Suggestion for Haskell-2 library module Random.
Haskell-98 declares
--------------------------------------------------------------------
class Random a where randomR :: RandomGen g => (a, a) -> g -> (a, g)
...
...
instance Random Int where ...
instance Random Integer where ...
...
* randomR takes a range (lo,hi) and a random number generator g, and
returns a random value
> uniformly distributed in the closed interval [lo,hi],
together with a new generator.
> It is unspecified what happens if lo > hi.
...
---------------------------------------------------------------------
- here i re-formatted the text and marked two lines with `>'.
I suggest to change this ">" part to
-- (h2) ----------------------------------------------------------
...
"uniformly distributed" in the "closed interval" [lo,hi],
together with a new generator.
It is required randomR (lo,hi) g == randomR (hi,lo) g
to hold for each g.
"Uniformly distributed" means the usual notion for the "numeric"
types Integer,Float,... - as well as for finite ones Bool,Char...
Defining the Random instance for other types
(say, T = (Random a, Random b) => (a,b) ),
the programmer has to rely on one's own understanding of
"uniform distribution" in domain T.
Similar is the notion "closed interval [lo,hi]".
For Integer, Float, ..., it is standard.
For other domains T, maybe even not supplied with ordering, it
means some subset in T defined by lo, hi :: T,
this subset specification is on the programmer.
----------------------------------------------------------------
I do not insist on this formulation literally.
This is only the idea.
Reason:
the programmer could define, for example,
instance (Random a, Random b) => Random (a,b) --instance (p)
where
randomR ((l,l'),(h,h')) g = case randomR (l,h) g of
(a,g') -> case randomR (l',h') g' of (b,g'') -> ((a,b),g'')
Then, the obstacles arise:
(1) what does might mean "closed interval [lo,hi]" in (a,b) ?
(2) applying instance (p) to, say, (Int,Int)
may cause evaluation of randomR (l,h) g with l > h :: Int.
The above suggestion (h2) provides some explanation for this.
Personally, my aim was to provide random polynomials f :: P a
over various coefficient domains `a', for the need of testing of
some functions. And the format
randomR (polynomial1,polynomial2) g
derived from what Haskell-98 Random provides, looks useful to
define random values inside various subsets of P a.
------------------
Sergey Mechveliani
[EMAIL PROTECTED]
