Proposal to deprecate and then drop fromJust

[Neil Mitchell]

I'm a big fan of fromJust, so a -1 to this proposal. I'm also a fan of
adding fromLeft and fromRight, but doubt anyone is going to agree with
me. These functions have an obvious meaning, and an obvious
implementation and an obvious name - they're handy. They should be
used with caution, and documented as to their downsides, but they're
still useful.

I also wouldn't describe fromJust as "unsafe", merely that "using
fromJust might be unsafe" - and that's something you might reasonably
want to check (and could, with the Catch tool,
http://community.haskell.org/~ndm/catch/, which is now thoroughly
dead).

As to the HLint warning, I'm not a great fan of it, since it's
basically saying "use explicit error since your compiler won't give
you a stack trace". I want to do configurable packages of HLint
replacements, so having it in there is a reasonable thing. I'll think
some more on that aspect.

Thanks, Neil


On Tue, Feb 24, 2015 at 1:09 PM, Oleg Grenrus <oleg.grenrus at iki.fi> wrote:
> And I did it myself, as it was about adding one single line rule:
>
> https://github.com/ndmitchell/hlint/pull/115
>
> Let’s see what Neil thinks about :)
>
> - Oleg
>
> On 24 Feb 2015, at 12:28, Oleg Grenrus <oleg.grenrus at iki.fi> wrote:
>
>
> On 24 Feb 2015, at 12:23, Erik de Castro Lopo <mle+hs at mega-nerd.com> wrote:
>
> Possibly what we need is a pragma to mark functions partial and a warning
> flag that warns on usage of functions that have been marked partial.
>
>
> I had this in mind too, but that is very heavy solution. I’d rather start
> with making hlint warnings?
>
> foo :: Maybe Int -> Int
> foo x = if isJust x then fromJust x else 0
>
> bar :: Maybe Int -> Int
> bar x = 1 + fromJust x
>
> It warns about `foo` nonsense. It could propose to change `bar` part into
> `fromMaybe (error “I’know what I’m doing”) x`  ?
>
> - Oleg
>
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