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

William Yager will.yager at gmail.com
Sun Nov 8 01:16:21 UTC 2015

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.


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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