<div><div dir="auto">This raises a good point: enum as it currently exists is pretty bad and not well behaved !</div></div><div><div><br><div class="gmail_quote"><div dir="ltr" class="gmail_attr">On Sat, May 15, 2021 at 10:54 AM Zemyla <<a href="mailto:zemyla@gmail.com" target="_blank">zemyla@gmail.com</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left-width:1px;border-left-style:solid;padding-left:1ex;border-left-color:rgb(204,204,204)"><div dir="auto">We can't write instances of Enum for (,) and Either until we have total versions of pred and succ.</div><br><div class="gmail_quote"><div dir="ltr" class="gmail_attr">On Wed, May 12, 2021, 20:11 Carter Schonwald <<a href="mailto:carter.schonwald@gmail.com" target="_blank">carter.schonwald@gmail.com</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left-width:1px;border-left-style:solid;padding-left:1ex;border-left-color:rgb(204,204,204)"><div dir="auto">*while we could</div><div dir="auto"><br></div><div dir="auto">Perhaps we should provide these flavors via new types and thus folks could still benefit from these instances by way of deriving via ?</div><div><br><div class="gmail_quote"><div dir="ltr" class="gmail_attr">On Wed, May 12, 2021 at 6:38 PM Carter Schonwald <<a href="mailto:carter.schonwald@gmail.com" rel="noreferrer" target="_blank">carter.schonwald@gmail.com</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left-width:1px;border-left-style:solid;padding-left:1ex;border-left-color:rgb(204,204,204)"><div dir="ltr"><div>isnt there also the issue that while we good provide newtypes for various choices of product of enum instance => enum of products, there literally quite easily 3 valid instances we could concoct (and many more besides)</div><div><br></div><div>"row by row"</div><div>"column by column"</div><div>"diagonalization"</div><div>etc?</div><div>that is: each of these is just as valid?<br></div></div><br><div class="gmail_quote"><div dir="ltr" class="gmail_attr">On Wed, May 12, 2021 at 5:46 PM Edward Kmett <<a href="mailto:ekmett@gmail.com" rel="noreferrer" target="_blank">ekmett@gmail.com</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left-width:1px;border-left-style:solid;padding-left:1ex;border-left-color:rgb(204,204,204)"><div dir="auto"><span style="color:rgb(0,0,0)">Nothing indicates that minBound is >= 0.</span><div style="color:rgb(0,0,0)"><br></div><div style="color:rgb(0,0,0)">So if you want to deal with a modulus properly you need to deal with a modulus of something like maxBound - minBound + 1 (after appropriately upcasting intermediate results to a large enough type to contain that range). </div><div style="color:rgb(0,0,0)"><br></div><div style="color:rgb(0,0,0)">You also run into range issues pretty early on, and then scenarios like the instances for Float that are even wonkier.</div><div style="color:rgb(0,0,0)"><br></div><div style="color:rgb(0,0,0)">If toEnum/fromEnum went to a larger type (Integer or Natural) and we didn't have the wonkier Float instances and the like this would be a much easier sell. As it is I find myself rather uncomfortable with the shakiness of the foundations this thing rests upon.</div><br><div dir="ltr">Sent from my iPhone</div><div dir="ltr"><br><blockquote type="cite">On May 12, 2021, at 10:53 AM, Sandy Maguire <<a href="mailto:sandy@sandymaguire.me" rel="noreferrer" target="_blank">sandy@sandymaguire.me</a>> wrote:<br><br></blockquote></div><blockquote type="cite"><div dir="ltr"><div dir="ltr"><div class="gmail_quote"><div><div>Hi all,<br><br></div><div>Found myself puzzled the other day when I wanted an <span style="font-family:monospace">(Enum a, Enum b) => Enum (a, b)</span> instance, and was distraught that it didn't exist.</div><div><br></div><div>The following is a reasonable implementation:<br><br><br><span style="font-family:monospace">instance (Bounded b, Enum a, Enum b) => Enum (a, b) where<br> fromEnum (a, b) = (fromEnum (maxBound @b) + 1) * fromEnum a + fromEnum b<br> toEnum n =<br> let bound = fromEnum (maxBound @b) + 1<br> b = n `rem` bound<br> a = n `div` bound<br> in (toEnum a, toEnum b)</span><br><br></div><div>And, while we're at it, might as well add canonical instances for Either:<br><br><br><span style="font-family:monospace">instance (Bounded a, Bounded b) => Bounded (Either a b) where<br> minBound = Left minBound<br> maxBound = Right maxBound<br><br>instance (Bounded a, Enum a, Enum b) => Enum (Either a b) where<br> toEnum i =<br> let bound = fromEnum (maxBound @a) + 1<br> in case i < bound of<br> True -> Left $ toEnum i<br> False -> Right $ toEnum $ i - bound<br> fromEnum (Left a) = fromEnum a<br> fromEnum (Right b) = fromEnum b + fromEnum (maxBound @a) + 1</span><br></div><div><br></div><div>Are there any reasons these instances are missing from base?<br><br></div><div>Cheers,<br></div><div>Sandy<br></div></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left-width:1px;border-left-style:solid;padding-left:1ex;border-left-color:rgb(204,204,204)"><div dir="ltr"><div><br></div></div>
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