Proposal to deprecate and then drop fromJust

[Chris Allen]

    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 
> <mailto: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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