[Haskell-cafe] ANN: random-fu

wren ng thornton wren at freegeek.org
Thu Jun 3 22:03:53 EDT 2010

Richard O'Keefe wrote:
> There's something in that package that I don't understand,
> and I feel really stupid about this.
> data RVarT m a
> type RVar = RVarT Identity
> class Distribution d t where
>     rvar :: d t -> RVar t
>     rvarT :: d t -> RVarT n t
> Where does "n" come from?

Presumably from universal quantification in rvarT? That is, the 
implementation of rvarT should be polymorphic in n, in which case the 
particular n doesn't matter (as well it shouldn't, since if it did 
that'd interfere with the composability of the transformer).

Though, since RVar is a synonym for RVarT, I can't imagine why rvar is a 
method instead of a shorthand defined outside of the class. (If RVar 
were primitive then I could imagine performance reasons, but since it 

Live well,

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