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

[Ivan Lazar Miljenovic]

On 28 December 2016 at 14:58, Simon Jakobi via Libraries
<libraries at haskell.org> wrote:
> 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

+1

> 2. Add readEither to the Prelude

-0.1 (because as you stated in your "Design Issues" section, I doubt
its usefulness).

> 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.

+1

>
>
> 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
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-- 
Ivan Lazar Miljenovic
Ivan.Miljenovic at gmail.com
http://IvanMiljenovic.wordpress.com