Proposal to deprecate and then drop fromJust

[David Feuer]

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" <cma at bitemyapp.com> wrote:

>  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 <hvr at gnu.org>
> wrote:
>
>> On 2015-02-24 at 06:42:22 +0100, Erik de Castro Lopo wrote:
>> > 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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