[Haskell-cafe] newbie help

[Abhijit Ray]

Thanks Tomasz and Cale, it worked now !


On 11/4/05, Cale Gibbard <cgibbard at gmail.com> wrote:
> On 04/11/05, Abhijit Ray <avizit at gmail.com> wrote:
> > Hi I just started on haskell , i am using the yet another haskell
> > tutorial by Hal Daume
> >
> > I wrote the following program and it dint compile.
> > what's wrong with it
> >
> > module Main
> >         where
> >
> > import IO
> >
> > main = do
> >         hSetBuffering stdin LineBuffering
> >         let testList = makeList
> >         let sum = foldr (+) 0 testList
> >         putStrLn "Sum is " ++ show(sum)
> >
> > makeList = do
> >         putStrLn "enter a num"
> >         num <- getLine
> >         let nbr = read num
> >         if nbr == 0
> >                 then do
> >                         return []
> >                 else do
> >                         all <-  makeList
> >                         return  (nbr : all)
> > ----------------------------------------------
> > the error i get is
> >
> > Chasing modules from: Sum.hs
> > Compiling Main             ( Sum.hs, Sum.o )
> >
> > Sum.hs:6:
> >     Couldn't match `[a]' against `IO ()'
> >         Expected type: [a]
> >         Inferred type: IO ()
> >     In the application `putStrLn "Sum is "'
> >     In the first argument of `(++)', namely `putStrLn "Sum is "'
> >
> > Sum.hs:9:
> >     Couldn't match `[b]' against `IO [a]'
> >         Expected type: [b]
> >         Inferred type: IO [a]
> >     In the third argument of `foldr', namely `testList'
> >     In the definition of `sum': sum = foldr (+) 0 testList
> > -----------------------------
> >
> > Thanks,
> > Abhijit Ray
>
> Hello,
>
> The first error is related to the way in which the following line parses:
> >         putStrLn "Sum is " ++ show(sum)
> which is like:
> (putStrLn "Sum is") ++ (show sum)
> It's complaining that you're passing an IO action to (++). It's an
> easy fix, add some parens, or a well-placed ($):
> putStrLn $ "Sum is" ++ show sum
>
> The second problem arises due to not running the IO action you wanted to:
> > main = do
> >         hSetBuffering stdin LineBuffering
> >         let testList = makeList
> >         let sum = foldr (+) 0 testList
> >         putStrLn $ "Sum is " ++ show sum
> What is the type of testList? Well, it must be the same as makeList --
> what is makeList's type? The definition of makeList is in the form of
> a do-block which carries out an IO computation producing a list of
> numbers, so it's (Num a) => IO [a]. Note that this is not a list, so
> it's unsuitable for passing to foldr.
>
> What do we do? We run the IO action using "<-":
> > main = do
> >         hSetBuffering stdin LineBuffering
> >         testList <- makeList
> >         let sum = foldr (+) 0 testList
> >         putStrLn $ "Sum is " ++ show sum
> This will cause the running of main to  *run* the IO action makeList,
> getting the result into the list testList, which now has type (Num a)
> => [a] (here a is going to be defaulted to Integer, since nothing says
> otherwise, and one needs to pick something to be able to read).
>
> Hope this helps, and good luck with Haskell :)
>  - Cale
>