Random Generator

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Random Generator

Shishir Srivastava
Hi,

I am trying to use the output value from the random function to generate
the random generator for the next value in list.

Below is the code -

----
import System.Random
myrandoms :: (RandomGen g, Random a) => g -> [a]
myrandoms gen = let (value, newGen) = random gen in value:myrandoms
(mkStdGen (value::Int))
----

however the compilation fails when the module is loaded -

[1 of 1] Compiling Main             ( myrandoms.hs, interpreted )

myrandoms.hs:3:80:
    Could not deduce (a ~ Int)
    from the context (RandomGen g, Random a)
      bound by the type signature for
                 myrandoms :: (RandomGen g, Random a) => g -> [a]
      at myrandoms.hs:2:14-48
      `a' is a rigid type variable bound by
          the type signature for
            myrandoms :: (RandomGen g, Random a) => g -> [a]
          at myrandoms.hs:2:14
    Relevant bindings include
      value :: a (bound at myrandoms.hs:3:22)
      myrandoms :: g -> [a] (bound at myrandoms.hs:3:1)
    In the first argument of `mkStdGen', namely `(value :: Int)'
    In the first argument of `myrandoms', namely
      `(mkStdGen (value :: Int))'

----------

Even though I am converting my 'value' parameter to Int in my new
generator, I am unable to see the error behind this.

Please can someone explain or even better provide a fix.

Thanks,
Shishir
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Random Generator

Brandon Allbery
On Wed, Apr 1, 2015 at 11:08 AM, Shishir Srivastava <
shishir.srivastava at gmail.com> wrote:

> myrandoms :: (RandomGen g, Random a) => g -> [a]
> myrandoms gen = let (value, newGen) = random gen in value:myrandoms
> (mkStdGen (value::Int))
>

You have declared a function that says that it can deal with any type `a`
that the *caller* chooses, then provided an implementation that only
supports Int.

Note that :: does not do conversion (as you said "Even though I am
converting my 'value' parameter to Int"); it declares that the type of
`value` *is* Int. Other types will be inferred to match, and this fails at
`value:` in a context which wants the type to be a caller-specified `a`,
not Int.

(I don't think you can coerce an unknown type `a` to Int given only the
context `Random a`. You must find a different way to implement this.)

--
brandon s allbery kf8nh                               sine nomine associates
allbery.b at gmail.com                                  ballbery at sinenomine.net
unix, openafs, kerberos, infrastructure, xmonad        http://sinenomine.net
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Random Generator

David McBride
In reply to this post by Shishir Srivastava
mkStdGen only accepts Ints as seeds.  But your random function, as you
typed it, can return any type of random.  You either have to restrict your
random function to returning ints, like so:

myrandoms :: (RandomGen g) => g -> [Int]
myrandoms gen = let (value, newGen) = random gen in value:myrandoms
(mkStdGen value)

Or you have to find a way to convert any Random a into an Int (not
possible), or put another constraint on it, such that you can return all
the types you might want that you have the ability to turn into ints, for
example:

myrandoms :: (RandomGen g, Random a, Intable a) => g -> [a]
myrandoms gen = let (value, newGen) = random gen in value:myrandoms
(mkStdGen $ convertToInt value)

class Intable a where
  convertToInt :: a -> Int

instance Intable Int where convertToInt = id
instance Intable Integer where convertToInt = fromIntegral
instance Intable Char where convertToInt s = undefined -- something

Which is obviously tedious, but may be worthwhile depending on your
application.



On Wed, Apr 1, 2015 at 11:08 AM, Shishir Srivastava <
shishir.srivastava at gmail.com> wrote:

> Hi,
>
> I am trying to use the output value from the random function to generate
> the random generator for the next value in list.
>
> Below is the code -
>
> ----
> import System.Random
> myrandoms :: (RandomGen g, Random a) => g -> [a]
> myrandoms gen = let (value, newGen) = random gen in value:myrandoms
> (mkStdGen (value::Int))
> ----
>
> however the compilation fails when the module is loaded -
>
> [1 of 1] Compiling Main             ( myrandoms.hs, interpreted )
>
> myrandoms.hs:3:80:
>     Could not deduce (a ~ Int)
>     from the context (RandomGen g, Random a)
>       bound by the type signature for
>                  myrandoms :: (RandomGen g, Random a) => g -> [a]
>       at myrandoms.hs:2:14-48
>       `a' is a rigid type variable bound by
>           the type signature for
>             myrandoms :: (RandomGen g, Random a) => g -> [a]
>           at myrandoms.hs:2:14
>     Relevant bindings include
>       value :: a (bound at myrandoms.hs:3:22)
>       myrandoms :: g -> [a] (bound at myrandoms.hs:3:1)
>     In the first argument of `mkStdGen', namely `(value :: Int)'
>     In the first argument of `myrandoms', namely
>       `(mkStdGen (value :: Int))'
>
> ----------
>
> Even though I am converting my 'value' parameter to Int in my new
> generator, I am unable to see the error behind this.
>
> Please can someone explain or even better provide a fix.
>
> Thanks,
> Shishir
>
>
>
> _______________________________________________
> Beginners mailing list
> Beginners at haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>
>
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Random Generator

Chaddaï Fouché
On Wed, Apr 1, 2015 at 5:32 PM, David McBride <toad3k at gmail.com> wrote:

> mkStdGen only accepts Ints as seeds.  But your random function, as you
> typed it, can return any type of random.  You either have to restrict your
> random function to returning ints, like so:
>
> myrandoms :: (RandomGen g) => g -> [Int]
> myrandoms gen = let (value, newGen) = random gen in value:myrandoms
> (mkStdGen value)
>



*Is there any good reason we're not using newGen for its intended purpose
here and instead weakening our randomness, maybe extremely (imagine if a
list a Bool is asked for...) ???*


>
> Or you have to find a way to convert any Random a into an Int (not
> possible), or put another constraint on it, such that you can return all
> the types you might want that you have the ability to turn into ints, for
> example:
>
> myrandoms :: (RandomGen g, Random a, Intable a) => g -> [a]
> myrandoms gen = let (value, newGen) = random gen in value:myrandoms
> (mkStdGen $ convertToInt value)
>
> class Intable a where
>   convertToInt :: a -> Int
>
> instance Intable Int where convertToInt = id
> instance Intable Integer where convertToInt = fromIntegral
> instance Intable Char where convertToInt s = undefined -- something
>
> Which is obviously tedious, but may be worthwhile depending on your
> application.
>
>
If this was really the way Shishir wanted to go, I would suggest simply
reusing the Enum typeclass rather than creating a new Intable typeclass,
since : fromEnum :: (Enum a) => a -> Int

Still a very bad and puzzling idea by the way...

--
Jeda?
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