[Haskell-cafe] Overriding a Prelude function?

[Ross Mellgren]

True enough -- if you really want to redefine the monadic operator,  
you have to use

{-# LANGUAGE NoImplicitPrelude #-}

import Prelude hiding ((>>), (>>=), return)

or something like it, although Michael's example didn't appear to be  
going quite that far.

-Ross

On Apr 22, 2009, at 12:37 PM, Dan Weston wrote:

> 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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