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

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 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 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/0192... ) 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 _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe

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