Hi Thomas,
this should be on the haskell-cafe or haskell-beginners mailing list. Haskell@... is mainly for announcements.
You have:
> f :: Monad m =>> Data.Traversable.mapM :: (Monad m, Traversable t) =>
> a -> m b
> (a -> m b) -> t a -> m (t b)
So, if you define g with
> g a = do Data.Traversable.mapM f aor 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@gmail.com>
> An: haskell@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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