Proposal to deprecate and then drop fromJust

Konstantine Rybnikov k-bx at
Wed Feb 25 06:25:20 UTC 2015

Maybe it would make sense to incorporate fromJustNote (as in `safe`
package) and highly recommend that one in docs? It makes it easier to debug
error and to realize that it will throw an error on Nothing.
25 лют. 2015 00:36, користувач "Daniel Díaz" <dhelta.diaz at>

> Hi.
> This function has been guilty (more than once) of making my debugging
> harder by providing
> too little error information. Sure, the programmer who wrote the code
> should have made
> sure that the argument could not possibly be Nothing, but that didn't
> stopped him/her
> to use fromJust. Yes, there is always a way to make an "error" happen, but
> without
> fromJust you'll have to do so explicitly. I think we should encourage good
> practices.
> Just my point of view.
> The downside I see are the current occurrences of fromJust all around. I
> guess they are
> a huge lot. Although I would love to get rid of them, this would require
> some work (even
> if it's very little) from a lot of developers. So if we want to do this, a
> consensus
> must be built first.
> Therefore, +1 from me.
> Best regards,
> Daniel Díaz.
> On 02/24/2015 06:42 AM, Erik de Castro Lopo wrote:
>> Hi all,
>> 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.
>> Comments?
>> Erik
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