[Haskell-cafe] Re: Overriding a Prelude function?

[michael rice]

I'm just copying code from this web page: http://en.wikibooks.org/wiki/Haskell/Understanding_monads

================

Using their initial seed I get this:

*Main> rollDie 362354 ~>> (rollDie ~>> rollDie)

<interactive>:1:0:
    Couldn't match expected type `t -> (t1, t2)'
           against inferred type `(Int, Seed)'
    In the first argument of `(~>>)', namely `rollDie 362354'
    In the expression: rollDie 362354 ~>> (rollDie ~>> rollDie)
    In the definition of `it':
        it = rollDie 362354 ~>> (rollDie ~>> rollDie)
*Main> 

===================

Passing their initial seed through successive calls to rollDie:

*Main> rollDie 362354
Loading package old-locale-1.0.0.1 ... linking ... done.
Loading package old-time-1.0.0.1 ... linking ... done.
Loading package random-1.0.0.1 ... linking ... done.
(3,1795116384)
*Main> rollDie 1795116384
(1,523309185)
*Main> rollDie 523309185
(4,1311937830)  <== ISN'T THIS SUPPOSED TO BE THE RESULT?
*Main> 

Michael


--- On Wed, 4/22/09, Achim Schneider <barsoap at web.de> wrote:

From: Achim Schneider <barsoap at web.de>
Subject: [Haskell-cafe] Re: Overriding a Prelude function?
To: haskell-cafe at haskell.org
Date: Wednesday, April 22, 2009, 9:52 PM

michael rice <nowgate at yahoo.com> wrote:

> OK, I changed the operator from (>>) to (~>>). When I try to use it I
> get this:
> 
> [michael at localhost ~]$ ghci rand
> GHCi, version 6.10.1: http://www.haskell.org/ghc/__ :? for help
> Loading package ghc-prim ... linking ... done.
> Loading package integer ... linking ... done.
> Loading package base ... linking ... done.
> [1 of 1] Compiling Main________________________ ( rand.hs, interpreted )
> Ok, modules loaded: Main.
> *Main> rollDie ~>> (rollDie ~>> rollDie)  
> 
> <interactive>:1:0:
> ______ No instance for (Show (Seed -> (Int, Seed)))
> __________ arising from a use of `print' at <interactive>:1:0-32
> ______ Possible fix:
> __________ add an instance declaration for (Show (Seed -> (Int, Seed)))
> ______ In a stmt of a 'do' expression: print it

Well, you obviously need an initial seed:

rollDie 0xdeadbeef ~>> (rollDie ~>> rollDie)

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