[Haskell-cafe] Funny State monad dependency

[Miguel Mitrofanov]

Before somebody noticed: I'm wrong.

It's not List monad, but also a "(->) x" monad, also defined in  
Control.Monad.

Therefore, "return y" is just "const y". Therefore,

x >>= (return y) = x >>= (const y) = x >> y


On 16 Apr 2008, at 17:04, Miguel Mitrofanov wrote:
> It has nothing to do with State; it actually works in List monad.  
> "return y" is just another way of writing "[y]".
>
> You don't need to import Control.Monad.State for this to work; you  
> only need Control.Monad (which is imported by the former).
>
> On 16 Apr 2008, at 16:56, Hans Aberg wrote:
>> When I load the State module in Hugs, then I can define the  
>> function f below, but I do not immediately see exactly what  
>> function "return" returns. Explanation welcome.
>>
>> For example:
>> > f [2..4] [6..9]
>> [6,7,8,9,6,7,8,9,6,7,8,9]
>> That is, it just repeats the second argument as many times as the  
>> length of the second argument.
>>
>> Hans Aberg
>>
>> --------
>> import Control.Monad.State
>>
>> f :: Monad a => a b -> a c -> a c
>> f x y = x >>= (return y)
>> --------
>>
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