[Haskell-cafe] Re: Fwd: Re: Simple game: a monad for each player

[Heinrich Apfelmus]

Heinrich Apfelmus wrote:
> Limestraël wrote:
>> Okay, I start to understand better...
>>
>> Just, Heinrich, how would implement the mapMonad function in terms of the
>> operational package?
>> You just shown the signature.
> 
> Ah, that has to be implemented by the library, the user cannot implement
> this. Internally, the code would be as Bertram suggests:
> 
>     mapMonad :: (Monad m1, Monad m2)
>              => (forall a . m1 a -> m2 a)
>              -> ProgramT instr m1 a -> ProgramT instr m2 a
>     mapMonad f (Lift m1)  = Lift (f m1)
>     mapMonad f (Bind m k) = Bind (mapMonad f m) (mapMonad f . k)
>     mapMonad f (Instr i)  = Instr i

Silly me! This can be implement by the user:

    mapMonad f = id' <=< lift . f . viewT
        where
        id' :: ProgramViewT instr m1 a -> ProgramT instr m2 a
        id' (Return a) = return a
        id' (i :>>= k) = singleton i >>= mapMonad f . k

and it would be a shame for the operational approach if that were not
possible. :)


Regards,
Heinrich Apfelmus

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