Re: [Haskell-cafe] View patterns and warnings about overlapping or non-exhaustive patterns

[Stephan Friedrichs]

Svein Ove Aas wrote:
> [...]
> 
> For the time being, it will *work*, you just won't get useful
> warnings. Hopefully it's going to be fixed for 10.2.
> 

Hmm I don't find #2395 anywhere on
http://hackage.haskell.org/trac/ghc/milestone/6.10.2 :(

//Stephan


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Früher hieß es ja: Ich denke, also bin ich.
Heute weiß man: Es geht auch so.

 - Dieter Nuhr
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Re: [Haskell-cafe] View patterns and warnings about overlapping or non-exhaustive patterns

[Neil Mitchell]

Hi Stephan,

> I'm working on a data structure that uses Data.Sequence a lot, so views
> are important and I tried to simplify my code using view patterns.
>
> The problem is, that I keep getting warnings about both overlapping and
> non-exhaustive pattern matches. A simple test case:

http://hackage.haskell.org/trac/ghc/ticket/2395

Add yourself to the CC list if it matters to you!

Thanks

Neil
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Re: [Haskell-cafe] View patterns and warnings about overlapping or non-exhaustive patterns

[Svein Ove Aas]

On Wed, Mar 11, 2009 at 5:22 PM, Stephan Friedrichs
 wrote:
> Hi,
>
> I'm working on a data structure that uses Data.Sequence a lot, so views
> are important and I tried to simplify my code using view patterns.
>
> The problem is, that I keep getting warnings about both overlapping and
> non-exhaustive pattern matches. A simple test case:
>
The view pattern implementation is currently incomplete, specifically
in that it is unable to decide whether a pattern match using them is
overlapping or non-exhaustive. Arguably the warnings should be
suppressed instead..

For the time being, it will *work*, you just won't get useful
warnings. Hopefully it's going to be fixed for 10.2.

-- 
Svein Ove Aas
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[Haskell-cafe] View patterns and warnings about overlapping or non-exhaustive patterns

[Stephan Friedrichs]

Hi,

I'm working on a data structure that uses Data.Sequence a lot, so views
are important and I tried to simplify my code using view patterns.

The problem is, that I keep getting warnings about both overlapping and
non-exhaustive pattern matches. A simple test case:

===T.hs===
{-# LANGUAGE ViewPatterns #-}

import Data.Sequence

test :: Seq a -> Seq b -> String
test (viewl -> EmptyL) (viewl -> EmptyL) = "empty, empty"
test (viewl -> EmptyL) (viewl -> _ :< _) = "empty, non-empty"
test (viewl -> _ :< _) (viewl -> EmptyL) = "non-empty, empty"
test _ _ = "non-empty, non-empty"
==

> ghci -Wall T.hs
GHCi, version 6.10.1: http://www.haskell.org/ghc/  :? for help
Loading package ghc-prim ... linking ... done.
Loading package integer ... linking ... done.
Loading package base ... linking ... done.
[1 of 1] Compiling Main ( T.hs, interpreted )

T.hs:6:0:
Warning: Pattern match(es) are overlapped
 In the definition of `test':
 test ((viewl -> EmptyL)) ((viewl -> EmptyL)) = ...
 test ((viewl -> EmptyL)) ((viewl -> _ :< _)) = ...
 test ((viewl -> _ :< _)) ((viewl -> EmptyL)) = ...
 test _ _ = ...

T.hs:6:0:
Warning: Pattern match(es) are non-exhaustive
 In the definition of `test': Patterns not matched:
Ok, modules loaded: Main.

*Main> test empty (singleton 'a')
"empty, non-empty"
*Main> test (singleton 'b') (singleton 'a')
"non-empty, non-empty"
*Main> test (singleton 'b') empty
"non-empty, empty"
*Main> test empty empty
"empty, empty"

There are warnings about non-exhaustive and overlapping pattern matches,
but the tests show that this isn't the case. So what's the problem? I
don't want to turn off or ignore warnings.

//Stephan


-- 

Früher hieß es ja: Ich denke, also bin ich.
Heute weiß man: Es geht auch so.

 - Dieter Nuhr
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