[Haskell-cafe] Re: Lists and monads

Bill Atkins watkins at alum.rpi.edu
Mon Jul 26 09:26:14 EDT 2010

Can you post an example of your code?  mapM and map are actually for pretty distinct purposes.

If you find yourself wanting to map over a pure list in monadic code, you should really look at applicative style, e.g.:

  import Control.Applicative

  data Struct = .... deriving (Read)

  readStructs :: IO [Struct]
  readStructs = map read . lines <$> getContents

It lets you apply a pure function (or a composition of pure functions) to a monadic value.  Note that the above is exactly equivalent to:

  readStructs = do
    contents <- getContents
    return . map read . lines $ contents

On Monday Jul 26, 2010, at 9:13 AM, Kevin Jardine wrote:

> On Jul 26, 3:00 pm, Vo Minh Thu <not... at gmail.com> wrote:
>> Also, just like with IO, maybe restructuring the code to separate
>> monadic code would help.
> The specific monad I am dealing with carries state around inside it.
> I could revert to a pure system in many cases by simply passing the
> state as a parameter but then that defeats the point of the monad and
> clutters up my function calls.
> Also, in other cases, I am using a module that defines its own monads
> and have no choice but to use them.
> I think I would prefer a style of programming where monads are equal
> citizens to pure function calls. There are various hints that such a
> style of programming is possible but as I say, I have not found any
> clear tutorials on it.
> _______________________________________________
> Haskell-Cafe mailing list
> Haskell-Cafe at haskell.org
> http://www.haskell.org/mailman/listinfo/haskell-cafe

More information about the Haskell-Cafe mailing list