I have this problem trying to define a function
inside a do expression. I tried this small code
to help me check. This works well:
I guess you intended to call printNumber in the quoted snippet?
(...)
{-# LANGUAGE RankNTypes #-}
{-# LANGUAGE ImpredicativeTypes #-}
After you pointed my dumb mistake, I was able to build
the first example -- without any of the extensions! Haskell
can be misterious some times.
Strange enough, I can't get the original (and, to my eyes,
equal) problem to work. This is the smallest I could get it
to be:
--- WARNING: wrong use of gtk, just to get an example
---
import Graphics.UI.Gtk ;
main = do {
initGUI ;
j1 <- drawingAreaNew ; j2 <- tableNew 1 1 True ;
notebook <- do {
note <- notebookNew ;
insertInNotebook <- let {
colocar :: (WidgetClass w) => w -> String -> IO Int ;
colocar wid texto = do {
lb <- labelNew Nothing ;
labelSetMarkup lb texto ;
notebookAppendPageMenu note wid lb lb
} } in return $ colocar ;
insertInNotebook j1 "J1" ;
insertInNotebook j2 "J2" ;
return note
} ;
putStrLn "Finish"
}
---
GHC says:
teste.hs:15:21:
Couldn't match expected type `DrawingArea'
against inferred type `Table'
In the first argument of `insertInNotebook', namely `j2'
In a stmt of a 'do' expression: insertInNotebook j2 "J2"
(...)
but I would like first argument of insert... to be any
instance of WidgetClass, be it Drawing... or Table.
Thanks,
Maurício
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