I think instead we should get rid of the error message and make
`fromJust` perform an unsafe memory reference.
data IM b a where
N :: IM 'False a
J :: a -> IM 'True a
fromJ :: IM 'True a -> a
fromJ (J a) = a
fromJust :: Maybe a -> a
fromJust m = fromJ (unsafeCoerce m)
On Tue, Feb 24, 2015 at 1:08 AM, Erik de Castro Lopo
Ivan Lazar Miljenovic wrote:
I've used fromJust in the past when I knew the value would be Just by construction.
I have too. And then a less experienced haskell dev saw that usage in code that I had written and used it elsewhere without the same assurances. I didn't notice and the Yesod webapp was deployed to production and two days later we had a report of "Exception: Maybe.fromJust: Nothing" showing up in web pages. Embarassing to say the least. We removed all usage of fromJust.
However, if *i* had used fromMaybe to begin with its *highly* likely that my colleague would have followed my lead and highly likely he would have *thought* about the Nothing case.
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).
Exactly. This:
fromMaybe (error "Module.functionName : unexpected Nothing") x
is hugely preferable to fromJust.
Erik -- ---------------------------------------------------------------------- Erik de Castro Lopo http://www.mega-nerd.com/ _______________________________________________ Libraries mailing list Libraries@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries