[Haskell-cafe] [Haskell] mapM with Traversables

[Ryan Ingram]

You can use Data.Sequence.fromList to go [a] -> Seq a, though.

So given

f :: Monad m => a -> m b

you have

import Data.Traversable as T
import Data.Sequence as S

g :: Monad m => [a] -> m (S.Seq b)
g = T.mapM f . S.fromList

  - ryan

On Wed, Sep 28, 2011 at 6:20 PM, Marc Ziegert <Coeus at gmx.de> wrote:

> Hi Thomas,
> this should be on the haskell-cafe or haskell-beginners mailing list.
> Haskell at ... is mainly for announcements.
>
>
> You have:
> > f :: Monad m =>
> >      a -> m b
> > Data.Traversable.mapM :: (Monad m, Traversable t) =>
> >                          (a -> m b) -> t a -> m (t b)
>
> So, if you define g with
> > g a = do Data.Traversable.mapM f a
>  or in short
> > g = Data.Traversable.mapM f
> , then the type will be
> > g :: (Monad m, Traversable t) =>
> >      t a -> m (t b)
> instead of
> > g :: [a] -> m (Seq b)
> .
>
> Try using ghci to find these things out. It helps to get not confused with
> the types.
>
>
> Besides the missing "Monad" context, g misses a generic way to convert
> between different Traversables, which does not exist. You can only convert
> from any Traversable (imagine a Tree) "toList"; not all Traversables have a
> "fromList" function.
> For conversion, you might want to use Foldable and Monoid, fold to untangle
> and mappend to recombine; but any specific "fromList" function will surely
>  be more efficient.
>
> Regards
> - Marc
>
>
>
> -------- Original-Nachricht --------
> > Datum: Wed, 28 Sep 2011 17:27:58 -0600
> > Von: thomas burt <thedwards at gmail.com>
> > An: haskell at haskell.org
> > Betreff: [Haskell] mapM with Traversables
>
> > Hi -
> >
> > I have a function, "f :: Monad m => a -> m b", as well as a list of a's.
> > I'd
> > like to produce a sequence (Data.Sequence) of b's, given the a's:
> >
> > g :: [a] -> m (Seq b)
> > g a = do Data.Traversable.mapM f a   -- type error!
> >
> > I see that "Data.Traversable.mapM f a" doesn't work... is this like
> asking
> > the compiler to infer the cons/append operation from the type signature
> of
> > g?
> >
> > Do I need to write my own function that explicitly calls the "append"
> > functions from Data.Sequence or can I do something else that would work
> > for
> > any "g :: Traversable t, Traversable u => t a -> m (u b)" given "f :: a
> ->
> > m
> > b"?
> >
> > Thanks for any comments!
> > Thomas
>
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