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?

On Tue, Mar 31, 2015 at 12:48 AM, Henk-Jan van Tuyl <hjgtuyl@chello.nl> wrote:

On Tue, 31 Mar 2015 04:14:35 +0200, Jeffrey Brown <jeffbrown.the@gmail.com> wrote:

:

I am trying to modify Layout.hs [1] to permit a variable number, rather
than a fixed number, of text entry boxes. I shrank the program, mostly by
removing features, until it was this:

main = start $ do
f <- frame [text := "Layout test"]
p <- panel f []
xinput <- textEntry p [text := "100"]
yinput <- textEntry p [text := "100"]
myVar <- varCreate [xinput,yinput]
set f [ layout := container p $ margin 10 $
column 5 [boxed "coordinates" (grid 5 5
[[hfill $ widget xinput], [hfill $ widget yinput]] -- replacing
) ] ]
return ()

I want to replace the line marked "replacing". Rather than hard-coding the
number of text entry boxes (2), I want it to deal with a mutable collection
of them, in myVar.

I tried this:
[ fmap (\e -> hfill $ widget e) $ varGet myVar ]

:

The result of varGet is of type "IO something", you must convert that to "something", by using "<-". You can do this by adding the line:
inputs <- map (\e -> [hfill $ widget e]) <$> varGet myVar
before the set command (note the square brackets). The <$> operator is from module Data.Functor and is defined as
(<$>) = fmap

The main function becomes this:
main = start $ do
f <- frame [text := "Layout test"]
p <- panel f []
xinput <- textEntry p [text := "100"]
yinput <- textEntry p [text := "100"]
myVar <- varCreate [xinput, yinput]
inputs <- map (\e -> [hfill $ widget e]) <$> varGet myVar
set f [ layout := container p $ margin 10 $
column 5 [boxed "coordinates" (grid 5 5 inputs)]
]
return ()

Regards,
Henk-Jan van Tuyl

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