[Haskell-cafe] Error Monad and strings
qdunkan at gmail.com
Wed Jul 28 03:50:51 EDT 2010
> I've always thought that being able to write:
>> catMaybes :: [Maybe a] -> [a]
>> catMaybes xs = [ x | Just x <- xs ]
> is really cool, which relies on:
>> fail _ = 
> being in the Monad instance for List.
Really? I thought that's just a feature of list comprehensions. List
comps are not monads, at least not any more. If you wrote it as:
catMaybes xs = do
Just x <- xs
Then yes, I believe that uses 'fail'.
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