Re: Proposal to deprecate and then drop fromJust

I am strongly against this, as sometimes you *do* want to use fromJust. For example: 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. 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. IMHO removing fromJust would be a bad idea; although it may be overused there are certainly times and places where it is useful. In addition, it seems like the *real* issue is that "error" needs to give better stack traces, so that you do not need to do as Erik suggests and write " fromMaybe (error "Module.functionName : unexpected Nothing") x". But I think that by 7.12 or 7.14 this will happen, as there is work being done in this direction. It may be a good idea to change the documentation of `fromJust` to say something like "please never use this unless you really know you want to". (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...) Strong -1. On Mon, Feb 23, 2015 at 9:51 PM, Chris Allen wrote:

I'm strongly in favor of removing this so as not to confuse new people with partial functions that have impossible type signatures.

+1

On 02/23/2015 11:49 PM, Ivan Lazar Miljenovic wrote:

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On 24 February 2015 at 16:42, Erik de Castro Lopo wrote:

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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?

I've used fromJust in the past when I knew the value would be Just by construction.

That said, I could always have written my own fromJust function with a custom error message, and did switch some of them to direct pattern matches (so that at least the error message said *where* the error occurred).

So I'm undecided; maybe (no pun intended!) -0.1.

Erik

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