Re: Proposal to deprecate and then drop fromJust

That's a great point! +1 to adding non-deprecation warnings to fromJust, ideally in the form of adding a warning to the re-export. This is quite nice, because then if the user really wants fromJust without a warning, they can import it from Data.Maybe.Partial. -Michael On Tue, Feb 24, 2015 at 11:21 AM, David Feuer wrote:

I think this is a great case for the proposed deprecated export feature. Instead of deprecating fromJust, we move it to Data.Maybe.Partial and deprecate its export from Prelude and Data.Maybe.

On Feb 24, 2015 2:07 PM, "Chris Allen" wrote:

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However, I think it'll require a very long deprecation cycle before actually removing it from base, as 'fromJust' is officially part of Haskell 2010, and there's probably a lot of code out there using 'fromJust'...

Terrifyingly, yes. I fired off a shell-script to cabal unpack all the libraries on Hackage last night and some cursory grepping has brought me:

1,176 instances of where fromJust is explicitly imported from Data.Maybe 7,431 instances of fromJust mentioned in a Haskell source file

I'm in favour (+1) of the deprecation (warnings) cycle proposal kicked around.

Also if anyone knows of a semantically aware (ie, can distinguish functions, imports, etc.) open source code search tool, I'd be very grateful if they'd point me in that direction.

On 02/24/2015 04:54 AM, Michael Snoyman wrote:

On Tue Feb 24 2015 at 12:47:07 PM Herbert Valerio Riedel wrote:

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On 2015-02-24 at 06:42:22 +0100, Erik de Castro Lopo wrote:

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We currently have in the Data.Maybe module from base:

fromJust :: Maybe a -> a

which newbies often find using hoogle or other search tools and is a huge trap. *Every* usage of the above can be replaced by using

fromMaybe :: a -> Maybe a -> a

which forces the user to provide a default value for the case where the Maybe a is a Nothing.

The idea would be to deprecate fromMaybe for 7.12 and remove it completely later.

I'm +1 for deprecating (or attach a warning pragma to) 'fromJust' as I've been bit by 'fromJust' myself in the past, when I was still mixing up 'fromMaybe' and 'fromJust'. Nowadays I don't reach out for 'fromJust' anymore at all.

However, I think it'll require a very long deprecation cycle before actually removing it from base, as 'fromJust' is officially part of Haskell 2010, and there's probably a lot of code out there using 'fromJust'...

Relatedly, I was thinking of a separate 'partial-functions' warning class, so we could annotate partial functions (head/last/...) in base, and have the associated warnings be tunable via a separate GHC -fwarn* flag. This could even be generalised to arbitrary warning-classes:

{-# WARNING[partial] fromJust "Use 'fromMaybe' instead" #-}

I'm sure we would come up with other useful warning-classes...

This sounds like a great idea, which I'd support fully. The idea of letting people opt-out of partiality warnings somehow would IMO address the concerns here perfectly.

Even though I'm strongly opposed to fromJust, I'm -1 on removing it any time soon due to ensuing breakage. I'm on the fence with a standard deprecation warning (since by what I just said we really *aren't* deprecating it), but a WARNING would be great.

Michael

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