Re: [Haskell-cafe] Re: Type errors, would extensions help?

15 Jan 2009

      I suggest you start using "let" in your do blocks; both of these
problems are solvable with let.

Binding with <- instead of "let" makes the type system work harder,
and will generally require type annotations & extensions for
polymorphic results.  And it's almost never what you want, anyways;
you don't often have an object of type "IO (forall a.  a -> a)"
instead of "forall a. IO (a -> a)" and this situation usually means
you should be using "let" instead.

Here's the Gtk example; the let on "mkNotebook" is not strictly
necessary but is just showing the concept in more places; I tend to
avoid x <- do ... in my code; I feel it means I should be abstracting
more.

main = do
  initGUI
  j1 <- drawingAreaNew
  j2 <- tableNew 1 1 True

  let mkNotebook = do
    note <- notebookNew
    let insertInNoteBook wid texto = do
      lb <- labelNew Nothing
      labelSetMarkup lb texto
      notebookAppendPageMenu note wid lb lb
    insertInNotebook j1 "J1"
    insertInNotebook j2 "J2"
    return note

  notebook <- mkNotebook

  putStrLn "Finish"

Also, is there a reason you hate the layout rule and are using
explicit semicolons everywhere?

  -- ryan

On Thu, Jan 15, 2009 at 10:46 AM, Mauricio  wrote:
...
...
...
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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Re: [Haskell-cafe] Re: Type errors, would extensions help?

Ryan Ingram