Re: [web-devel] Yesod. Handle Exception.

Thanks. it's convenient. 2011/1/25 Michael Snoyman

2011/1/25 Антон Чешков :

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Hello Michael. Please explain how to handle exceptions in Yesod applications. For example i would to catch the error generated by unique constraint on table. runDB $ insert $ Table field1 field2

In general, starting with Persistent 0.4, you will want to use the monad-peel package[1], which provides facilities for exception catching in wrapped monads. In Persistent 0.3, you can use the MonadInvertIO functions available in the neither package[2]. MonadInvertIO and MonadPeelIO are very similar in concept, but MonadPeelIO is just better[3].

But in your particular case, you don't need to resort to runtime exceptions: just use insertBy[4] or checkUnique[5].

Michael

[1] http://hackage.haskell.org/package/monad-peel [2] http://hackage.haskell.org/packages/archive/neither/0.1.0/doc/html/Control-M... [3] I wrote MoandInvertIO, Anders Kaseorg wrote MonadPeelIO, and he's smarter than me. [4] http://hackage.haskell.org/packages/archive/persistent/0.3.1.3/doc/html/Data... [5] http://hackage.haskell.org/packages/archive/persistent/0.3.1.3/doc/html/Data...

-- Best regards, Cheshkov Anton Phone: +7 909 005 18 82 Phone: +7 931 511 47 37 Skype: cheshkov_anton