On 16.12.2013 14:12, Roman Cheplyaka wrote:
The purpose of 'fail' is to handle monadic pattern-match failures. I consider explicit use of 'fail' in user code a hack.
I treat 'fail' as a semi-private method of the Monad class. I may override it, but I should never call it. It only exists for the compiler.
This is useful information. Please add it to the documentation. That would definitely help newcomers to get some orientation in the design space of exceptions... http://hackage.haskell.org/package/base-4.6.0.1/docs/Control-Monad.html#t:Mo... fail :: String -> m a Fail with a message. This operation is not part of the mathematical definition of a monad, but is invoked on pattern-match failure in a do expression. -- Andreas Abel <>< Du bist der geliebte Mensch. Theoretical Computer Science, University of Munich Oettingenstr. 67, D-80538 Munich, GERMANY andreas.abel@ifi.lmu.de http://www2.tcs.ifi.lmu.de/~abel/