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@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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