[Haskell-cafe] Stacking monads
Andrew Coppin
andrewcoppin at btinternet.com
Thu Oct 2 15:53:24 EDT 2008
Jonathan Cast wrote:
> On Thu, 2008-10-02 at 18:18 +0100, Andrew Coppin wrote:
>
>> After an insane amount of time making my head hurt, I disocvered that
>> the type "Either ErrorType (ResultSet State)" is actually a monad.
>>
>
> It's a monad if you can write a function
>
> join :: Either ErrorType (ResultSet (Either ErrorType (ResultSet alpha)))
> -> Either ErrorType (ResultSet alpha)
>
> (which follows from being able to write a function
>
> interleave :: Either ErrorType (ResultSet alpha)
> -> ResultSet (Either ErrorType alpha)
>
> satisfying certain laws). Otherwise not, as you noticed.
>
Er... OK. Yes, I guess that kind of makes sense...
>> Since ResultSet *just happens* to also be in Functor,
>>
>
> It doesn't just happen to be one. liftM is *always* a law-abiding
> definition for fmap, when used at a law-abiding monad.
I'm lost...
(What does liftM have to do with fmap?)
> (This is why
> posters here are always bringing up head-hurting category theory, btw.
> Absorbing it sufficiently actually teaches you useful things about
> Haskell programming.)
>
That would be a surprising and unexpected result. After all, knowing
about set theory doesn't help you write SQL...
>> At this point I am sorely tempted to just change ResultSet to include
>> the error functionallity I need. However, ResultSet is *already* an
>> extremely complicated monad that took me weeks to get working
>> correctly...
>>
>
> What does it look like?
A list, basically. (But obviously slightly more complicated than that.)
> Quite possibly it can be factored out into
> smaller pieces using monad transformers. (In which case adding error
> handling is just sticking in another transformer at the right layer in
> the stack --- that is, the layer where adding error handling works :).
>
Well I'm *already* trying to layer an error transformer on the top and
it's failing horribly. I don't see how splitting things up even more
could do anything but make the program even *more* complex.
>> Does anybody have any idea how this whole monad stacking craziness is
>> *supposed* to work?
>>
>
> No. [1]
>
Ah, good. :-)
> But we know how it *can* work; this is what monad transformers exist to
> do. You want to either change ResultSet to be a monad transformer,
> or
> you want the monad ErrorT ErrorType ResultSet. Very little can be said
> in general without knowing what ResultSet looks like.
>
I thought ErrorT was a class name...?
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