[Haskell-cafe] Re: Lists and monads
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.:
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.
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