[Haskell-cafe] More death to partial functions: Should -fwarn-incomplete-uni-patterns be enabled with -Wall?

[Apostolis Xekoukoulotakis]

Just learning Haskell here but I also agree. Rust gives a compile error by
default for non-exhaustive patterns.
I expect Haskell to be more secure than Rust.

https://doc.rust-lang.org/book/match.html

On Sun, Nov 8, 2015 at 3:16 AM, William Yager <will.yager at gmail.com> wrote:

> 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.
>
> --Will
>
> On Sat, Nov 7, 2015 at 6:10 PM, Niklas Hambüchen <mail at nh2.me> wrote:
>
>> Hello everyone,
>>
>> to my dismay, a -Wall compiled program I deployed today crashed with
>>
>>   Exception: Non-exhaustive patterns in lambda
>>
>> and I learned from http://dev.stephendiehl.com/hask/ that
>>
>>   A more subtle case is when implicitly pattern matching with a single
>>   "uni-pattern" in a lambda expression. The following will fail when
>>   given a Nothing.
>>
>>     boom = \(Just a) -> something
>>
>>   GHC can warn about these cases with the
>>   -fwarn-incomplete-uni-patterns flag.
>>
>> And in fact:
>>
>>   ghci -Wall
>>   > map (\Nothing -> "ok") [Just 'a']
>>   ["*** Exception: <interactive>:2:6-21: Non-exhaustive patterns in lambda
>>   > :set -fwarn-incomplete-uni-patterns
>>   > map (\Nothing -> "ok") [Just 'a']
>>   <interactive>:4:6: Warning:
>>       Pattern match(es) are non-exhaustive
>>
>> It really surprised me that -fwarn-incomplete-uni-patterns is not
>> included in -Wall; I've felt really safe with my -Wall so far,
>> especially about totality and similar things that will almost certainly
>> lead to crashes.
>>
>> In an older mail from 2010
>> (
>> https://mail.haskell.org/pipermail/glasgow-haskell-users/2010-September/019237.html
>> )
>> I saw Simon mention that it wasn't planned to warn about inexhaustive
>> patterns like this.
>>
>> I feel like there has been a strong push towards more totality in the
>> last years, and I would like to ask if enabling
>> -fwarn-incomplete-uni-patterns in -Wall may be reconsidered for a coming
>> next GHC version, or if there are still opponents to that.
>>
>> Niklas
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>
>
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