[Haskell-cafe] comprehension problem
Ross Mellgren
rmm-haskell at z.odi.ac
Wed Jun 3 16:27:57 EDT 2009
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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