Proposal to deprecate and then drop fromJust

[Edward Kmett]

FWIW- I would very much support a notion of a "partiality inference" as
long as we had something like a {-# PARTIAL #-} pragma for when something
like a useful error message is masking an otherwise actual partial function
and a {-# TOTAL #-} pragma for where GADT inference is just failing to
realize some cases are impossible.

If folks wanted to go down that road, a lot of user code could benefit from
this sort of system, and I could see a nice separate class of warnings for
the issue.

It'd be a damn sight more work, but I think it'd be worth it.

This would let folks opt in or out, depending on which way we went to this
extra social contract.

-Edward


On Tue, Feb 24, 2015 at 2:32 PM, Edward Kmett <ekmett at gmail.com> wrote:

> -1
>
> I'm personally pretty strongly against removing this function on mere
> proscriptive grounds and a deprecation is effectively removal for most
> users who care about warnings.
>
> I wouldn't have voted to add it in the first place, but now that it is
> here and in wide use asking several thousand occurrences on hackage to
> change to suit your sensibilities and use more verbose alternatives seems
> to be rather brutal.
>
> That said, I'd fully support putting in strong wording in the
> documentation about the partial nature of the function.
>
> -Edward
>
>
>
>
> On Tue, Feb 24, 2015 at 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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