[Haskell-cafe] Overriding a Prelude function?

[michael rice]

Here's what I get:

[michael at localhost ~]$ ghci
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.
Prelude> import Prelude hiding ((>>))

<interactive>:1:0: parse error on input `import'
Prelude> 

=====

I was passing seed0 to rollDie and getting back (r1,seed1)
         passing seed1 to rollDie and getting back (r2,seed2)
         passing seed2 to rollDie and getting back (r3,seed3)

Just based on the problem text, I would guess that
        passing rollDie and seed0 to (>>) I would get back (r3,seed3),
losing the intermediate random numbers r1 and r2 along the way, at
least that's what I understood it to say.

So, I know that next I'm probably going to have to do something to 
remedy that, but I haven't gotten to that next step yet. What is unsugar?

Thanks in advance for your patience.

Michael


--- On Wed, 4/22/09, Dan Weston <westondan at imageworks.com> wrote:

From: Dan Weston <westondan at imageworks.com>
Subject: Re: [Haskell-cafe] Overriding a Prelude function?
To: "Ross Mellgren" <rmm-haskell at z.odi.ac>
Cc: "michael rice" <nowgate at yahoo.com>, "haskell-cafe at haskell.org" <haskell-cafe at haskell.org>
Date: Wednesday, April 22, 2009, 12:37 PM

Be aware that the do unsugars to (Prelude.>>), not your (>>), even if you hide (Prelude.>>):

import Prelude hiding ((>>))
m >> f = error "Call me!"
main = putStrLn . show $ do [3,4]
                            [5]

The desugaring of the do { [3,4]; [5] } is (Prelude.>>) [3,4] [5] = [5,5], whereas you might have hoped for [3,4] >> [5] = error "Call me!"

Dan

Ross Mellgren wrote:
> I think
> 
> import Prelude hiding ((>>))
> 
> does that.
> 
> -Ross
> 
> On Apr 22, 2009, at 11:44 AM, michael rice wrote:
> 
>> I've been working through this example from: http://en.wikibooks.org/wiki/Haskell/Understanding_monads
>> 
>> I understand what they're doing all the way up to the definition of (>>), which duplicates Prelude function (>>). To continue following the example, I need to know how to override the Prelude (>>) with the (>>) definition in my file rand.hs.
>> 
>> Michael
>> 
>> ==============
>> 
>> [michael at localhost ~]$ cat rand.hs
>> import System.Random
>> 
>> type Seed = Int
>> 
>> randomNext :: Seed -> Seed
>> randomNext rand = if newRand > 0 then newRand else newRand + 2147483647
>>     where newRand = 16807 * lo - 2836 * hi
>>           (hi,lo) = rand `divMod` 127773
>> 
>> toDieRoll :: Seed -> Int
>> toDieRoll seed = (seed `mod` 6) + 1
>> 
>> rollDie :: Seed -> (Int, Seed)
>> rollDie seed = ((seed `mod` 6) + 1, randomNext seed)
>> 
>> sumTwoDice :: Seed -> (Int, Seed)
>> sumTwoDice seed0 =
>>   let (die1, seed1) = rollDie seed0
>>       (die2, seed2) = rollDie seed1
>>   in (die1 + die2, seed2)
>> 
>> (>>) m n = \seed0 ->
>>   let (result1, seed1) = m seed0
>>       (result2, seed2) = n seed1
>>   in (result2, seed2)
>> 
>> [michael at localhost ~]$
>> 
>> 
>> _______________________________________________
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> 




      
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