Proposal to deprecate and then drop fromJust

[Ivan Lazar Miljenovic]

On 24 February 2015 at 21:06, Oliver Charles <ollie at ocharles.org.uk> wrote:
> Everyone is talking about wanting a function to do this, but I'd argue an
> inline case statement is better. Whenever you have
>
>   f (fromJust x)
>
> You can write
>
>   f (case x of Just x' -> x')

Which is even more fugly if you have more function composition after
the usage of fromJust (though I suppose LambdaCase makes it a tad
nicer).

>
> Its a little more typing, but the big wins are:
>
> - a warning that you're relying on partiality
> - a useful error message if your assumption fails, giving you the line
> number of the pattern match failure.
>
> These are both superior to fromJust, so I can see no reason for it to exist.
> If its useful in GHCI then add it to your .ghci. If you think everyone
> should have it by default, then maybe we should have a conversation about a
> "GHCI prelude".
>
> +1, death to hidden partiality!
>
> ocharles
>
> On 24 Feb 2015 09:36, "Jon Fairbairn" <jon.fairbairn at cl.cam.ac.uk> wrote:
>>
>> Andrew Gibiansky <andrew.gibiansky at gmail.com> writes:
>>
>> > I am strongly against this, as sometimes you *do* want to use fromJust.
>> > For
>> > example:
>>
>> What people want isn’t necessarily good language design.
>>
>> > 1. Quick and dirty scripting/examples where you don't care about
>> > error-proofing your work and just want it to be done quickly. In that
>> > context, fromJust is a good function.
>>
>> but you can have your own library that contains “fromMaybe
>> undefined” as well as all the other quick and dirty functions
>> useful for one-off programmes.
>>
>> > 2. You know by construction that it is a Just constructor. This is
>> > usually
>> > not a good design, but sometimes it's useful. Writing an (error
>> > "fromJust")
>> > is possible, but annoying.
>>
>> Programming languages accrue all sorts of bad practices from
>> that sort of argument. Good Haskell programming involves making
>> the type system move as many errors as possible from run time to
>> compile time.
>>
>> > (That said, I do wish that `head` and `tail` were replaced by `headMay`
>> > and
>> > `tailMay`, which you could then use with `fromJust` if you wanted to.
>> > But
>> > that's another discussion for adding things from `safe` to `base`,
>> > unrelated...)
>>
>> I’m with you on that.
>>
>> So I’m +1 on the proposal.
>>
>> --
>> Jón Fairbairn                                 Jon.Fairbairn at cl.cam.ac.uk
>>
>>
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>
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-- 
Ivan Lazar Miljenovic
Ivan.Miljenovic at gmail.com
http://IvanMiljenovic.wordpress.com