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<p>+1, and I apologize for not reading the full discussion before
suggesting the same solution. Ignore my other message please.<br>
</p>
<div class="moz-cite-prefix">On 2021-06-08 1:36 p.m., Richard
Eisenberg wrote:<br>
</div>
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cite="mid:010f0179ecb22739-f3098517-5397-4b60-be87-a28be5273242-000000@us-east-2.amazonses.com">
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I've been very much of two minds in this debate: On the one hand,
having these constraints is very practically useful. On the other,
what we're doing here is very un-Haskellish, in that we're letting
operational concerns leak into a declarative property (a
function's type). The reason we're doing this is another
un-Haskellish thing -- partiality -- but that ship has sailed.
<div class=""><br class="">
</div>
<div class="">So, may I propose a slightly different way forward?</div>
<div class=""><br class="">
</div>
<div class="">Instead of adding a HasCallStack constraint on these
functions, add an IsPartial constraint. For example:</div>
<div class=""><br class="">
</div>
<div class="">> head :: IsPartial => [a] -> a</div>
<div class=""><br class="">
</div>
<div class="">This is slightly awkward, still, because IsPartial
is a class-constraint-like-thing, but it has no parameter. But
it has a few very nice properties:</div>
<div class="">* IsPartial is declarative: it describes a property
of the function without worrying about its operation.</div>
<div class="">* If we think about the way constraints propagate,
IsPartial has the right semantics: the caller of a partial
function would itself become partial.</div>
<div class="">* We have some room in how we relate IsPartial to
HasCallStack. We could say that IsPartial is just a synonym for
HasCallStack (e.g. with type IsPartial = HasCallStack). But
perhaps better would be to somehow give users control over
whether they want the HasCallStack mechanism to be able to solve
IsPartial constraints. Maybe some users would prefer not to be
able to satisfy IsPartial constraints immediately, but instead
to require an acknowledgement in their code that they're doing
something partial. For example:</div>
<div class=""><br class="">
</div>
<div class="">partialityIsOK :: String -> (IsPartial => r)
-> r</div>
<div class="">elements xs = map (partialityIsOK "lists returned by
`group` are always non-empty" head) (group xs)</div>
<div class=""><br class="">
</div>
<div class="">The partialityIsOK function has a more involved type
than I would like, but it's very usable in practice. Of course,
such a thing only makes sense if IsPartial cannot automatically
be satisfied. Getting this to work properly probably needs an
extra language feature (maybe make IsPartial magically
built-in?), but it might provide a declarative, yet
operationally practical way forward here.</div>
<div class=""><br class="">
</div>
<div class="">Richard</div>
<div class="">
<div class="">
<div><br class="">
<blockquote type="cite" class="">
<div class="">On Jun 6, 2021, at 12:49 PM, Dominic
Steinitz <<a href="mailto:dominic@steinitz.org"
class="" moz-do-not-send="true">dominic@steinitz.org</a>>
wrote:</div>
<br class="Apple-interchange-newline">
<div class="">
<meta http-equiv="Content-Type" content="text/html;
charset=windows-1252" class="">
<div style="word-wrap: break-word; -webkit-nbsp-mode:
space; line-break: after-white-space;" class="">-1 for
the reasons Henrik has listed
<div class=""><br class="">
<div class="">
<div style="caret-color: rgb(0, 0, 0);
font-family: Helvetica; font-size: 12px;
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font-weight: normal; letter-spacing: normal;
text-align: start; text-indent: 0px;
text-transform: none; white-space: normal;
word-spacing: 0px; -webkit-text-stroke-width:
0px; text-decoration: none;" class="">Dominic
Steinitz<br class="">
<a href="mailto:dominic@steinitz.org" class=""
moz-do-not-send="true">dominic@steinitz.org</a><br
class="">
<a href="http://idontgetoutmuch.org" class=""
moz-do-not-send="true">http://idontgetoutmuch.org</a><br
class="">
Twitter: @idontgetoutmuch</div>
</div>
<div class=""><br class="">
<blockquote type="cite" class="">
<div class="">On 5 Jun 2021, at 11:10, <a
href="mailto:libraries-request@haskell.org"
class="" moz-do-not-send="true">libraries-request@haskell.org</a>
wrote:</div>
<br class="Apple-interchange-newline">
<div class="">
<meta charset="UTF-8" class="">
<span style="caret-color: rgb(0, 0, 0);
font-family: Helvetica; font-size: 12px;
font-style: normal; font-variant-caps:
normal; font-weight: normal; letter-spacing:
normal; text-align: start; text-indent: 0px;
text-transform: none; white-space: normal;
word-spacing: 0px;
-webkit-text-stroke-width: 0px;
text-decoration: none; float: none; display:
inline !important;" class="">Re: RFC: Add
HasCallStack constraint to partial Data.List</span><br
style="caret-color: rgb(0, 0, 0);
font-family: Helvetica; font-size: 12px;
font-style: normal; font-variant-caps:
normal; font-weight: normal; letter-spacing:
normal; text-align: start; text-indent: 0px;
text-transform: none; white-space: normal;
word-spacing: 0px;
-webkit-text-stroke-width: 0px;
text-decoration: none;" class="">
<span style="caret-color: rgb(0, 0, 0);
font-family: Helvetica; font-size: 12px;
font-style: normal; font-variant-caps:
normal; font-weight: normal; letter-spacing:
normal; text-align: start; text-indent: 0px;
text-transform: none; white-space: normal;
word-spacing: 0px;
-webkit-text-stroke-width: 0px;
text-decoration: none; float: none; display:
inline !important;" class=""> functions.</span></div>
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