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

[Mario Blažević]

On 2017-01-05 11:22 AM, Kris Nuttycombe wrote:
> On Thu, Jan 5, 2017 at 7:01 AM, Phil Ruffwind <rf at rufflewind.com 
> <mailto:rf at rufflewind.com>> wrote:
>
>     Rather than deprecating a function, it would be nice to have some sort
>     of {-# PARTIAL #-} pragma to warn the user that 'read' is a partial
>     function (and similarly for 'head', 'tail' and its friends), much like
>     how GHC already warns about partial case-blocks.  That being said,
>     it's
>     probably best to delay this until there is a way to explicitly turn of
>     the warnings individually at the call site for those who want
>     warning-free code.
>
>
> This strikes me as the right avenue to pursue. While I'd personally be 
> happy to expunge partial functions from base entirely, I recognize 
> that others have different needs - and a {-# PARTIAL #-} pragma would 
> have broad utility. Coupled with a -fno-warn-partial flag, users would 
> be able to opt-in to the status quo, but the default would be to steer 
> people away from partial functions, which seems like the right thing. 
> For new users, the warnings would be educational, since for many 
> people coming from languages where many "functions" are partial the 
> idea of totality is something that needs to be learned.

I like this idea the best so far. Two more points:

- haddock should generate some warning in documentation from the {-# 
PARTIAL #-} pragma, so there should probably be an optional warning 
message, as in {-# PARTIAL "`head` expects a non-empty list" #-}
- -fno-warn-partial should be called -Wno-partial to fit the other 
warning flags

     Also, GHC already generates partiality warnings with 
-Wincomplete-patterns, they're just not on by default. Would these be 
affected? Should haddock generate warnings for incomplete patterns in 
documentation, and is there any relation between -Wno-partial and 
-Wno-incomplete-patterns?