Proposal: Add readMaybe (and possibly readEither) to Prelude, make Haddocks for read more cautionary

[Henrik Nilsson]

Hi,

Yuras Shumovich said:

 > >> you may be happy to not
 > >> add a new
 > >> import line but you force your library users to the newest version
 > >> of GHC
 > >> and you risk to make his programs uncompilable because it may
 > >> depend on
 > >> other packages that are not (yet) updated to the newest GHC. If you
 > >> care for
 > >> multiple versions of GHC you have to make much more cumbersome
 > >> import
 > >> statements or add multiple lines of preprocessor.
 > >>
 > > These concerns apply to any change to the Prelude.

 > Yet it is a valid argument. We should be careful with Prelude.

I can only second this. It is *very* frustrating when code fails
to compile with an slightly out-of-date version of GHC just because
someone, perhaps a beginner, perhaps mostly by accident, use a new
"nifty" add on to the prelude just because it happens to be there.

I recently had this experience with code written for a recent version
of GHC I needed to make work with GHC 7.8. I had to make rather a lot
of edits that ultimately turned out to be entirely trivial, in many 
cases because new, alternative names had been added for existing
features. When I compared the "before" and "after" versions of the
code, I didn't find the new version an iota more readable or elegant
than the old one on balance because the number of affected lines in
relative terms was small so the only real effect of the additions I had
to get rid of was to increase the cognitive burden of the reader in 
terms of a larger "Prelude vocabulary".

I wouldn't have minded half as much if the new features had been
signalled by explicit imports.

And most likely, in many cases, the new features would not have been
used at all as they were not particularly important for this particular
piece of code (in terms of increasing readability etc). Which
is not to at all to say that these features might not be very
useful for other pieces of code. In which case they could just have
just be imported explicitly.

So indeed, as Yuras said: "We should be careful with the Prelude".

/Henrik





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