[Haskell-beginners] ListT + Writer

[Baa]

@Francesco, yes, it completely answers my questions. Thank you very much
for clarification!! I got it.

В Thu, 25 May 2017 20:44:57 +0200
Francesco Ariis <fa-ml at ariis.it> пишет:

> On Thu, May 25, 2017 at 07:02:58PM +0300, Baa wrote:
> >
> > В Thu, 25 May 2017 17:43:49 +0200  
> > > Should be as easy as:
> > > 
> > >     import Control.Monad.List
> > >     import Control.Monad.Writer
> > > 
> > >     type Prova = ListT (Writer String) Int  
> > 
> > Yes! And this is the source of my questions.. 1) How to call it? 2)
> > What does mean to provide argument of type ".. Writer .."? As
> > result, it's good: you can run it with "runWriter", but what is
> > "writer" in input argument? Fake writer (which is ignoring)?  
> 
> I am not sure for question #1. If we look at the constructor of ListT
> (`runListT :: m [a]`) and MonadWriter (`(a, w)`) and we compose the
> two we get:
> 
>     ([a], w)
>     -- or ([Integer], String) in our example
>     -- e.g. ListT (writer ([1,2], "ciao")) <-- to construct
> 
> No idea what is the correct name of this critter.
> 
> As we can see by running it, the argument in `fn` (both the list part
> and the string part) is not ignored:
> 
>     λ> fn $ ListT (writer ([1,2], "ciao"))  
>     ListT (WriterT (Identity ([3],"ciaohey baby")))
> 
> Does this answer your questions?
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