Proposal: die to System.Exit (and/or Prelude)

[Malcolm Wallace]

On 16 Dec 2013, at 13:12, Roman Cheplyaka wrote:

> * Ivan Lazar Miljenovic <ivan.miljenovic at gmail.com> [2013-12-17 00:00:16+1100]
>> On 16 December 2013 22:15, Roman Cheplyaka <roma at ro-che.info> wrote:
>>> * Herbert Valerio Riedel <hvr at gnu.org> [2013-12-16 10:53:21+0100]
>>>> Why not simply use the existing `fail :: String -> IO a` method instead?
>>> 
>>> The purpose of 'fail' is to handle monadic pattern-match failures.
>>> I consider explicit use of 'fail' in user code a hack.
>> 
>> Or for use with parser-combinator libraries (though I suppose this
>> could be seen as a pattern-match failure...)?
> 
> I'd consider that an abuse, too. E.g. Parsec provides a parserFail
> function that you can call instead.

I respectfully disagree.  Whilst it was probably a mistake originally to include "fail" as a method of the Monad class, it has always been a useful additional method.  Personally, I would have it in a separate class (called MonadFail, or similar).  But wherever it lives, we would always have had it.

I see no good reason to distinguish between runtime pattern-match failure, and a direct user call of "fail".  It is quite common to write parsers using pattern-matching, and why should one kind of backtracking be privileged over another?  Their effects are of exactly the same kind.

e.g.

    keyword = ( do "let" <- word -- fails if the string does not match
                   b <- binding
                   return (LET b) )
               `onFail`
              ( do "case" <- word
                   exp   <- expression
                   "of"  <- word `onFail` fail "missing keyword 'of'"
                   alts  <- many1 alternative
                   return (CASE exp alts) )

etc.