[Haskell-cafe] Newbie type question for wxHaskell learner

[b1g3ar5]

Thanks for your reply but it doesn't quite solve the problem. This:

plist <- mapM (\x-> (panel nb [])) my_list

returns [Panel()] and works as you say, but:

elist <- mapM (\x-> (textCtrl (panel nb []) [text := contents x]))
my_list

still won't work because the function panel returns a IO (Panel()) and
so won't do as a parameter to textCtrl.

I can get round this by applying mapM to a list of the indices (of
my_list and plist:

elist <- mapM (\ix-> (textCtrl (plist!!ix) [text := contents (my_list!!
ix)])) [1..4]

but this seems a bit crap. There must be a neat way of doing this.

N


On Jan 7, 4:21 pm, Daniel Fischer <daniel.is.fisc... at googlemail.com>
wrote:
> On Friday 07 January 2011 17:09:11, b1g3ar5 wrote:
>
> > I've tried to solve this but I am failing.
>
> > I can do this:
>
> > p0<-panel nb []
> > e0<-textCtrl p [text:=my_list!!0]
>
> > but I want to do this on all of my_list, so I tried:
>
> > let es = map (\x-> textCtrl (panel nb []) [text:=x]) my_list
>
> > Now, this won't work because the panel nb [] is IO (Panel()) and the
> > parameter to textCtrl needs to be a Panel()
>
> > How do I get out of IO?
>
> What you need is mapM:
>
>     es <- mapM (\x -> textCtrl (panel nb []) [text:=x]) my_list
>
> mapM :: (Monad m) => (a -> m b) -> [a] -> m [b]
>
> applies the function to each list element, runs the resulting action and
> collects the results. If you don't need the results but only the effects of
> running the actions (common in IO), use
>
> mapM_ :: (Monad m) => (a -> m b) -> [a] -> m ()
>
> mapM and mapM_ are compositions of
>
> sequence :: (Monad m) => [m a] -> m [a]
>
> resp.
>
> sequence_ :: (Monad m) => [m a] -> m ()
>
> with map (mapM f list === sequence (map f list)), those are useful on their
> own too.
>
>
>
> > Thanks.
>
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