Warn against using 'fail' directly in user-code

[Andreas Abel]

On 16.12.2013 18:37, Niklas Hambüchen wrote:
>> (Fwiw, I've tested attaching such a warning to `fail` in GHC HEAD's
>> libraries/base/GHC/Base.lhs and it seems to work just fine so far... so
>> if there's consensus, we could add that for GHC 7.8)
>
> That sounds great!
>
> And as far as I remember, we will have MonadFail, as Malcolm mentioned,
> soon.
>
> On 16/12/13 13:54, Andreas Abel wrote
>>    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.
>
> This seems to go well with your change.

Above I was just quoting the current documentation.  It should be 
formulated sharper, to say explicitly "do not use `fail' to throw your 
own errors!"



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