Re: [Haskell-beginners] return (user defined type) confusion?

12 Apr 2012

      It has to do with the scope of "where" in your mainloop.
See comments inline.

On Thu, Apr 12, 2012 at 04:08:10PM +0100, umptious wrote:
...
import Data.Map as Map
data Pt2 = Pt2 {x::Float, y::Float} deriving (Show, Read)
data Shape =   Circle   {origin::Pt2, radius::Float}
             | Square   {origin::Pt2, side  ::Float}
             | Rect     {origin::Pt2, other ::Pt2}
               deriving (Show, Read)
enterCircle :: IO(Shape)
enterCircle = return (r) -- needs to rtn io (shape) because will take input
here
       where r = Circle{origin=Pt2{x=1,y=2}, radius=4}
mainLoop :: [Shape] -> IO (String)           
mainLoop shapes = do
    putStrLn "?"
    cmd <- getLine
    if cmd=="q"
      then
        return ("done")
      else do
        y <- enterCircle
This "y" shades the Pt2 accessor "y".
...
mainLoop shapes'
    where shapes' = y : shapes
This where thinks that "y" is actually the Pt2 accessor "y".

An easy fix would be to rewrite this line as:
...
mainLoop $ y: shapes
Or mess with tabs to get the right scope for "where"...

hth,
L.

-- 
Lorenzo Bolla
http://lbolla.info