Proposal to deprecate and then drop fromJust

Herbert Valerio Riedel hvr at
Tue Feb 24 10:47:00 UTC 2015

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

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...


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