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

[Simon Jakobi]

read [1] is an easy way to introduce runtime exceptions into programs,
but its documentation doesn't sufficiently warn of this danger. read's
safe alternatives, Text.Read.readMaybe [2] and Text.Read.readEither
[3], are relatively unknown and too hard to find.

To guide users (particularly newcomers) away from read and towards
readMaybe and readEither, I propose to

1. Add readMaybe to the Prelude
2. Add readEither to the Prelude
3. Change the documentation for read to point out the partiality and
to recommend the above alternatives:

    The haddocks for read currently read:

    > The read function reads input from a string, which must be
completely consumed by the input process.

    I propose to add a paragraph roughly like this:

    > Note: read will throw an error if the parse fails. If there's
any uncertainty w.r.t. the shape of the input, readMaybe or readEither
should be used instead.


Name clashes:

A cursory code search on Github for readMaybe [4] reveals many
existing definitions, mostly with the same type (Read a => String ->
Maybe a) and definition. The same search for readEither [5] shows
mostly GHC forks.

Design issues:

I am somewhat doubtful about the benefit of readEither over readMaybe:
While readEither does give additional info on the kind of parse
failures, that information is encoded in a String error message, from
which it must be parsed if it is needed in the program.


As the different parts of the proposal can be implemented
independently of each other, please vote separately on each part of
the proposal.

Discussion period: 4 weeks.

Cheers,
Simon

[1] http://hackage.haskell.org/package/base-4.9.0.0/docs/Prelude.html#v:read
[2] http://hackage.haskell.org/package/base-4.9.0.0/docs/Text-Read.html#v:readMaybe
[3] http://hackage.haskell.org/package/base-4.9.0.0/docs/Text-Read.html#v:readEither
[4] https://github.com/search?l=Haskell&q=readMaybe&type=Code
[5] https://github.com/search?l=Haskell&q=readEither&type=Code