz-cite-prefix">Op 31-10-2015 om 12:10 schreef Roel van
Dijk:<br>
</div>
<blockquote
cite="mid:CABw4ky7zduT8cS_vpYgrH4WHC41Uovo1=h+oM5rb=R0t8__YWg@mail.gmail.com"
type="cite">
<div dir="ltr">Small nitpick, but I would generally put the
"exception" or "error" in the Left part of an Either and a
correct result in the Right part.
<div><br>
</div>
<div>This has some advantages. </div>
<div>1 - Right is right as opposed to wrong. Easy to remember
mnemonic.</div>
<div>2 - It fits neatly with the <font face="monospace,
monospace">Monad (Either e)</font> instance.</div>
<div><br>
</div>
<div>Roelof, a nice exercise is to first implement your<font
face="monospace, monospace"> f2 </font>function with the <font
face="monospace, monospace">Integer -> Maybe Integer</font>
type and then with <font face="monospace, monospace">Integer
-> Either String Integer</font>.</div>
<div>If you realize that both <font face="monospace, monospace">Monad
Maybe</font> and <font face="monospace, monospace">Monad
(Either e)</font> you can use almost the same code.</div>
</div>
<div class="gmail_extra"><br>
<div class="gmail_quote">2015-10-31 12:04 GMT+01:00 Darren Grant
<span dir="ltr"><<a moz-do-not-send="true"
href="mailto:dedgrant@gmail.com" target="_blank">dedgrant@gmail.com</a>></span>:<br>
<blockquote class="gmail_quote" style="margin:0 0 0
.8ex;border-left:1px #ccc solid;padding-left:1ex">
<p dir="ltr">I'd expect most Haskellers to recommend
something like,</p>
<p dir="ltr"> f2 :: Integer -> Either Integer
ErrorString</p>
<p dir="ltr">where ErrorString is some specific error value
type. (String may suffice for you.)</p>
<p dir="ltr">This is a safe general solution, but there are
many potentially more specific possibilities that might
make your program simpler depending on how this function
relates to the context it will be used in.</p>
<p dir="ltr">Cheers,<br>
Darren<br>
</p>
<div class="HOEnZb">
<div class="h5">
<div class="gmail_quote">On Oct 31, 2015 21:46, "Roelof
Wobben" <<a moz-do-not-send="true"
href="mailto:r.wobben@home.nl" target="_blank">r.wobben@home.nl</a>>
wrote:<br type="attribution">
<blockquote class="gmail_quote" style="margin:0 0 0
.8ex;border-left:1px #ccc solid;padding-left:1ex">Op
31-10-2015 om 11:40 schreef Darren Grant:<br>
<blockquote class="gmail_quote" style="margin:0 0 0
.8ex;border-left:1px #ccc solid;padding-left:1ex">
<br>
Unfrotunately the answer to this is not simple:<br>
<br>
<a moz-do-not-send="true"
href="http://stackoverflow.com/questions/4243117/how-to-catch-and-ignore-a-call-to-the-error-function"
rel="noreferrer" target="_blank">http://stackoverflow.com/questions/4243117/how-to-catch-and-ignore-a-call-to-the-error-function</a><br>
<br>
'error' more or less terminates the program in an
unreasonable way.<br>
<br>
It would be preferable for f2 to result in a type
that can contain the error result to be parsed.<br>
<br>
Cheers,<br>
Darren<br>
<br>
<br>
</blockquote>
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<br>
<br>
Here my try for the Maybe <br>
<br>
<br>
f2Maybe :: Integer -> Maybe Integer <br>
f2Maybe n <br>
| n > 0 = Nothing <br>
| n == 0 = Just 1 <br>
| even n = Just (f2Maybe ( n `div` 2) ^ 2)<br>
| odd n = Just ((f2Maybe ( n `div` 2) ^ 2) * 2) <br>
<br>
<br>
But it will not compile , the clauses for even and odd do not work.
<br>
<br>
Both maybe and either are not explained in the first 4 chapter of
the Craft book. <br>
<br>
Roelof<br>
<br>
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