[GHC] #13013: Add readMaybe to Prelude (possibly readEither too), make Haddocks for read more cautionary

20 Dec 2016

      #13013: Add readMaybe to Prelude (possibly readEither too), make Haddocks for read
more cautionary
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           Reporter:  sjakobi        |             Owner:
               Type:  feature        |            Status:  new
  request                            |
           Priority:  normal         |         Milestone:
          Component:  Core           |           Version:
  Libraries                          |
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 = Motivation =

 `read` 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` and `Text.Read.readEither`, are
 relatively unknown and too hard to find.

 = Proposal =

 == Summary ==

 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.

 == 3. Improve the documentation for `read` ==

 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 the following paragraph:
...
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.
== Qualifications ==

 Ragarding point 2 (`readEither`), I have some qualms whether `readEither`
 ([http://hackage.haskell.org/package/base-4.9.0.0/docs/Text-
 Read.html#v:readEither docs])  offers sufficient additional benefit over
 `readMaybe` to also warrant inclusion in `Prelude`. 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.

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13013
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