Laziness on new "random" package

[Carter Schonwald]

You want to use a stateT esque monad that does a split within the bind.
Quickcheck should have an example of this.

On Fri, Jul 17, 2020 at 2:47 AM Dannyu NDos <ndospark320 at gmail.com> wrote:

> Though it is reasonable to make utilities monadic, they lack a crucial
> property: *laziness*.
>
> I have the following datatype:
>
> newtype ArbReal = ArbReal (Word -> Integer)
>
> This represents arbitrary real numbers by being able to compute arbitrary
> decimal places after the decimal point. For example, to compute pi, let f
> be the function passed to constructor ArbReal. Then f 0 = 3, f 1 = 31, f 2
> = 314, and so on.
>
> I can implement instance Random ArbReal:
>
> instance Random ArbReal where
>     random g = let
>         (h, i) = split g
>         d:digits = randomRs (0 :: Word, 9) h
>         getNum [] = 0
>         getNum (d:ds) = toInteger d + 10 * getNum ds
>         takeDigits n = getNum (reverse (take n digits)) + toInteger
> (fromEnum (d >= 5))
>         in (ArbReal (takeDigits . fromIntegral), i)
>     randomR (lo, hi) g = let
>         (x, h) = random g
>         in (lo + x * (hi - lo), h)
>
> But I see no way to implement an instance of UniformRange ArbReal, for it
> relies on randomRs, which is lazy. Neither ST nor IO is able to contain
> such laziness.
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