Re: [Haskell-cafe] Re: Debugging partial functions by the rules

17 Nov 2006

      dons:
...
dmhouse:
...
On 16/11/06, oleg@pobox.com  wrote:
...
And if we are absolutely positive that the value is (Just x),
we can always write
       maybe (assert False undefined) id v
It should be pointed out that Data.Maybe does export a less well-known
function, fromMaybe:
fromMaybe z = maybe z id
This can be used to make the 'maybe X id' case a bit tidier (although
technically it's not a save on characters).
How controversial would a proposal to {-# DEPRECATE fromJust #-} be, in
favour of:
Just _ = x  -- which will give you the precise line number
maybe (assert False)
maybe id
fromMaybe
It seems to me this is one cause of mysterious newbie errors we 
could easily discourage, with little harm.
Btw, I'm not seriously suggesting removing it ;) 
It could be discouraged ever so slightly in the haddocks though.

I'd encourage those worried about fromJust not to use it though, and
perhaps try:

    import Debug.Trace.Location
    fromMaybe (failure assert "my just was nothing") 
              (Nothing :: Maybe ())

    $ ./a.out
    *** Exception: A.hs:5:34-39: my just was nothing

instead of:

    fromJust (Nothing :: Maybe ())

    $ ./a.out
    *** Exception: Maybe.fromJust: Nothing

While Dana and Neil work on more tools for spotting these for us.

-- Don

Re: [Haskell-cafe] Re: Debugging partial functions by the rules

dons＠cse.unsw.edu.au