#2309: containers: specialize functions that fail in a Monad to Maybe

Jonathan Cast jonathanccast at fastmail.fm
Thu May 29 09:57:48 EDT 2008

On 29 May 2008, at 1:30 AM, Neil Mitchell wrote:

> Hi
>>> Following this line of reasoning: why is  1  syntactic sugar for
>>>  fromIntegral 1? If 1 has type Integer then the generic value is  
>>> just a
>>> fromIntergal away!
>> That's a good point the other way --- I suspect, like most things  
>> Num,
>> numeric literals were rather grand-fathered in than designed  
>> intentionally
>> that way.
> fromIntegral is essential if you have two "integer" types, namely Int
> and Integer. If you are looking for something to blame, then its the
> premature optimisation that is Int. Int is really just a performance
> hack around Integer.

Um, that de-railed quickly...  ‘fromIntegral’ in the original is a  
typo  or mis-understanding for ‘fromInteger’.  My comment was re:  
implicit fromInteger (and polymorphic numeric literals).  GP was  
arguing for making Map.lookup polymorphic in the failure monad; I  
replied I thought, in the case of numeric literals, that no-one  
designing Haskell without the tradition that 3 `member` IR, no-one  
would have thought that 3 :: Integer and 3 :: Double both made  
sense.  Explicit conversions are not really the issue here.


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