[Haskell-beginners] Parsec and Validation

[Brandon S Allbery KF8NH]

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On 7/31/10 15:57 , Vladimir Solmon wrote:
> time :: Parser TimeOfDay
> time = choice [ tTimeHourMin,
>                           t24hrClock
>                        ]

If the parse of tTimeHourMin fails after reading some characters (most
probably, at the oneOf because it has been fed a t24hrClock value), those
characters remain read and t24hrClock will pick up where the oneOf failed,
then itself fail because all the digits were read by the many1 in range.  To
prevent this, resetting to where tTimeHourMin started its parse, wrap it in
a try:

> time = choice [ try tTimeHourMin
> 		, t24hrClock
> 		]

- -- 
brandon s. allbery     [linux,solaris,freebsd,perl]      allbery at kf8nh.com
system administrator  [openafs,heimdal,too many hats]  allbery at ece.cmu.edu
electrical and computer engineering, carnegie mellon university      KF8NH
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