[Haskell-cafe] one-way monads

Dan Weston westondan at imageworks.com
Wed May 21 14:14:52 EDT 2008

Dan Doel wrote:
> On Tuesday 20 May 2008, ajb at spamcop.net wrote:
>> Actually, it's true less than 50% of the time.  In particular, it's
>> not true of any monad transformer.
> Sure it is. Any particular transformer t typically comes with some particular 
> way of writing a function of type t m a -> m a (you may have to throw away 
> some t-related stuff, of course).
> Since a specific transformed monad is built from a specific monad, and a 
> specific transformer, and specific transformers are likely to have a function 
> of type t m a -> m a, and specific monads are likely to have functions of 
> type m a -> a, you can compose them to get a function of type t m a -> a for 
> the specific monad t m. And so on for transformed-transformed monads. :)
> That only fails if either of the specific pieces fails to have the right 
> function, which happens well under 50% of the time, I think (IO and STM are 
> the ones that immediately occur to me (barring a certain evil function), 
> although you could make a case for ST by technicality; no failing 
> transformers come to mind (except CCT if we're counting ST), but I haven't 
> wracked my brain very hard).
> -- Dan

The claim was "less than 50% of the time", not "less than 50% of the 
monads in the standard libraries". I wonder what fraction of monads in 
real code the IO monad alone accounts for? 50% does not seem implausible 
to me.

Dan Weston

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