<div dir="ltr">Just learning Haskell here but I also agree. Rust gives a compile error by default for non-exhaustive patterns.<div>I expect Haskell to be more secure than Rust.</div><div><br></div><div><a href="https://doc.rust-lang.org/book/match.html">https://doc.rust-lang.org/book/match.html</a><br></div></div><div class="gmail_extra"><br><div class="gmail_quote">On Sun, Nov 8, 2015 at 3:16 AM, William Yager <span dir="ltr"><<a href="mailto:will.yager@gmail.com" target="_blank">will.yager@gmail.com</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div dir="ltr">Absolutely agreed. The only time it is generally safe to write pattern matching lambdas is if the type only has a single constructor. It's very possible that someone re-factors their code so the type has multiple constructors, but forgets that they wrote some of these lambda expressions. -Wall should warn them about this.<br><div><br></div><div>--Will</div></div><div class="HOEnZb"><div class="h5"><div class="gmail_extra"><br><div class="gmail_quote">On Sat, Nov 7, 2015 at 6:10 PM, Niklas Hambüchen <span dir="ltr"><<a href="mailto:mail@nh2.me" target="_blank">mail@nh2.me</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">Hello everyone,<br>
<br>
to my dismay, a -Wall compiled program I deployed today crashed with<br>
<br>
Exception: Non-exhaustive patterns in lambda<br>
<br>
and I learned from <a href="http://dev.stephendiehl.com/hask/" rel="noreferrer" target="_blank">http://dev.stephendiehl.com/hask/</a> that<br>
<br>
A more subtle case is when implicitly pattern matching with a single<br>
"uni-pattern" in a lambda expression. The following will fail when<br>
given a Nothing.<br>
<br>
boom = \(Just a) -> something<br>
<br>
GHC can warn about these cases with the<br>
-fwarn-incomplete-uni-patterns flag.<br>
<br>
And in fact:<br>
<br>
ghci -Wall<br>
> map (\Nothing -> "ok") [Just 'a']<br>
["*** Exception: <interactive>:2:6-21: Non-exhaustive patterns in lambda<br>
> :set -fwarn-incomplete-uni-patterns<br>
> map (\Nothing -> "ok") [Just 'a']<br>
<interactive>:4:6: Warning:<br>
Pattern match(es) are non-exhaustive<br>
<br>
It really surprised me that -fwarn-incomplete-uni-patterns is not<br>
included in -Wall; I've felt really safe with my -Wall so far,<br>
especially about totality and similar things that will almost certainly<br>
lead to crashes.<br>
<br>
In an older mail from 2010<br>
(<a href="https://mail.haskell.org/pipermail/glasgow-haskell-users/2010-September/019237.html" rel="noreferrer" target="_blank">https://mail.haskell.org/pipermail/glasgow-haskell-users/2010-September/019237.html</a>)<br>
I saw Simon mention that it wasn't planned to warn about inexhaustive<br>
patterns like this.<br>
<br>
I feel like there has been a strong push towards more totality in the<br>
last years, and I would like to ask if enabling<br>
-fwarn-incomplete-uni-patterns in -Wall may be reconsidered for a coming<br>
next GHC version, or if there are still opponents to that.<br>
<br>
Niklas<br>
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