Type error in Haskell program

Question

User can give id, width, height and description rectangle and then I write it to a file. Now I would like to load this content from the file to my program but I have error:

Couldn't match expected type [RectangleType] against inferred type IO [Rectangletype]. In the first argument of menuRectangles namely db. In the expression menuRectangles db. In a do expression menuRectangles db.

What is going on ? This is content of my file: [Rectangle 2 5 6 "abcabc",Rectangle 1 2 4 "abcabc"]

This is code:

import IO
import Char
import System.Exit
import Maybe


data RectangleType = Rectangle Int Int Int deriving(Show, Read)


loadFile :: FilePath -> IO [RectangleType]
loadFile fname =
    catch (do fileContent <- readFile fname
              return (read fileContent)
    ) errorHandler
    where
        errorHandler e = do putStrLn ("Error file")
                            exitFailure

db = loadFile "db.txt"


main = do
    putStrLn "Choose option:"
    n <- getLine
    case n of
        "1"         -> do menuRectangles db; main
        "2"         -> putStrLn "bye, bye"
        otherwise   -> do putStrLn "Bad option"; main


menuRectangles :: [RectangleType] -> IO [RectangleType]
menuRectangles rs = do
    putStrLn "Please choose option:"
    putStrLn "1 - Add rectangle"
    putStrLn "2 - Show rectangle"
    putStrLn "3 - Quit"
    putStr "Number: "
    n <- getLine
    case n of
        "1"         ->  do { {- rs_new <- addRectangle rs; -} menuRectangles rs };
        "2"         ->  do { {- showRectangle rs; -} menuRectangles rs }
        "3"         ->  do { putStrLn "Quitting"; return rs }
        otherwise   ->  do { putStrLn "The End"; return rs }

EDIT: correct code:

import IO
import Char
import System.Exit
import Maybe


data RectangleType = Rectangle Int Int Int deriving(Show, Read)


loadFile :: FilePath -> IO [RectangleType]
loadFile fname =
    catch (do fileContent <- readFile fname
              return (read fileContent)
    ) errorHandler
    where
        errorHandler e = do putStrLn ("Error file")
                            exitFailure


main = do
    db <- loadFile "db.txt"
    mainMenu db


mainMenu rs = do
    putStrLn "Choose option:"
    n <- getLine
    case n of
        "1"         -> do menuRectangles rs; mainMenu rs
        "2"         -> putStrLn "bye, bye"
        otherwise   -> do putStrLn "Bad option"; mainMenu rs


menuRectangles :: [RectangleType] -> IO [RectangleType]
menuRectangles rs = do
    putStrLn "Please choose option:"
    putStrLn "1 - Add rectangle"
    putStrLn "2 - Show rectangle"
    putStrLn "3 - Quit"
    putStr "Number: "
    n <- getLine
    case n of
        "1"         ->  do { {- rs_new <- addRectangle rs; -} menuRectangles rs };
        "2"         ->  do { {- showRectangle rs; -} menuRectangles rs }
        "3"         ->  do { putStrLn "Quitting"; return rs }
        otherwise   ->  do { putStrLn "The End"; return rs }

Presumably so that the question and answers still make sense. — Paul Johnson
– Paul Johnson, Commented Apr 20, 2011 at 16:27

ninjagecko · Accepted Answer · 2011-04-19 16:26:10Z

5

In Haskell there exists a concept called pure code. Pure code does not have any of the following: user-input values, system calls, pseudorandom number generation, calls to non-pure code, etc.

Pure functions are guaranteed to always always always have the same behavior based on the lexical content of your program (e.g. they can return different values, but the reason they return different values cannot depend on "the world").

This is a very powerful policy in functional languages, and allows very powerful code. For example, you know that calling a function won't change some unrelated global variable or the state of some other data structure. I often apply this policy to python code.

The way Haskell enforces the purity-impurity thing is with the IO monad.

Any of things that "the world" touches are wrapped up in the IO monad, which represented that the values have been "tainted". If anything touches these "tainted" values, the values they return must also be tainted.

You will need to run your pure code within the IO monad if you want it to have access to those values. Once within the IO monad, you can "unwrap" the value you've read and pass it into your pure code, then your pure code returns value, then you can print your value. Everything works as expected, but you have to do it in the IO monad.

Note that it is good form to ensure that most of your code is written in a pure, functional form outide of the IO monad. e.g. a pure doStuffWithRectangles function, that is then called from within the IO monad.

The beautiful thing about it is that your pure code doesn't need to know the Rectangle value has been tainted by being of type IO Rectangle. As long as you work in the IO monad, your pure code will think it is just a normal Rectangle.

And just to make this answer more explicit: readFile returns things wrapped in the IO monad, because those things come from "the world" (the filesystem) and for example if you changed the file during program execution, it might return a different value.

--db = loadFile "db.txt" REMOVED--

main = do --within the IO monad
    putStrLn "Choose option:"
    n <- getLine
    **DB <- LOADFILE "db.txt"** --db is now a pure value
    case n of
        "1"         -> do menuRectangles db; main
        "2"         -> putStrLn "bye, bye"
        otherwise   -> do putStrLn "Bad option"; main

More information: http://www.haskell.org/tutorial/io.html

A good online/offile read: http://learnyouahaskell.com/

Also a good online/offile read: http://book.realworldhaskell.org/

edited Apr 19, 2011 at 16:26

answered Apr 19, 2011 at 15:56

ninjagecko

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3 Comments

fuz Over a year ago

As far, as I can see from the question, the OP already knows the concepts of IO. Maybe he just forgot of that single signature.

mrquestion Over a year ago

ninjagecko unfortunatelly Your code doesn't work for me. I have changed my code and now it works (unfortunatelly i don't know how I can write code in comment - why all code is in one line ?):

main = do         db <- loadFile "db.txt"         mainMenu db               mainMenu rs = do         putStrLn "Choose option:"         n <- getLine         case n of             "1"         -> do menuRectangles rs; mainMenu rs             "2"         -> putStrLn "bye, bye"             otherwise   -> do putStrLn "Bad option"; mainMenu rs

ninjagecko Over a year ago

I assume you were aware the line I added **DB <- LOADFILE "db.txt"** is not valid Haskell code due to formatting and capitalization; I cannot tell unfortunately because I cannot read the way stackoverflow is formatting your code, and the fact that a 'mainMenu' has been added in many places which was not there before. Glad you managed to fix it, though it would be great to know the minimal changes necessary. =)

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