[Haskell-beginners] Mutable collection of gui objects (wxHaskell)

[Henk-Jan van Tuyl]

On Tue, 31 Mar 2015 22:11:47 +0200, Jeffrey Brown  
<jeffbrown.the at gmail.com> wrote:

> It works! And I don't understand it! Particularly this line:
>
>   inputs <- map (\e -> [hfill $ widget e]) <$> varGet myVar
>
> I'm considering the type signatures:
>   map :: (a -> b) -> [a] -> [b]
>   (<$>) :: Functor f => (a -> b) -> f a -> f b
>   varGet :: Var a -> IO a
> map and <$> (fmap) are nearly synonymous; map seems like a special case  
> of
> fmap. I gather the fmap must be to map inside the IO type that varGet
> returns, and the map must be mapping across some list. But what about  
> their
> function arguments? Is map using the lambda expression, or is fmap? What
> function argument is the other one using? Are they both somehow sharing  
> the
> lambda expression?

The expression
   f <$> x
is equal to
   fmap f x
. If
   f :: a -> b
   x :: IO a
then
   fmap f :: IO a -> IO b
. You could say, fmap lifts f to the IO monad (you could also use liftM  
for this).

In your case, f is
   map (\e -> [hfill $ widget e])
and x is
   varGet myVar
You stored a list of textEntrys in myVar, the lambda expression is mapped  
over this list.

Another way to look at it: you can replace the line
   inputs <- map (\e -> [hfill $ widget e]) <$> varGet myVar
with
   xs <- varGet myVar
   let inputs = map (\e -> [hfill $ widget e]) xs

Regards,
Henk-Jan van Tuyl


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