[Haskell-cafe] comprehension problem

[ptrash]

Cool, thanks a lot.



Ross Mellgren wrote:
> 
> P Float is the constructor to create a value of this type, similar to  
> data declarations.
> 
> That is, 0.5 :: Float, P 0.5 :: Probability
> 
> The {} notation after D creates a record accessor, also similar to  
> data declarations. It's equivalent to making an unD that unwraps the  
> value yourself:
> 
> newtype Dist a = D { unD :: [(a, Probability)] }
> 
> is the same as
> 
> newtype Dist a = D [(a, Probability)]
> 
> unD :: Dist a -> [(a, Probability)]
> unD (D x) = x
> 
> a in Dist a is a type variable, for example you could have Dist Float  
> (containing [(Float, Probability)]), or Dist String (containing  
> [(String, Probability)])
> 
> -Ross
> 
> On Jun 3, 2009, at 4:01 PM, ptrash wrote:
> 
>>
>> Hi,
>>
>> what does this to code rows mean:
>>
>> newtype Probability = P Float
>> newtype Dist a = D {unD :: [(a, Probability)]}
>>
>> newtype definies a new type called Probability. But what does P  
>> Float mean?
>>
>> And what is the a in Dist a?
>> What does D {...} mean?
>>
>> Thanks for your help.
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